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Topics - phatt

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Schematics and Layouts / LM386 H/phone amp options
« on: April 18, 2021, 09:55:38 AM »
The recent Honey amp got me searching my backups and I found The Nobels Mycro Amp circuit.
I don't know if this is of any use but this is a Headphone circuit using the LM386 chip.
Gives clues as to how to implement a preamp with clean OD and Dist.
Of course it does require more parts than the honey amp but the second stage has filtering which may help with small speakers. :-\

Tubes and Hybrids / Texas tone made simple
« on: October 24, 2019, 12:23:19 AM »
Hi all,
I've been messing with this for a few years so thought I'd show folks this simple trick.
If you are lucky enough to own an old BF Fender or similar Valve amp from that era this may help. This is unlikely to work with a lot of amplifiers made now days (both SS and Glass) but I have had some success with my all SS setup,,so some experimentation may be required to work with your particular SS setup.

The Video should be fairly clear for those who understand the basics of how tone is created.

Note the audio circuitry in my little camera compresses quite hard so you won't hear just how much the bass splatters when the trick is bypassed at high volume,
When engaged you will loose perceived power but the whole point of doing this is to get the amp running at full volume without the bass splattering, read clean headroom. 8|
With those old amps the bass is big and sounds full at low volume but the bass collapses with many of those old valve circuits if you try to get to full volume,
Even worse if you have overwound PU's. xP

So here is what just one small cap can do when used in the right place.
Keep in mind this is all done on the cheap,,So no Tx special pu's and the small amp is old valve radio junk.
the pu's in this guitar are 4k ceramic (read,, about as cheap a copy strat as you can buy)

Enjoy, Phil

Amplifier Discussion / MaxTurbo mods
« on: June 19, 2019, 09:24:04 PM »
I picked this up cheap and took the chance it might be an easy fix but proved to be quite a task.
It worked but had very low output for the claimed 40~50Watts. more like 10Watt :duh
Obviously a no name import job but as it was a small 10Inch speaker, just the size I was looking for as a small grab and go amp.

It says *RSGA5010R* on the front and a small badge on the grill that says *Max Turbo*
I only found one small reference to it on the net giving the specs but nothing more.
Power chip is TDA7295 and runs on +/-40volt rails.
I doubt there would be many of these around but if you have one it's likely got the same design problems so if you need all the mods I did to get it working I'll up load the schematic of all the mods.
meantime I've traced the main preamp circuit and added the main mod that fixed the notch filter issue.

It took a while to work out the circuit because double sided PCB's are a royal PITA to back track.  :grr
I'm sure I will have missed something but these schematics will be fairly close. I've tried to label the part numbers as found on pcb.
I have not drawn up the power section as it seems like it's taken from the data sheets of TDA7295.

After a while I found some of the missing power was due to a bent pin of an IC that was never soldered. Wooden chop sticks are a great fault finding tool when you
probe around. ;)

Improved power but winding the treble up past 3/4 invoked some really harsh hi freq distortion more like a crackle which was evident from the start but now even moreso.

Along with the bass treble there is a notch filter knob but it did not seem to work in any meaningful way.
So I finished drawing out the schematic (after many attempts) and simulated the circuit.
And sure enough just as my ears had noted, that Notch filter network seemed to have little effect on the tone shaping.

Up shot,, U5 (which is what the notch control pot connects to) seems to make no sense.
Be aware I'm still not sure how this circuit is *supposed to work* so I'm guessing a lot here. :-[ :-\
After many sims I took a gamble and connected the *top of VR5b* to the *input of U7* via a 10k resistor and Bingo the notch sweeped and worked as I would expect and
the bonus was that horrible treble issue was gone.  :) :)

The cab is cheap compressed cardboard crap so keep it away from water and it should be ok.
This thing had 10 miles toooo much bass so I replaced most of interstage 100nF caps with 10nF which made the amp usable at full volume.
I plugged the ports on the front which also helped the sonic result.

As I had a short Reverb tank laying around doing nothing so added a Reverb circuit between preout and power in,, a nice addition. 8)
The send return on the front is useless as it only works on the clean selection so when OD is engaged the FX loop is bypassed. WTF???
I'm stuffed as to why they made it so, maybe another design stuff up??

Amplifier Discussion / Marshall G100R-CD blown
« on: August 28, 2018, 04:32:20 AM »
Hi All, I need some help :-\ with a Marshall G100R CD head.
Owner purchased it Sc/hand, worked ok but now keeps blowing internal 1A fuse.

