Amperes required by LM1875 Guitar Power Amp

[exztinct01]

Ouch!! that "free" Fritzing software is f***ing EXPENSIVE!! :loco

:lmao:
I actually considered their Fritzing Fab service but it really is soooooo...... expensive

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.

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.

[exztinct01]

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)

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)


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.


this is my actual pcb, components are not yet soldered

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?

[phatt]

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.
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.

[exztinct01]

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

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|

[exztinct01]

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.

[phatt]

Woah,, as you are asking about Mains Earth Then it's a Safety issue so I'll let someone qualified to answer that Question. 
Phil.

[Vitrolin]

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,

[J M Fahey]

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 )

[exztinct01]

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.

[exztinct01]

I just put a dedicated faston terminal in the star ground for the speaker return, another one for the transformer's 0 volts.

[J M Fahey]

Cool and congratulations. :dbtu:
I just posted about transformer size on the other thread (both should really be merged in to 1)

[exztinct01]

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?

[exztinct01]

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

[exztinct01]

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 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.

    Fourth: Etch using Muriatic acid and hydrogen peroxide solution

    Fifth: Drill


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:

[J M Fahey]

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 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:

also found at Health/New Age/incense stick suppliers.

I suggested a metallic cap jam glass bottle

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.

    Fourth: Etch using Muriatic acid and hydrogen peroxide solution

    Fifth: Drill


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: