Suppose I build an LM1875 power amp, how much current rating should my traffo have if I run it at +-25 volts? how bout at +-12v?
Let's say I'll be using the circuit in the ESP pages (project 72)
please don't give me VA, I don't fully understand it yet plus traffos sold here are rated in amperes not VA
For +/-25V rails and 8 ohms speaker, transformer rating is 18+18VAC , maybe you find it labelled as 35 or 36V CT (center tapped) , it's the same, and current rating must be 1A to 1.5A
VA is no mystery, it's simply V * A , here we are going from 36*1=36VA to 36*1.5=54VA
If your amp is stereo, double the current in A or the VA rating (V stays the same).
If you get a somewhat lower voltage for a *very* good price, or pulled from a dead amplifier (old junked 80's HiFi amp or receiver) for free, go for it, you'll lose a little power , almost unnoticeable ... and nobody can argue the price ;)
FWIW when somebody brings for repair an old Sansui/Technics/Yamaha/whetever old faithful amp and balk at the price, (hey!!! I paid $89.99 for it, NEW!!!! ... in 1989 ... >:( ), they leave it.
I pull the transformer and sometimes the heatsinks, also the fancy knobs, and junk the rest, such tgransformers can always be used for projects similar to yours.
I build nothing with them, although might help me to replace a dead one, but donate them to enthusiastic Musicians who want to build "something".
As of powering it with 12+12V (I suppose DC) amp will work but it won't longer put out 20W RMS , think 5 or 10W depending on load, is that fine with you?
A very common 12+12VAC 1A transformer will give you raw 16+16V rails and around 15W power, not bad.
i do have transformers taken from an old amp and it has about 3 output windings. Is it okay to leave some windings unused? (since I'll only be using the 18+18 ct)
I already have these:
LM1875 chipamp
35A bridge rectifier
phenolic paper copper clad (is this ok or should i use fr4)
36vac ct traffo
large heatsink taken from a 300W amp connected to TIP41's and 42's
resistors (carbon film only)
caps (mylars and electrolytics)
please comment if these are enough
Edit: i don't know the fuses to use for the ps. And I already bought the Eminence Patriot Red White and Blues. I'm planning to use my amp with two speakers in parallel but impedance would not allow. Should I go for stereo?
Quote from: exztinct01 on March 15, 2016, 05:55:28 PM
i do have transformers taken from an old amp and it has about 3 output windings. Is it okay to leave some windings unused? (since I'll only be using the 18+18 ct)
IF the main windings were 18+18 (hint, check wire diameter, I mean copper wire not output plastic covered cable) meaning thickest wire, other secondaries are not that important and can be ignored.
Tell us: what was the original power amp rating?
if a 30/60W MI amp or, say, 25+25W Hifi, fine.
You *may* build 2 25W power amops inside the same chassis, if Power Supply and Heat sink agree.
You might build it on the table, un cased, just 1 channel first, and test it.
Then you decide whether to build it 25W or 25+25 .
QuoteI already have these:
LM1875 chipamp ...check
35A bridge rectifier ... check
phenolic paper copper clad (is this ok or should i use fr4) ...check
**I use nothing but fr4, so I can drill with regular 1mm bits, which last only 50 holes or so with glassn fiber; hard metal bits are incredible fragile and I'm sick of replacing them all the time**
36vac ct traffo ... check wire diameters
large heatsink taken from a 300W amp connected to TIP41's and 42's ... check
**parts: buy what you need, regular parts, no magic dust**
resistors (carbon film only)
caps (mylars and electrolytics)
please comment if these are enough
QuoteEdit: i don't know the fuses to use for the ps. And I already bought the Eminence Patriot Red White and Blues. I'm planning to use my amp with two speakers in parallel but impedance would not allow. Should I go for stereo?
measure transformer EI iron stack, please use mm if at all possible.
Quote from: J M Fahey on March 15, 2016, 07:50:18 PM
Tell us: what was the original power amp rating?
if a 30/60W MI amp or, say, 25+25W Hifi, fine.
I can't tell yet since I left it at home but the amp is a karaoke stereo amp using two TIP41's and two TIP42's
The traffo has 3 output winding, 18-0-18, 0-9, and 0-5-20. Looking at it, I think it can power at least two 100W channels (I doubt if it's RMS since the amp's cheap)
Quote
You might build it on the table, un cased, just 1 channel first, and test it.
Then you decide whether to build it 25W or 25+25 .
can i build it on a solderless breadboard (except the PS and the chipamp itself)
Quote
measure transformer EI iron stack, please use mm if at all possible.
I'll do it later at home
Should I go with panel mount fuse holder for the mains fuse and pcb mount for those between the ps and the powr amp?
Timely thread for me. I'm looking at some pt iron I got when my local RadioShack liquidated, and trying to figure out how to use it.
The big two for current are a 25.2v ct 2A and a 12.6v ct 3A. The 25.2v should be plenty good for a home-use chip amp power amp.
The lower-volt 12.6v doesn't obviously serve any project I can find, though the VA math is just fine for similar low power use thanks to the current. Ideas?
To J.M.:
I can't measure that iron stack. Is that the part where the wire is being wound? I can't seem to see it. My traffo is this one
(http://s20.postimg.org/715j8olrt/WP_20160228_12_05_25_Pro.jpg)
Also, besides my 35A panel mount bridge rectifier, I do have some PCB mount bridge rectifiers rated at 4A and 6A. Can I use them instead? How should I know what rating of bridge rectifier to use?
To Bear:
I think both can be used for home amps. Refer to J.M. Fahey's reply above
Quote from: J M Fahey on March 15, 2016, 12:32:33 PM
As of powering it with 12+12V (I suppose DC) amp will work but it won't longer put out 20W RMS , think 5 or 10W depending on load, is that fine with you?
A very common 12+12VAC 1A transformer will give you raw 16+16V rails and around 15W power, not bad.
Add:
Quote from: J M Fahey on March 15, 2016, 12:32:33 PM
FWIW when somebody brings for repair an old Sansui/Technics/Yamaha/whetever old faithful amp and balk at the price, (hey!!! I paid $89.99 for it, NEW!!!! ... in 1989 ... >:( ), they leave it.
I pull the transformer and sometimes the heatsinks, also the fancy knobs, and junk the rest, such transformers can always be used for projects similar to yours.
I build nothing with them, although might help me to replace a dead one, but donate them to enthusiastic Musicians who want to build "something".