So I setup my good ol 60Watt lamp limiter and no DC Voltage at psu.
AC at secondary is ok 32-0-32VAC (no load).
I Removed links to the 2 power transistors and I got the two 45Volt rails back.
So likely power units are boofed.
They are stamped with *Marshall T64 and T65*. So I'm wondering what would be the equivalent?

I've dug up a schematic which states the power units as 2SC 3519 (NPN) & 2SA 1386 (PNP)
both are 15A/160V/130W, They are TO218 devices.

Now to save this chap some expense I've got 2 pairs of 2SC5200 & 2SA1493 (15A/230V/150W) sitting here doing nothing,, 
But they are TO264 type which will fit by drilling new mount holes in the heatsink.
I'm assuming they may work but I'm a bit iffy :-X

I've attached the PDF of what looks like the G100R CD Schematic (Found it on DIY Electronics)
but the power stage is not the same so I'm a little unsure of how this circuit works.

I'm ok with the basic power stages but the schematic is not well drawn and has some mistakes and junction points are not clear.
Rectifier is drawn wrong for a start. :duh

Also TR14 & TR15 (which would be the Voltage stage of a normal diff amp) are not present on this PCB so I'm wondering if there is another schemo floating around somewhere. I'm a little lost as to how this works  xP
This amp states Made in India.

*EDit; Sorry TR14&15 are predrive for output Tr's NOT the VAS :duh

Another point worth note, this amp has been repaired before.
R112 (330R 7W) has been replaced. (Obvious as it's been mounted under the PCB)
This is the Drop resistor for the -15V rail.
But upon inspection it has failed again.  (I did not realize this at first ???)
There is no sign of heat stress on the replaced R112 it reads open circuit.
only a slight dis-coloration on PCB under the two 15V Zeners which is to be expected.
Everything else looks as per normal.

As this power stage requires both 45V rails and 15V rails to function then would it be fair to assume that failure of either 15v Rails could cause the power stage to blow something as well as blow the main fuse?
Any help would be most helpful, Phil.

Preamps and Effects / RF switchmode Fix
« on: August 30, 2016, 09:21:21 AM »
It seems the world is destined to be run by Switch Mode power supplies and as iron core transformers
are now rare to find one has to put up with the short comings of this new  technology.
Yes they are light and cheap but I very much doubt their life span will even come close to the
old copper and iron transformer which can last indefinitely.

I'm posting my finding as I've had a few Smode supplies give me grief.

The main issue with these type of supplies is the high frequency switching introduces
some very aggressive RF hash and although some better designed units work OK for Audio,, a lot don't. :grr

I use a small Boss brand 9VDC 1Amp SM supply to run a few pedals which to its credit has worked
well for 3 years now but I have no spare supply pak and I know one day it will fail.
I recently acquired a 9VDC 2 Amp SMode wallwart,, $2 at a jumble sale but it comes with the dreaded RF whine :duh :grr
So I thought I'm going to research this problem as my Boss unit works noise free,,, sooo,,, there must be a way to fix this confounding problem.
After a couple of fruitless nights searching for some plain and simple explanations of the problem
and how to fix it I finally found this page;

Well worth a read if you wish to understand what the problem is and How to fix it for minimal cost.

I'm in the process of testing this idea and after a few trips down the local electrical repair shop I've snaffled enough bits from scrapped white goods to make the two filters. Just ask before you raid there rubbish bins,, just watch out for micro waves and like white goods as some store voltages. 8|

It only took a couple of hours to establish that the idea works. :dbtu:
If any of you technical folks wish to add advice feel free as the maths involved for inductance is a bit over my head. The small donut inductors read 22mH on my meter ,, if I'm reading it correctly? I used 2 in series and the capacitors are 1,000uF. I was pushed for space so I left out the 100nF ceramics.

Meanwhile,,here is some pics of the process.
cheers all ,Phil.

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Schematics and Layouts / Zobel Question
« on: July 18, 2016, 06:47:38 AM »
Tekky Q,
          This Blown LM3886 circuit snippet is from a BiAmp powered speaker.
Supply rails are ~  +/-40VDC
The output is latched to the rail and too hot to touch within a minute of turn on.
The other two LM3886 are driving the woofer which is still working so half tempted to have a go at fixing this. I figure there is no point in trying to replace the power chip if it's just going to happen again.

Would I be right to think that with no Zobel network on the output it may well have been oscillating causing the chip to fail?  The tweeter is fried. :'(

Thanks, Phil.

(R14, R15 and D9 just go to the limit/overload circuit)

Amplifier Discussion / Trifelex unit
« on: April 06, 2016, 02:51:05 AM »
Just had a Peavey Triflex PA rig here,, only problem was the intermittent "Speakon connectors" that connect to the 2 satellite speakers. Just dirt cheap chassis mounted connectors by the looks of it so a quick bypass and other connectors fixed the problem :tu:

My Q; Does anyone know how they get this thing to work? :loco
Claims 250W at 4 Ohm for each remote speaker out and then drives the built in subwoofer. I was expecting to see at least 2 power amps but this unit has what looks like a very basic maybe ~50Watt power amp and then some tricked up circuitry to run all 3 speakers.
Only 2 power transistors clamped onto the thin alloy case running from 35Volt rails, heck and not a heat sink in sight, WTF? :o :loco

Even if it's only 50 watts, I'd still be interested to know how they drive a sub and then two 4 Ohm speakers as well, all off one rather over rated mono amp. Even with no heat sink This thing does not seem to run hot.

The output of the power amp goes through a bunch of inductors and caps (Middle PCB) and then to each speaker output and that part has me confused as the remote speakers have a normal X-over system for 10inch and horn.

The two power transistors are 2SC3263 and 2SA1294 which seem to be a 230V/15A/130W Pwr transistor. pre-drivers are MJE15032g and MJE15033g.
The 2 emitter resistors are .8 Ohm which seem rather high but might help driving very low Z but then that would reduce the power somewhat. It certainly is not anywhere near 200watts the sub breaks up if you try to push it hard. so it's rather gutless.
Thanks, Phil.

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Amplifier Discussion / RIP Roly Roper
« on: February 21, 2016, 07:24:41 PM »
With great loss to us all,,,Roly has moved on to the other side.

Comments from someone close to him here;

Just thought you would all like to know. :'( :'( :'(

Schematics and Layouts / New pedal project
« on: December 28, 2015, 03:24:11 AM »
Well after a very long and interesting journey into compressors and ODrive setups I'm having a ton of fun with this one. I've never liked compressor pedals as they sound wrong to my ears.
Unlike a lot of big name guitar compressor pedals this one adds the missing sizzle. 8)
Bear in mind I built this for a solid state amplifier which does not have the dedicated guitar circuitry so may not work so well in front of some guitar amplifiers, all the mojo is implemented in the pre stages and the power amplifier section is essentially flat and clean. 
I have found that a lot of dirt pedal circuits no matter how good often don't work so well when used in front of many ss guitar amplifiers.

Older valve Amplifiers always had a fair amount of distortion *even when played clean* there was always a little, shall I say Hair on the peaks of the signal this small distortion added a distinct sizzle to notes played giving the instrument a more dynamic feel.
When cranked up higher or boosted with a pedal that clipping is accentuated, helped along by some supply sag and other quirks or short comings of early Valve amp design. So they effectively had a built in compressor with no bypass.

Most SS amps don't have that luxury and one is forced to use compressors and all sorts of gizmos to replicate that magical feel and obviously some work better than others.

This circuit uses an optical compressor > a simple OD > cab sim.

The first A/B test is just some plane chords with no efx.
Signal path is; PhAbbtone > compressor > *OD OFF* > Cab sim > out to Laney Amp.
Mic at 600mm (2 foot) straight to computa no EQ or tone tweaks.
(I note some bass loss on my small desktop speakers.)

When I engage the compressor you get compression along with just a bit of sizzle.
engaging the OD simply increases the sensitivity but signal is still fairly clean so you still have plenty of scope for OD pedals for more serious lead work.

The second clip I whipped up on the fly;
Comp on, OD on for a little extra edge, Cab on, playing over a simple loop pedal. Engaging an OS2 for some solo. (using the Ditto Looper borrowed from a good mate). <3)

This circuit will work on 9Volts but performance won't be as good. :-\

Played on Sunday at the local open mic jam and everything went well I had no trouble with control over clean and dirt levels. The compressor really helps keep the SPL within a civilized range when jumping on dirt pedals.


Amplifier Discussion / Laney Amp power boosted
« on: December 23, 2015, 01:21:17 AM »
I finally finished this project,, original posting here;
So some pictures of the finished unit, looks like spaghetti but no one looks inside anyway. :P

First picture has Original back panel for comparison.