I do love donations and I am an
ENTHUSIASTIC MUSICIAN
Quote from: exztinct01 on March 16, 2016, 10:21:07 AM
To J.M.:
I can't measure that iron stack. Is that the part where the wire is being wound? I can't seem to see it. My traffo is this one
(http://s20.postimg.org/715j8olrt/WP_20160228_12_05_25_Pro.jpg)
All I ask are external dimensions, I can get internal ones from an EI lamination chart , knowing 2 of them .
(http://www.nyp.com.hk/Photos/EI%20Dimensions.gif)
I need A , B , and the thickness of the piled EI sheets, you have a mounting strap around them, no big deal, I'll substract a couple mm from the thickness.
QuoteAlso, besides my 35A panel mount bridge rectifier, I do have some PCB mount bridge rectifiers rated at 4A and 6A. Can I use them instead? How should I know what rating of bridge rectifier to use?
The 4A and 6A ones should be ample, since the amp(s) will pull 1A each.
Quote
FWIW when somebody brings for repair an old Sansui/Technics/Yamaha/whetever old faithful amp and balk at the price, (hey!!! I paid $89.99 for it, NEW!!!! ... in 1989 ... >:( ), they leave it.
I pull the transformer and sometimes the heatsinks, also the fancy knobs, and junk the rest, such transformers can always be used for projects similar to yours.
I build nothing with them, although might help me to replace a dead one, but donate them to enthusiastic Musicians who want to build "something".
QuoteI do love donations and I am an ENTHUSIASTIC MUSICIAN
Oh , no doubt, I can see that ;)
You're welcome, but remember I am in Buenos Aires, some 6000 miles away :o
That said, maybe a local Tech junks similar unrepairable (for $$$ reasons) amps, or you can get them "untested" (might even actually work, but that's not the point) at garage sales or from a junk (unsellable) pile at Salvation Army or even Aunt Emma's attic.
Of course, as is, you pull the useful bits yourself.
Any shipping is a deal killer, transformers are heavy, but search locally.
haha, I do realize that :cheesy:
Okay, A is about 78 mm, B 64 mm, thickness is 35 mm
Ok
1) let's deduct ~3mm from each because of strap metal thickness plus it fits loosely, so we have:
A=75 mm ; center leg C is 1/3A=25mm=1"
2) stack thickness is net 32mm=1 1/4"
In general, commercial EI sizes follow a definite scale, no random values in between (same as caps and resistors also follow a standard scale).
In general if American they grow in 1/4" steps, so popular values are, say, 3/4 " , 1" , 1 1/4" , 1 1/2" and so on so I'm reasonably certain about the 1 1/4" thickness value you found.
By the same token, the 1" center core width fits the standard table.
3) so your core is 1" * 1 1/4" , a very common size, but I'll calculate using mm or cm because it's way easier (when will you Infidel heathens convert to Metric? :trouble )
4) Basic power handling for EI cores: Area (in sq. centimeters) squared.
Easy, huh?
Good iron will handle more, cheap one will handle less, but that's the general purpose estimate.
So for your core:
2.54*3.2=8.13 cm squared. Round down because of airspace between laminations: 8 cm^2 or 8 cm sq or sq cm , pick one.
basic VA (Power) handling: 8*8=64VA .
You can easily pull 80% of that or some 50W .
A Stereo 25+25W amp, 50W total would need some 40% more, if it were subject to continuous tone testing, but on a Hi Fi amp, current demand is not continuous if you play regular Music.
And for Guitar amps?
Well, if you overdrive the amp, make the guitar squeal or feedback for an hour or two, non stop, you'll probably end with a quite hot transformer.
Under normal use, even if playing loud Rock?
No big deal.
I assume that being the highest rated, the 18+18V winding has most of the copper in that window, and other windings were low power, meant for preamp duty.
So it looks like you have a suitable PT for 2 x LM1875 amp :dbtu:
Quote from: J M Fahey on March 16, 2016, 05:12:23 PM
So it looks like you have a suitable PT for 2 x LM1875 amp :dbtu:
+1 ☺
Okay, now I can start building.
Oh wait, what size of wires should I use for the amp?
Can I use typical 22 gauge hookup wires with no shielding or is it very important to use shielded ones?
Just found some time to do a few pics;
This is one of those small LM1785 practice amps sold by the thousands.
The transformer sec is 12-0-12 VAC measures about 50mm x 40mm x 30mm
So the DC rails are around 16VDC.
The front panel says Casino 12watts and it's loud enough to annoy someone in the next room.
Even moreso through a large 12 inch speaker. :tu:
On most of these small amps there is only one fuse on the mains input.
I took the fuse off the PCB and mounted a separate holder on the back as I don't like the idea of mains potential on the audio PCB.
Hope it helps.
(http://s5.postimg.org/m2hm54l93/131.jpg) (http://postimage.org/)
free image hosting (http://postimage.org/)
The other pics are a couple of kits I picked up cheap but they never got used.
This might help you with layout of LM3886 power amp if you end up doing a larger build.
The PCB kit came with LM3876 chips but the supplier kindly swapped these for the LM3886. <3)
So hence the board reads 40watts.
Worth a mention; note how the heat sink has a thick base and the thinner fins radiate out from that mass. This is way better than a thin base with a lot of fancy fins.
(http://s5.postimg.org/6ia8elb4n/121.jpg) (http://postimage.org/)
free photo upload (http://postimage.org/)
(http://s5.postimg.org/5u1dvncev/118.jpg) (http://postimage.org/)
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Phil.
wow, really big heatsink for the 3886
Guess I have to make some friends in the aluminum and glass supply industry here to have those for cheap
The fuse for the LM1875 above is 500mA quick blow.
Don't over thunk it all,, this is a small amplifier and in rush is not really much of an issue fast blow is fine. :dbtu:
If a 500mA fuse keeps blowing then up it to 620mA,, if 800mA or more blows that tells you that you have a serious problem in the circuit,, time for deeper search. Then you use a limiter bulb tester otherwise you will destroy the whole unit. :'(
I've seen some with no fuse just a thermal cutout unit inside the transformer.
Yes that heat sink is over kill you don't need it that big but gives you a clue as to how a good heat sink is designed. BTW,, there are 2 power amps on that heat sink hence I used a larger one. 8|
Phil.
Toroidals require slow fuses because of their much higher inrush current , even if unconnected to any load, it takes some power to magnetize all that iron.
A fast fuse blows on turn on.
EI are lossier so in a way somewhat self protected.
@ phatt, after originally liking that Jaycar PCB, it looks nice, I noticed input and output are within millimeters of each other, WTF? :duh and also that output Zobel (2r7 + 100nF) are printed but not fitted.
Chipamps often work without it ... until you connect speakers with a long cable that is 8|
accdg. to Rod Elliott "no heatsink is too big"
Quote from: J M Fahey on March 20, 2016, 10:19:19 AM
Toroidals require slow fuses because of their much higher inrush current , even if unconnected to any load, it takes some power to magnetize all that iron.
A fast fuse blows on turn on.
EI are lossier so in a way somewhat self protected.
@ phatt, after originally liking that Jaycar PCB, it looks nice, I noticed input and output are within millimeters of each other, WTF? :duh and also that output Zobel (2r7 + 100nF) are printed but not fitted.
Chipamps often work without it ... until you connect speakers with a long cable that is 8|
Re the Zobel,
Those parts were moved so to be closer to the pin. I recall our dear friend Roly mentioning best to keep the Zobel as close to output as possible so I altered the PCB slightly.
(The big resistor standing on end covering the Jaycar name) :tu:
Yes I did notice the input/out issue but they were cheap. :-X
I doubt this PCB design would win awards for the best layout, even with my DIY skill I would have done that layout in a different way and I don't like the narrow traces.
I've not yet fired these units up so I can't make judgement as to how well they work. ??
Phil.
that one rod elliott's referring to that comes close to providing proper heatsink to their amps
from here http://sound.westhost.com/articles/guitar-amps.htm
"Of all the brands, there is one US maker who seems to generally get most things more or less right. There have been some spectacular blunders with early valve amps and some of the re-issues, and the continued use of completely unshielded pickups and wiring inside many of their guitars is a constant source of irritation. However, they do seem to enjoy comparatively better overall reliability than many of the others, but there will still be exceptions. Many of their transistor amps are borderline IMO, but don't often fail - mainly because modern power transistors are extremely rugged and regularly outperform their datasheet maximum ratings and published safe operating area.
Hey Phil, in your LM1875 layout, I see that you had a separate pcb for the chip. Besides the chip, what components are also located in that pcb?
Just the 5 jumper wires with a small cap across pin 3 and 5 which are the supply rails. :tu:
The power chip was originally just bolted to a small plate of Alloy which obviously worked until it blew up,, which is why I ended up with it. by chance I had a spare chip so I got it working again. This time with a much larger heat sink. :dbtu:
If you read the whole ESP page you will note he mentions chip amps getting too Hot.
If you read the data sheet for LM3886 the fine print reveals that you have to limit supply to +/- 28VDC if you want to drive 4 Ohm Loads. Hum not so great after all :-\
As to what Amp Brand ESP refers to,, well you will have to ask Rod. :tu:
Some of the early peavey stuff was good but later maybe not so good. Then again you could probably say the same about a lot of brand name gear now :-X
Because he mentions guitars with no shielding cable then I'd guess Fender would be the obvious brand as they are still wiring up Strats without coax cable which is Plain dumb. :loco
Phil.
Quote from: phatt on March 21, 2016, 04:36:46 AM
Because he mentions guitars with no shielding cable then I'd guess Fender would be the obvious brand as they are still wiring up Strats without coax cable which is Plain dumb. :loco
Phil.
What do you mean by "dumb" young Australian kid?
I have been playing my unshielded wiring 1956 Stratocaster plugged into this 1 knob (volume) all of 5W beauty since forever
(https://lectrolab.files.wordpress.com/2010/10/troub-amp-11-03-10-01.jpg)
and I have never ever had a problem although I play LOUD.
At least that's what Nurse Emilia at the Retirement Home always complains about.
Oh!!!! I see!!!! you mean that radio which is heard in the background?
Hey !!!! , a guy playing alone needs a little backing track to play along, doesn't he?
And if I set tone control to "dark" I
almost make it disappear :o
Shielded wiring!!!!!!!
You young kids are all spoiled kids !!!!!!!
I alway say: "spare the rod and spoil the child!!!!!! " :trouble
Hey I'll take the "Young" as a compliment. :tu:
Yes now that I'm older and wiser I've dramatically reduced most of the Dumb stuff I do. :P
I read a good one the other day;
Intellect is God given,, Wisdom comes by learning from all your dumb mistakes. :lmao:
Phil.
Quote from: phatt on March 22, 2016, 04:05:16 AMIntellect is God given,, Wisdom comes by learning from all your dumb mistakes. :lmao:
Phil.
If that is true then I thought I would be a wise genius by now... :duh :lmao:
i wasted about 5 hours today trying to figure out (draw) how to make a layout for my lm1875 amp based on ESP Project 72. i looked at the national datasheet and found only small difference in the schem. Someone mentioned that the bypass caps and the zobel should be as close as possible to the chip but how far is not advisable?
Phil, I've seen your lm1875 layout where the chip is separated from other components of the power amp. Is it okay if the chip is a bit far from those bypass caps?
Also, which traces or wires should I avoid getting close to each other?
I'm assuming you are using some kind of software to design the pcb?
If So, ideally mount all the power amp parts on the same PCB and keep the Zobel close to output pin of chip, within 20~50mm.
Don't use that LM1875 Casino amp I posted as a good example as it's far from perfect layout wise. But it does show that in spite of crap layout they do work.
Star grounding for common is not just for the pcb of power amp layout,,
(look at incurably_optimistic star ground layout)
You have to realize it starts at the rectifier supply,, Not at the common of the PCB.
Usually it's the Com1 Join at the first 2 main filter capacitors.
Sure if the Rectifier and Filter caps and Poweramp parts are all on one PCB then it's easy to make the Com 1 point but if supply is on a separate board or chassis mounted then it's easy to loose sight of the Common path.
Somewhere on ESP site Rod mentions this point and although it won't stop the circuit from working,, it makes it harder to track down hums and buzzes. I'll have a hunt for it later.
If your rectifier and Main filter caps are separate from power amp PCB then keep the wire connections as short as you can. Say no more than 150mm,, 6 inch long.
Oh yes try and keep input and output on other ends of the Pcb is good design.
That issue is a big problem with hiZ preamp circuits and I've walked into that trap when breadboarding circuits,,, but hey,,that is how us diy folks learn,, I come highly qualified,,,,,,,,from the School of Hard Knocks. :lmao:
Hope it helps,, Phil
Thanks Phil. BTW I'm using Fritzing for designing the PCB and I'm having a hard time designing coz I don't have the LM1875 part. And I also didn't know what components should be far from each other. But your post helped me a lot.
+1 ;)
All good,,
Adding
Oh yeah,, another point regarding PCB design; Most of them are geared towards computer Mother board type circuits (low voltage) so track width and pad sizes are set small.
That is not what you want for diy home drilled and hand soldered pcb. :-[
So find the "global parameters" and setup some wider tracks and larger pad sizes before you go joining the dots.