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More clean headroom at big volume so now my keyboard sound is much improved.
Guitar also has a bit more punch.

Ideally the fins on the heatsink should be vertical but it's a big chunk of alloy and it was free junk.
Except for a few odd bits the whole power amp and supply was from my pile of parts.
Merry Xmas to all  ;D

Amplifier Discussion / Hot rodding a simple SSPoweramp
« on: June 11, 2015, 05:30:32 AM »
Modifying My old Laney Amp which I've owned now for ~10 Years.

It's been a great little work horse which has been flogged to death but yet never given much trouble.
The power amp never gets Hot no matter how hard it's driven, luke warm at best even on the hottest days of summer. 8) 8) 8)
But I feel it could deliver just a bit more volume at times when gigs get loud.
Anyway rather than take a chance on a quick and dirty hot rod trick I thought I'd build a test circuit first.

The test has shown R11 (10k) feedback resistor can be doubled without issue which reaps an obvious and useful jump in volume and does not upset the circuit, exactly what I had hoped for.  <3)

The Laney transformer delivers 35-0-35 VDC with 2x 3,300uF caps while the test circuit is driven from 32-0-32 VDC but has slightly larger transformer and filtered with 6,800Uf caps.

Except for the 2N3442 power transistors the test circuit components are much the same, I/m using MJ15015 which have more power. I'm assuming they won't alter the result much?
2N3442,  140V- 10A- 117W
MJ15015, 120V- 15A- 180W

The test circuit is maintaining stable measurements and supply rails only dropping 2 volts at full power.
With audio signal input after 2 hours at full volume into a soak the heat sink still refuses to get past luke warm.  The tabs on Q3,5, and 6 are also only just warm. R9 (2k2) is 1 Watt also warm which I expected. Yay 2 wins in one day <3)

Of course a scope would come in handy right now but I've never owned one and hardly worth it for the little use it would get from me.
I just simulate this stuff and take some intelligent readings on the test bed.
I don't have all the correct models for my simulation program so it maybe a bit skewed but
a simulation shows doubling the FB resistor from 10k to 22k the output wattage jumps from 5 Watts to 20 Watts which seems a lot but the actual test Amp seems to cope fine with clear signal up until 3/4 volume when distortion starts to become obvious.   

All going well so far but a couple of things bother me which I'd like to resolve.

1/ While cross referencing actual Amp PCB to Schematic it seems there might be an issue with D7 and D9,, something not right? (marked on Drawing attach)
D9 looks wrong on the drawing yet is reversed on the actual board, which I think is Right.
While D7 seems drawn right on schematic but is reversed on board, which I think is Wrong. Head scratch? :duh

(This is the over current section and although it does not function in normal use I'd like to make sure it's correct in case of a short circuit.)

2/ The bias trim pot;
Am I right in assuming that, IF the wiper ever goes open circuit then the amp will likely self destruct.
If so, is there a better way to wire up the bias pot so that the Amp runs cold bias if the the pot fails.

Any help with that will be great.

My Amp model is a KD50 but the only schematic I found was for the KB80 but they are very similar.
Note 1; I have no idea why R2 (100k) is there but it just goes to ground and seems to do nothing.
Note 2; My test circuit does not have the protection system installed as it is not needed for this test.

Amplifier Discussion / Transformer Tricks
« on: April 17, 2015, 07:51:41 AM »
Could not decide where to put this one?
But how's this for thinkin outside the box?

I don't fully grasp this idea but I do know that transformers are clever and hold many secrets. 8|

A bit more here,, which does make some sense;
works like a relay to drive a lamp.

But the audio amp part has got me a bit baffled,,,  xP So I'm Open to any explanations about the Trans Amp idea.

Schematics and Layouts / Old Aussie Jade Amplifier
« on: March 22, 2015, 08:36:40 AM »
Here is and old Aussie SS rig from way back, came to me with dead poweramp.
Hey surface mount "old style" Which does actually make working on it quite easy in some ways. :tu:

After firing up with new parts I became concerned about that broken PCB which is only held in place by the power transistor sockets and a lump of foam glued on the back of the floating pcb so I added some extra stand offs for support.