I use the lazer printer and hot iron technique for making my PCB's and found that tracks less than .5mm wide gives me lotsa trouble. :grr
I use 1mm drills as .8mm drills break real easy so Pads less than 2mm leave only
.5mm of copper to solder. Larger components get 3mm pads where possible.
Phil.
Quote from: exztinct01 on March 27, 2016, 06:51:14 AM
Thanks Phil. BTW I'm using Fritzing for designing the PCB and I'm having a hard time designing coz I don't have the LM1875 part. And I also didn't know what components should be far from each other. But your post helped me a lot.
+1 ;)
Ouch!! that "free" Fritzing software is f***ing EXPENSIVE!! :loco
I was about to download and try it when I read how to use it after design: they use a proprietary file format only understood by them, must be sent to their own fabrication house, prices are crazy :duh
They charge U$ 1 a sq cm, or ~U$6 a sq in, so my typical 100W board, 10x15 cm (4"x6") would cost U$150 :grr :trouble
I make it for about $2 :lmao:
Check that you can "export" or save the final design as a printable *graphic* file (.gif / .jpg / .bmp / etc.) so you can print it on paper (or transparent/drafting paper) and use it as a real size guide to make your own PCB
Or use pedal maker oriented http://diy-fever.com/software/diylc/ which is simplest and fastest to learn.
It also designs perfboard and Veroboard, besides etched PCBs.
Also, since it is VERY popular, there is a arge free library of user contributed designs (in fact, you are encouraged to add yours after successfully tested) , I suggest you look at all designs to have ideas on how others, with similar background and problems, solved theirs.
Just one small detail: there are 3 versions, first one Windows only, other two try to be "universal" (Mac, Linux, etc.) by running on clumsy Java code ... buggy and full of problems so stick to V1
https://github.com/bancika/diy-layout-creator/releases/download/REL_3_11_0_BETA/diy_layout.exe
http://diy-fever.com/wordpress/wp-content/uploads/2009/12/layouts-01-19-2010.zip
You don't need to have an actual LM1875 in your hand to design for it, software package *must* have a part symbol called something like "TO220 5 leg" because
many chips have same size and pinout: TDA2002/2003/2006/2030/2040/2050 , LM1875, etc.
If not, draw it yourself, chip datasheet usually shows case size, leg diameter and separation, etc.
http://www.ti.com/lit/ds/symlink/lm1875.pdf
check page 13 for "mechanical dimensions" .
They also suggest a couple generic PCB designs which you can copy or use as a guide, on pages 9 and 10 .
This is what you may end up with ;) :
https://www.youtube.com/watch?v=Fq56qW4gP98
Quote from: J M Fahey on March 27, 2016, 01:38:06 PM
Ouch!! that "free" Fritzing software is f***ing EXPENSIVE!! :loco
:lmao:
I actually considered their Fritzing Fab service but it really is soooooo...... expensive
Quote
Check that you can "export" or save the final design as a printable *graphic* file (.gif / .jpg / .bmp / etc.) so you can print it on paper (or transparent/drafting paper) and use it as a real size guide to make your own PCB
Yeah, fritzing has options to export as PDF (actual size), SVG and Gerber.
Quote
Or use pedal maker oriented http://diy-fever.com/software/diylc/ which is simplest and fastest to learn.
It also designs perfboard and Veroboard, besides etched PCBs.
I also have this, the old and the new versions. I used it already for some of my pedal designs. I downloaded the old version to be able to view those old layouts compilation. So I guess I just have to find its users' community for parts. Thanks.
okay I have started the LM1875 already but I have some questions:
1. Rod Elliott's schem shows R6 as 10 (10 ohms maybe?) (Encircled PURPLE)
(http://s20.postimg.org/9xcrc3dq5/LM1875_Schem.png)
but in the following text, he stated that the 1 ohm resistor (R6) should be a 1 watt type. So what really is the value of R6? 10 ohms or 1 ohm? (Text is highlighted in the next image)
(http://s20.postimg.org/t69u23xvh/Project_72.png)
2. Also, I don't know what came to my mind in creating the layout. Rod's schem doesn't have a 10 ohm resistor in between the common of the input and the common of the power amp (Traced RED in the first image). But I decided not to directly connect the two traces and then provided holes for a 10 ohms resistor, which I have seen in most LM1875 gainclone projects.
(http://s20.postimg.org/kmqg4cpj1/LM1875_amplificatoare.png)
this is my actual pcb, components are not yet soldered
(http://s20.postimg.org/8nexx1jy5/WP_20160423_23_00_20_Pro.jpg)
should I put a 10 ohms resistor? what difference would it make? (NOTE: The amp is to be used as a power amp for guitar, coupled with Rod's P27 preamp)
3. Lastly, Rod warned not to connect the speaker return to the amplifier's earth bus, coz it will get oscillation. My actual pcb above has a 2 contact terminal block on the right for speaker out (+ and -), well I think it can be seen that the (-) goes to the middle of the 3 contact terminal block, which will return to my star ground located near the filter capacitors of the PS. However, that speaker return is connected to the amp's earth bus, will it be ok since the other signals' paths are not that large to allow the speaker return current to pass through easily? Or should I just connect the speaker return directly to the star ground, or use a 5 watts 0.1 ohm resistor on the way back to the star ground?
Can I use the schem below for the speaker returns? One goes to the amp's ground, another goes to 0R1 5W resistor then to star ground?
(http://s20.postimg.org/6f0rfpcu5/Screenshot_90.png)
Arrh ,, Why don't you Email Rod and ask him for conformation? Looks like a typo error. :tu:
I'd Simply connect the speaker Neg back to the star ground point which is the joint where the 2 First Main filter caps join to common point. That same point should then go to Chassis case.
As far as the .1 Ohm resistor (sometimes written as 0R1) used to create a bit of current feedback,,,
it's up to you.
Every technical person will have their own opinion on this subject. :-X
I personally have tried this both ways on many different poweramp circuits and the difference is very hard to actually hear. Be aware there is a slight loss of wattage and on such a small amp circuit I just don't see the point.
My best observation is it may help on bigger amplifiers where power is plentiful and then it may only help a little at full power other than that,,, there is not much to be gained. YMMV.
That 10 Ohm ground lift resistor to isolate the preamp is also another thing that may end up creating more problems than it solves. I'd just bridge it to the common and be done with it.
If it was a big 400watt power amp designed to run a complex system there might be some benefit but hey it's a small 10~15 watt amp,, so don't over think it.
As you have built this in modular form you can easily change things after it's built so no big deal. :tu:
It may interest you to know that I've just built a 2x10 inch speaker box and for kicks I ran it from that little Casino 12 watt amplifier (LM1875) Posted on page 1. Not bad considering it's only running on 17 volt supply rails. I could use it for small gigs as long as the drummer does not slam the kit. :grr
For home use this would be more than you would need. Which proves again what has been said many times; small amps are severely ham strung by the tiny speakers as it's just not economical to make small wattage amps with big speakers.
Those little 1 watt smokey amps through a quad box are quite loud. 8|
Phil.
Quote from: phatt on April 24, 2016, 07:52:31 AM
Arrh ,, Why don't you Email Rod and ask him for confirmation? Looks like a typo error. :tu:
ah maybe, I'll just ask him then
Quote
Those little 1 watt smokey amps through a quad box are quite loud. 8|
Phil.
Yeah, and this is how it would look like if I'm the one building it 8|
(http://s20.postimg.org/kkb5v9b99/WP_20160424_13_14_27_Pro.jpg)
without Rod's reply yet and having just read about the zobel, I computed the resistor needed for my LM1875 amp. The Red White and Blues has an Re of 6.42 so multiplying it by 1.25 gives me a value of 8.025. Therefore, I'm gonna use a 10 ohms resistor for the zobel. Confirmed?
PS: Since the Philippines' electrical system doesn't have grounding (true earth) after entering residential meters (i heard that one of our two prong outlets is actually grounded but not sure which one), if I am to just let my circuit's common float, of course common to chassis connection is a BIG NO. Aside from that, do you have any suggested modifications, additions or other reminders for a floating ground circuit? I still have a plan to add true grounding in the future if it's safer.
Woah,, as you are asking about Mains Earth Then it's a Safety issue so I'll let someone qualified to answer that Question. 8)
Phil.
Quote from: exztinct01 on April 24, 2016, 04:02:52 PM
PS: Since the Philippines' electrical system doesn't have grounding (true earth) after entering residential meters (i heard that one of our two prong outlets is actually grounded but not sure which one), if I am to just let my circuit's common float, of course common to chassis connection is a BIG NO. Aside from that, do you have any suggested modifications, additions or other reminders for a floating ground circuit? I still have a plan to add true grounding in the future if it's safer.
neutral (N), most likely blue cord, should be equal to earth, if there isn't a 3 prong outlet, leave it floating don't ever connect chassis to another cord than ground, someone WILL get electrocuted if phase and neutral gets switched around in a way that phase is connected to chassis (and guitar player).
earth isn't connected through the meter but most probably to the frame where meter and fuse boxes are mounted. or a in houses a ground spear hammered in the ground.
a ground connection is safer it removes the accumulated voltage difference between appliances, so no more minor electric shocks around the house,
1) Starting with the dangerous stuff, agree that NO part of the 2 pin mains wiring shouod ever contact chassis, even though one of them is probably grounded at the ustility box.
Point is you use the same 2 round pin old European style as we did (and is still used in all homes built before 1990) and it can be easily plugged the other way and kill you.
2) the problem with the design you linked is that they run a long ground wire from power amp to external supply , shared between speaker (amperes of current) and feeble audio signals, a mess.
They will NOT solve that with the cheesy 10 ohms resistor "separating" grounds because there is not such a separation ::)
I have made thousands of amplifiers, never worried too much about star grounding and other fashion ideas, hardly ever have problems, BUT I **always** return speaker "-" terminal to best ground available: the short fat stub joining both main filter caps with its own wire or track, period.
Once you take care of the Lion, mosquitoes are not that important.
So dedicate an extra wire for speaker grounding or a track if supply is on same PCB.
And locally decouple amp with at least 2 x 100uF x 25V caps, same with preamp.
"Keep trouble inside each home".
3) 1 or 10 ohms? .... no need to ask Rodd , just download the LM1875 datasheet and see what they suggest.
10 ohms is a general purpose classic, but I have seen quite a few chipamps using 1 ohm (and one case: 0 ohms, just a 0.1uF ceramic :loco )
Here's my power amp and ps almost ready for testing. Power amp is exactly from ESP Project 72, PS used 2 caps 2200 uF 35v only since it's what's available in my storage. I won't be using my newly purchased transformer since I don't trust it's rating, so I think I'll stick with the transformer I showed in the first page.
(http://s20.postimg.org/oi82vitrx/WP_20160426_12_25_09_Pro.jpg)
I just put a dedicated faston terminal in the star ground for the speaker return, another one for the transformer's 0 volts.
Cool and congratulations. :dbtu:
I just posted about transformer size on the other thread (both should really be merged in to 1)
sorry, I really should have merged the two threads. I'll stop posting there then. Anyway, it was suggested that because of the clipping at the output of the P27 preamp, power amp should have a specific input sensitivity ( I forgot the value), how then can I find the input impedance of my power amp?
I don't think this is a safe way to test my amp but after determining that my output doesn't have a significant amount of dc, I proceeded to connecting it to a 4 ohms 5 watts small speaker (i added a 10 ohms resistor in series with the speaker), the input to my laptop, and then fired it up. I haven't used a fuse yet :) but I'm glad it worked. I won't be doing this setup again, so dangerous, and my table is a mess
(http://s20.postimg.org/5ncc53tq5/WP_20160426_15_34_50_Pro.jpg)
I've looked everywhere for a PCB layout of the P27 preamp and I can't find any, only to discover this thread http://www.ssguitar.com/index.php?topic=2856.0 (http://www.ssguitar.com/index.php?topic=2856.0) hiding here.
I hate GOOGLE sometimes :grr
Some thoughts:
1. Roly reminded in this thread time and again the importance of connecting true earth to the amp's common since that common is connected to the guitar's strings. But if that common might have very high potential, why are guitar amps in some countries such as the Philippines being used without that true earth connection (floating ground)? In what possible scenario might that voltage go to the guitar's strings?
2. How 'bout showing my layout and etching method, for a good laugh <3)
First: Create a pcb layout in Fritzing, exporting it as etchable pdf, and printing its mirror image actual size on a bond paper.
Second: Cover my copper clad with masking tape, then paste the image to the tape with glue
Third: Use cutter to remove the parts of the paper and tape which covers the copper to be etched away.
(http://s20.postimg.org/ln46g54jh/WP_20160423_14_52_18_Pro.jpg)
Fourth: Etch using Muriatic acid and hydrogen peroxide solution
(http://s20.postimg.org/xrjfx4hfh/WP_20160423_17_33_17_Pro.jpg)
Fifth: Drill
(http://s20.postimg.org/ctd9z1hkt/WP_20160423_17_32_44_Pro.jpg)
Doesn't give me a beautiful pcb layout but it gets the job done, plus I can always adjust the tracks and pads width, diameter and position according to the actual components I have on hand. Also, I think it's the cheapest method for etching :tu:
Quote from: exztinct01 on April 26, 2016, 10:34:54 AM
I've looked everywhere for a PCB layout of the P27 preamp and I can't find any, only to discover this thread http://www.ssguitar.com/index.php?topic=2856.0 (http://www.ssguitar.com/index.php?topic=2856.0) hiding here.