The speaker wire is/was hard wired to top corner of pcb track with no other clamp or support so any hard reef on the speaker cable would just snap the pcb which quite obviously has happened before and would likely happen again.
So looking at the vertical quad box that goes with the amplifier and a very short speaker cable,,, hum? I added a block connector and clamp at back of cabinet and added a longer speaker cable. Replaced a couple of dud pots and All fixed now.

I thought I'd post this because I've not seen this track side mounting done before and also I traced the OD channel as it actually sounds quite good, very responsive. Just backoff the guitar volume and it cleans up quite nice.
It's never going to put heavy metal rigs out of work but through the Quad box it does a convincing rock sound.

Any comments on the poweramp design are welcome as I've not seen this type before and I'm wondering if it's any good,, pro's and con's?
Cheers, Phil.

Schematics and Layouts / Watt-zit-do?
« on: March 01, 2015, 06:21:56 AM »

Anyone wanna have a go at explaining this odd ball circuit?

Judging by the lamp I figure it's a limiter for the tweeter of some sort
but the diodes and transistor have got me stumped. Head,scratch?

Also I'm not fluent in Glockenspiel type languages so I can't even read it.   xP

I have been experimenting with even more limiting ideas for my Cruize Control concept and dug this up from one of my very first hunting trips on the net, which I saved. While cleaning out the bottom draw I found it again and I would like to at least understand how it works as it might hold some ideas for my quest to limit,sag or compress the signal in some way.
Thanks, Phil.

Schematics and Layouts / PhAbb Cruize Control
« on: October 29, 2014, 03:04:16 AM »
Hi all,
       I Finally built my limiter circuit into a working unit. yoo hoo
It's Called the "Cruize Control".
Think power amp sag control or an Overdrive limiter, might help explain what it does.

Works like a charm with no pumping and produces the effect reminicant of old cathode biased Valve amps which tend to sag/limit/compress the sound at high volume while adding a fair amount distortion.

So just by using the on-board volume control on most electric guitars you can go from clean to scream without the need for a dashboard of fancy pedals.

For those who missed my other thread,
A very good example of the effect I'm talking about is here;

Sadly Joe Bonna does not explain to the masses that this only happens with certain types of valve amplifiers (look at Fender 5E3 and similar schematics for clues)

As a lot of Valve amps use fixed bias and stiff psu the power stage does NOT sag much and most of the fancy dirt distortion is done inside the preamp sections. In my experience this is not as convincing as a saggy old valve amp from a forgotten era.
Most SS power amps are even more clinical, staying clean all the way up with very little sag and rely even more on preamp tricks to get the distortion to work.

So rather that trying to re-invent power amplifiers with no idea of how to find the square root of the universe  :duh I thought I would try something simple and to my surprise it works far better than I expected. <3)

with my "Cruize Control" The difference between half volume and full volume is not a huge jump, the sound level remains between a fixed range so you don't get a massive jump in volume level, it just sags and distorts a lot more while raising the volume just enough to play over the top of clean rhythm parts.

If you are using this after a pedal just change R2= 1Meg and R5= 1k.

Trim 1 sets the gain for the limiter section and can be a front panel control but I've got enough knobs to Ef up the sound so one less knob on the panel means less to go wrong. I've set mine around 30~50k.
VR2 is the main knob and turns down (Clamps/Limits) the output level as you turn it up so set for what ever works for you.

If you read the other thread I've dropped the transformer as it's not needed. I also realized the extra parts on the limiter drive circuit were not needed. so this makes it dead simple to build. :tu:

It runs from a 9Volt AC plug pak and with a trick little rectifier circuit delivers 15/0/15VDC split rails. (If there is any interest I'll post the supply I used.)

I've used 3 different LDR's on the test circuit and it seems these can make a bit of difference as they can have slightly different response times.

For those in Australia, the Jaycar part for the LDR is RD3480, cost $3.
The torch is likely overkill with 9 LED's but the torch was cheaper that buying just one white Led and came with an alloy housing to mount the LDR. to easy :)
The Torches I found in the "Reject shop" cost $4
If you go with a led torch be aware that some of those torch leds won't work, my guess is the type with a single Hi intensity led are a no go, usually a tiny yellowish led, Obviously not all leds are equal.

While testing I removed my dedicated PhAbbZone preamp and raised the gain (as mentioned above) and played direct and it works fine. I then inserted a Boss OS2 dirt pedal (unaltered) and got some very long sustain to happen so it's no slouch if asked to do big distortion. :dbtu:
The circuit is noise free and dead quite.
Have fun.

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