I hate GOOGLE sometimes :grr
1) that design has errors, triple check it against the schematic.
First one I detected, which makes me think there may be more: output clipping fiodes are two in series pointing up, in parallel with two in series pointing down (look at the schematic and this becomes clear) , now look at bthe PCB: he has 2 in parallel pointing up, and 2 in parallel pointing down ... not the same.
2) that design has way too wide tracks, way too close.
I suggested as a general purpose guide to start with 40 mil tracks (1 mm) separated by 40 mils (1 mm) at least, and pads 100/125 mils in diameter (2.5 to 3mm) , logic is that leg holes in general are 1 mm so you want at least 1mm copper around for proper soldering.
In fact part legs would be happy with 0.75mm or 0.8mm holes, but such drills must be special ordered (hint, I get them at Jeweller's supply houses, but they are expensive) while 1mm is a common value which can be found in the corner shop.
3) I was worried about your printed picture which showed blank areas along some tracks, or a thick one with a very thin parallel one, maybe it was just a picture reflection, but etchings look very good.
Kust polish copper shiny with steel wool before drilling; it's harder after because surface is no longer flat and steel wool catches in the holes.
Wash and dry your hands well before touching the PCB, touch them only by the edges, avoid touching bare copper because skin salts attack it,repeat hand washing every 15 minutes (no kidding).
Make some protective varnish with a small stone of tree rosin inside a marmalade/jam flask, then cover with alcohol or, worst case, paint thinner, let it dissolve fully (a couple days)
Brush it on copper , when dry will protect it against oxidation and act as flux when soldering, win win situation.
You may have to grab PCB by one end (always by the edges) and slam it against a table (cover it in newspaper first) so holes are not filled with liquid flux which might block them later.
Some people spray a fine mist of Krylon acrylic **instant dry** artist's varnish (found in Art supply shops, what they spray on pencil drawings to avoid later smearing) but it's not "flux", just a protective anti oxidation coat, molten solder will melt it away but it will need a hot iron and does not really help soldering as good old rosin.
Colophony pine rosin:
(http://incensemaking.com/images/ingredients/pine-colophony.jpg)
also found at Health/New Age/incense stick suppliers.
I suggested a metallic cap jam glass bottle
(https://www.freemanharding.co.uk/img/p/2-5-thickbox.jpg)
because it's hermetic and durable, a plastic container will let contents evaporate and not available when you need them, remember it takes a couple days for the stone to dissolve.
If too thick add some more alcohol or thinner.
Also very good to slightly dip solder wire tip in it to help in difficult soldering jobs.
Some thoughts:
1. Roly reminded in this thread time and again the importance of connecting true earth to the amp's common since that common is connected to the guitar's strings. But if that common might have very high potential, why are guitar amps in some countries such as the Philippines being used without that true earth connection (floating ground)? In what possible scenario might that voltage go to the guitar's strings?
2. How 'bout showing my layout and etching method, for a good laugh <3)
First: Create a pcb layout in Fritzing, exporting it as etchable pdf, and printing its mirror image actual size on a bond paper.
Second: Cover my copper clad with masking tape, then paste the image to the tape with glue
Third: Use cutter to remove the parts of the paper and tape which covers the copper to be etched away.
(http://s20.postimg.org/ln46g54jh/WP_20160423_14_52_18_Pro.jpg)
Fourth: Etch using Muriatic acid and hydrogen peroxide solution
(http://s20.postimg.org/xrjfx4hfh/WP_20160423_17_33_17_Pro.jpg)
Fifth: Drill
(http://s20.postimg.org/ctd9z1hkt/WP_20160423_17_32_44_Pro.jpg)
Doesn't give me a beautiful pcb layout but it gets the job done, plus I can always adjust the tracks and pads width, diameter and position according to the actual components I have on hand. Also, I think it's the cheapest method for etching :tu:
Quote from: J M Fahey on April 26, 2016, 11:46:33 AM
1) that design has errors, triple check it against the schematic.
2) that design has way too wide tracks, way too close.
I haven't reviewed that layout yet, I just assumed it was good but I actually am going to check it tomorrow. Also, when I see a layout like that, I first build it on a software pcb designer to be able to edit or improve the design.
Quote3) I was worried about your printed picture which showed blank areas along some tracks, or a thick one with a very thin parallel one, maybe it was just a picture reflection, but etchings look very good.
:) There are no blank areas there, etching was actually good except for some unintended track bridges coz I didn't recognize them soon. I was too careless that time. But I removed those bridges using my handrill.
Also, wow! for the protection advice. I already knew that pcb's needed protection from oxidation but I somewhat kept on ignoring them since all I've been doing before are small projects with cheap components. But for a power amp, especially a guitar amp which I intend to use for a long time, durability and reliability is very important. Also, I think it's a good practice to protect my pcb even though I'm only a beginner since complex circuits with more costly components are surely coming in the future.
Although, I do pray for a permanent math instructor job so I can afford a professionally made pcb. It's hard to spend money when after every semester, you don't know if you will be rehired. :(
question: that tree rosin you showed me, does that call for a specific tree (e.g. pine)? i think we have some type of tree rosin here we call "kamanyang", but it's not pine. Still, if that calls for a specific tree, I'll be searching for it here.
Add: Here is another shot of the pcb I etched. Some solders don't look too good since I don't have a good iron tip right now, but I tested the amp and it worked and sounded good. Also, I already tested for shorts and dc in the output but found none.
(http://s20.postimg.org/smi8fi4hp/WP_20160427_08_44_37_Pro.jpg)
Well, you tried to solder on very dirty and oxidized copper, that makes it very hard work for the soldering iron.
You should have polished it bright with steel wool, or *very fine* sandpaper.
In fact, I recommend you disassemble that board, pull out as much solder as you can (melt it and *quickly* swipe it out with a dry cotton cloth) and lightly sand it with 600 grit sandpaper, what car paint shops use.
No kidding.
Otherwise if you have any problem, you won't know whether it's a real problem or just poor soldering.
I checked "kamanyang".
It *is* a tree rosin, and should work, but the name applies to the most expensive there is, the incense rosin used in Church ceremonies, specially Catholic and Orthodox Church.
Not sure what you mention is the real thing (which would be like using $80 a bottle Stolichnaya Vodka when a project calls for $1 a bottle "denatured alcohol") but **maybe** in Philippines the word became generic, meaning any cheap rosin.
The one I mention must be available for cheap , a couple U$ a kilo at the neighbourhood hardware shop, plumbers use it to help solder lead plumbing pipes, go figure.
It should also be available for cheap at a local small chemical supply shop, the kind which also sells unbranded detergent and clothes washing soap in bulk, rat poison, floor cleaner/deodorant, caustic soda, muriatic acid, etc.
Real kamanyang should be available at a Religious objects/Santeria shop or a New Age one, to be burnt over coals and "send bad spirits away".
IF some Spanish use remains, what you want is "resina de pino" , while kamanyang is "incienso".
Quote from: J M Fahey on April 27, 2016, 08:26:07 AM
Well, you tried to solder on very dirty and oxidized copper, that makes it very hard work for the soldering iron.
xP
I didn't really know it was oxidized by how it looks :duh
how stupid of me :loco
and our kamanyang is not expensive, well maybe the one used in churches here are, but many are sold cheap and bought by some healers
so I guess the name just became generic here
Update: Just finished desoldering components from and sanding my amp and PS pcb. Still waiting for the tree rosin to dissolve completely.
BTW, I bought that kamanyang and what they gave me are two different items sold together. Vendors say they are burnt together. In the attached image, the darker ones on the left feels and looks like wood bark or anything related to wood, and the left ones colored yellow looks like crystals but are brittle. the woodlike ones are called kamanyang and the crystal like ones are incenso according to the vendors
(http://s20.postimg.org/56ern3t4t/WP_20160428_13_20_35_Pro.jpg)
(http://s20.postimg.org/9gtfioy7x/WP_20160428_13_24_11_Pro.jpg)
(http://s20.postimg.org/6wxsolanx/WP_20160428_21_14_47_Pro.jpg)
I put them inside containers separately and poured ethyl alcohol, and will try to see which is the tree rosin, or maybe both will give me what I need,
I just tested my P27 preamp in breadboard, connected to my LM1875 power amp and it worked good. But I used old, used and all linear pots for the preamp so I'm planning on purchasing some pots for it tomorrow before I put it in pcb. So, which among those 5 pots should be log and which should be linear?
Also, is it necessary to shield the preamp from the power amp?
In general, volume/bass/treble/master should be Log/Audio, Mid is usually Linear.
As of shielding preamp, not high gain here so it's not *mandatory* but it never hurts, it may keep unintended FM radio away.
in ESP P27A, there is a 10 ohms resistor in series with the return path from the preamp. What's its purpose? Is it for isolating the preamp's common?
Also, is it necessary to connect the preamp's and the power amp's common? or can I just use a shielded wire, connect the shield ONLY in the power amp's common, NOT in the preamp's common? This way, their only common connection is through the power supply.
first, thanks to all your help for finishing my first guitar amp. It's not yet in a chassis since I don't have the tools for building it but when I get some money, I will immediately go to a metalwork shop to order a custom build one.
My take on my first build (again, it consists of P27 preamp using TL072 connected via shielded wire to LM1875 single channel where the shield connects only to the power amp, common connection is through the PS)
- Noise is very low, I barely hear it when not playing, even when both Volume and Master Volume are at maximum.
- We actually used it (uncased) in a praise and worship setting since our guitarist was so excited about the amp. Honestly, our drummer plays so loud that our soundman finds it hard to bring the vocals and keyboard up to its level, but the guitar in my DIY amp sounded so clear throughout the hall, with leads singing ;)
- After about 45 minutes of playing (our guitarist was not using any effect pedals that time, just the amp and guitar at maximum volume both), I touched the heatsink and the IC and HONESTLY, it did not even get warm (being out in the open air of course helped).
- Our guitarist even asks me why there is no gain pot and I don't know what to answer him. All I know is that the preamp already has two volume controls and the Master Volume at max produces a little bit distortion. But he asks for more, I don't know how to add a circuit that will integrate a gain control there.
- After our service concluded, I put my DIY MXR Distortion Plus in front of the preamp, set it with maximum gain and Volume at about 3 then played a song with distorted guitar. Turning the pedal on makes a dramatic increase in volume compared with the pedal off so I don't know how to make the transition from clean to distorted using the DS+ sound good. Some suggested that I try different diodes than the 1N4148's.
- Also, when I tried to increase the pedal's volume (I think it was at 7 and above), the speaker suddenly squealed. That high pitch sound disappears when I lowered the volume. So, how do I remove that squeal? Should I maintain my pedal at low gain or low volume? I avoid having my pedal at low gain and volume coz I don't think I hear distortion at those settings. I believe I hear those distorted sounds clearer at high volume.
- Also, after playing with the pedal, my LM1875 IC and heatsink got warm. Not too hot, just warm. :)
Any suggestions, opinions, recommendations, mad reactions are accepted :trouble
Quote from: exztinct01 on May 22, 2016, 09:50:17 AM
- Our guitarist even asks me why there is no gain pot and I don't know what to answer him. All I know is that the preamp already has two volume controls and the Master Volume at max produces a little bit distortion. But he asks for more, I don't know how to add a circuit that will integrate a gain control there.
I've build this preamp and I think of the "volume" control on the schematic as a gain control. This control sets the level of the signal before the second op-amp stage with a set gain and clipping diodes, so using this pot not only changes the volume of the pedal, but also amount of clipping.
If you want more distortion out of this preamp, I think the easiest way is to put some booster pedal before the amp in the signal chain - with hotter signal at the input the diodes would clip more easily. If you want to be able to control the gain of specific op-amp stages, as those work as non-inverting amplifiers, you can lower the values of R11 or R6 to 2,2k (that's the lowest value you should use according to Rod) and wire a 5k resistor in series. Although in my opinion that would be redundant to the volume control.
But keep in mind that I'm as new as you to the area of building amps so take this comment with a grain of salt! Better wait for someone more qualified to answer before you make any changes (and the best idea is to put the circuit on a breadboard and then experiment)
oh I know you're still way better than me at this :lmao:
I don't think I'll be integrating a drive in the amp. If he wants distortion, then I'll build him a TS808 or a Big Muff Pi aside from my Distortion Plus. Or like you said, I may build an AMZ Booster.
I find it a bit bassy though, the preamp. How about you?
I didn't have in mind putting a booster in the same box, just add a booster pedal to the end of your signal chain if you want to get your distortion from overdriving the amp rather than from a distortion pedal. I prefer to get distortion from my pedals rather than amp, but I liked the overdrive from this preamp so it's just an idea.
Well, I like a lot of bass from the guitar amp so I actually increased the amount of bass in this preamp :duh but if you want less, try a bigger value for C6 capacitor for starters.
Think twice before building him a BMP if you don't like bass as this thing has TONS of it and not much of treble response. :grr Of all the overdrives/distortions I've build one of my favorites is BSIAB II, check it out if you haven't already!
BSIAB II, I'll definitely try that ;)
In a system composed of a preamp, power amp, them sharing a power supply, which is better? Isolating preamp from power amp or isolating preamp from direct connection to power supply which means its ground connection would pass through power amp first before going to star ground?
I don't even know if I'm asking the right question :lmao:
I thought maybe if both preamp's and power amp's common are connected directly to PS, and also with each other, there will be a loop and it will do no good for the system if power amp's return will pass through preamp. Am I getting it right? Please help me fix my brain. :loco
If the ground points of the preamp and power amp are connected to each other directly and also there are two separate connections from the preamp and power amp to the common point, then you're asking yourself for trouble - like you said, you're creating a ground loop and also there's a danger that a high current return from the power amp would travel to the common point through the pre-amp stage, which of course is not desired. You can read more about this in this excellent post: http://www.ssguitar.com/index.php?topic=288.0
What * incurably_optimistic* said :dbtu:
just adding;
The idea is to keep low current high sensitivity preamp ground returns picking up High Current of the power stage.
Think of the ground plane as a second power supply rail and needs as much attention as the voltage supply rails.
In schematics the ground path is often assumed for the sake of clarity but that can be rather confusing for the novice.
Opamps have fairly good PSR (power supply rejection) so you can get away with a bit more but the old days of discrete (both Valve and Transistor) you had to be very mindful of ground path.
Look up Valve Wizard's site for some common ground plane gotcha's,,on Valve layouts.
Phil.
in a power amp pcb (e.g. P27 power amp), is the signal common connected to its supply common? Mr. Fahey stated in his post they should return to star ground in different paths. P27 power amp have only one return path and I think both signal and supply commons pass there since preamp is isolated by a 10 ohms resistor from the common point.
this is my amp layout, let's say it's high current amp. Which connection should I cut?
Power amp's pcb has signal common and supply common directly connected, same with preamp. I already found some errors like my center tap terminates after filter caps. Another, loops. May I also say that I don't have true earth there so grounds' floating. I can't make any fix yet for that floating ground since it will require redesigning my house's electrical system.
Forget that 10R resistor as it is not needed,,, then ESP drawing will make more sense.
Ground path goes from centre of main filter caps to power amp then onwards to preamp then to input sockets.
That common wire can be one or split it up (Star ground)
Rod mentions the input socket wireup option but it's debatable as to which way is best. :-X
You can connect *circuit Com point* (at Main filter caps) directly to metal Chassis or you can connect the input socket to chassis *BUT NOT Both* as that will introduce a ground loop and hum may increase.
No matter how good your design even experts end up testing several ground options with a fly lead and an alligator clip testing to find what ground path gives the best hum rejection.
In Rods design the gain is not massive so I doubt you will have much issue with the ground layout. :tu:
That fancy ground lift idea is perfectly valid but as most guitar players use pedals nowadays which are often powered via a *non grounded* wallwart supply it can cause issues. No medals for guessing why I know ?? :P
And if your wallwart/plugpak is one of those cheap switchmode types they can introduce a lot of noise problems if your amp chassis is not well grounded. At some point in all those connections you have to have a ground otherwise noise and humm WILL BE a problem :'(
Frankly you are better off just grounding the chassis to circuit common or if you must have it make it a switchable ground lift.
Phil.
oh sorry, I missed attaching the image in my last post.
Here it is
(http://s20.postimg.org/pjmucfav1/WP_20160607_19_29_45_Pro.jpg)
you have to remove either a ground path from the preamp to the power amp (to achieve more of a "star grounding"), or you can remove a ground path from the preamp to the PS filter caps (to get something in the mold of "Ground bus" - you will have a return path starting at the lowest current part (input), then gradually increasing the current - preamp, then power amp, then power supply).
Like it was mentioned, you cannot keep both of those connections.
Yes I have been trying to find a clear and well defined example of how to wire up but I can't find anything so I'll use your drawing.
It's easy to get it wrong,, I've been building stuff for a long time and i still fluff it up sometimes. :-[
When you interconnect boards like this you only ground one end of the shielded cable otherwise you make a ground loop.
If the input socket is plastic then just connect both ends of shielding.
hope it helps, Phil.
thanks a lot to both of you :dbtu:
illustrations are much better than text, since I honestly don't understand some ideas or terms here.
Another question, what if the two equipments have different power supply, is it still okay to cut that ground path? For example I use a pedal, is it enough to just connect the signal path from pedal to preamp, and not the common path?
In general you cannot do this - you need to have grounds of all the devices connected one way or another. The reason is that the voltage by definition is not an electric potential, but it's a difference of potentials between two points (that's what you mean when you say "voltage across a resistor" - it's a difference of potentials between two ends of a resistor). We always assume a point of "zero potential" to be the ground point of our circuit, but in reality this is rather artificial - you could say that the ground is at 100v potential, and it still wouldn't change anything because what we're measuring is a difference of potentials. It's just convenient to say that the ground has zero potential.
You want the reference points in all the circuits (by that we usually mean grounds of the circuits) to be at the same potential, so you need to connect them by a piece of wire - then you cannot say that the ground of one circuit is at 100v and the ground of another circuit is at 200v, because they are electrically connected, so they must have the same potential.
The problem with your circuit wiring was that the preamp and power amp grounds were connected not only by one path (that is good), but by TWO paths: one going straight from preamp to power amp, and another one going like this: preamp->power supply caps->power amp - which introduces a ground loop which may cause trouble (look at the attached image).
When you remove one of those two connections there is still ONE left, so all is good! Ideally you want all of ground points of all the circuits in your amp (power amp ground, preamp ground, power supply ground and what not) connected to one, chosen by you, ground point of your amp (usually the point between the filter caps) by just one path.
Back to the pedal-preamp case: of course it may happen that the pedal power supply is not "floating", which means that it's ground is in some way connected to the mains ground and also your amp common is connected to the mains ground - then you don't need the ground part of the guitar cable going from the pedal to the amp, but it's a rather rare situation so don't cut that cable :trouble
Man, I fell like I'm really awful at explaining anything so I'll just wait for Phil or someone to clear all the confusion I surely introduced :loco
it's clear enough for me :tu:
@ *incurably_optimistic*
Well judging by that posting you don't need my help. :dbtu:
Phil.