A couple weeks ago, I was gifted a Frahm PS200 (8 ohms, 30W, 4" + tweeter, studio speaker), and installed a TDA8932B inside. It sounds great with my selection of headphone amps through it, both with bass and guitar, but I wanted to have a built-in preamp I could use and, potentially, turn it into a battery powered portable amp at some point.
I've been trying to find something simple, maybe that could get some dirt off of, if I wanted to. Gain-Tone-Volume is plenty good for me. Maybe even Gain-Bass-Treble-Volume. I just don't want a dozen knobs.
I've been looking around but haven't really seen anything that clicked, so I'm asking here in case anyone else can direct me to something interesting.
Thanks y'all.
Well first thing you gotta know is what power supplies are available to run the preamp ?
Quote from: Loudthud on July 01, 2025, 09:31:23 PMWell first thing you gotta know is what power supplies are available to run the preamp ?
Initially, I'm thinking just 12v, both for the power amp and preamp, as I already have a 12v PSU that works well with the amp, and I could easily add a 3x 18650 battery holder to it in the future.
I also have some XL6009 boost converters, so I could use one for higher voltages on the preamp if needed.
The TDA8932B board I have can work with up to 30v, but I'd probably limit it to 20v max just for the sake of not going over the speaker's max 30w rating.
Quote from: blackcorvo on June 30, 2025, 02:09:48 AMA couple weeks ago, I was gifted a Frahm PS200 (8 ohms, 30W, 4" + tweeter, studio speaker), and installed a TDA8932B inside. It sounds great with my selection of headphone amps through it, both with bass and guitar, but I wanted to have a built-in preamp I could use and, potentially, turn it into a battery powered portable amp at some point.
I've been trying to find something simple, maybe that could get some dirt off of, if I wanted to. Gain-Tone-Volume is plenty good for me. Maybe even Gain-Bass-Treble-Volume. I just don't want a dozen knobs.
I've been looking around but haven't really seen anything that clicked, so I'm asking here in case anyone else can direct me to something interesting.
Thanks y'all.
If you want something interesting then I would build a C2CE pedal to drive that powered speaker you made, you might not be after something with a tube in it but all the C2CE pedals are made to be run on a normal 9V-12V pedal power supply so your 12V would be ideal for these pedals.
The Nobelium is a clone of the Noble Bass preamp pedal so that would work great, the BassDude is a Fender Bassman preamp and the Wrecking Ball is a Mesa/Boogie dual Rectifier preamp in a pedal.
You will not find anything more interesting then a real Tube pedal running the tubes at proper high voltage, and they are fun to build as well.
If you just want to buy a pedal then look at Sushi Box FX pedals, Nathan from Sushi Box is also the designer for the C2CE pedals since he owns both.
Nathan is away on holiday in Mongolia right now but he will be back in 3 weeks.
The Tube pedals do use a bit of power so might not be the best for a battery powered setup though.
I would look at any other solid state pre-amp pedal as well if I was you, if the pedal is a copy of the pre-amp of a real amp then it should be able to drive the TDA IC ok.
Any Sunn BETA preamp pedal should be able to do clean and crunch and would be one of my favorite pedals to build
It all depend on what sort of music you are into, what exactly do you want to play ?
https://c2celectronics.com/
https://www.sushiboxfx.com/
https://aionfx.com/project/beta-preamp/
Quote from: blackcorvo on July 02, 2025, 03:47:59 AMQuote from: Loudthud on July 01, 2025, 09:31:23 PMWell first thing you gotta know is what power supplies are available to run the preamp ?
Initially, I'm thinking just 12v, both for the power amp and preamp, as I already have a 12v PSU that works well with the amp, and I could easily add a 3x 18650 battery holder to it in the future.
I also have some XL6009 boost converters, so I could use one for higher voltages on the preamp if needed.
The TDA8932B board I have can work with up to 30v, but I'd probably limit it to 20v max just for the sake of not going over the speaker's max 30w rating.
Marshall Lead 12 is a KILLER little SS amp.
You can build just the preamp to drive your power amp.
Full schematic
Marshall Lead 12 Full.png
Preamp only:
Marshall Lead 12 Preamp.png
2 "problems" to solve:
1) you will have to draw your own PCB
There are a couple ones on the Net, also Veroboard versions, but I do not trust them.
Also designers get "creative" and add unnecessary features as diode clipping, switchable gain stages, different EQ, etc. which actually degrade it.
Best is original one which is also the simplest.
2) it needs symmetrical supplies.
Originally +/-16V but works fine with +/-12V
You already have +12V and it is easy to get -12V out of it, using a charge pump IC.
I´ve done that many times for my 12V Lead Acid battery powered "Callejero" (Street Musician) busking amps.
Didn´t even use a charge pump IC (not available way back then in Argentina) but a humble 555 oscillator, go figure.
3) Plan B: most any Distortion pedal can work as a preamp for your Chipamp, even a humble MXR Distortion + , a Tube Screamer, etc. and all those work straight from single +9V supplies, and are happy with +12V.
Or you can pad +12V down to +9V by using an LM7809.
To broaden the scope: there are a few "Amp in a Box" pedals, sporting full tone controls, search the usual "PCB Pedal" sites.
But read all comments and build only those which are "checked".
Marshall Guv´nor is a Marshall preamp channel in a pedal format, works out of single +9V and there is a tested iron-on PCB layout available. search for it.
Don´t you have a pedalboard? Or a Zoom (or similar) pedal?
Those can straight drive your chipamp.
Thanks for the suggestions!
Quote from: Tassieviking on July 03, 2025, 03:18:38 AMIt all depend on what sort of music you are into, what exactly do you want to play ?
https://c2celectronics.com/
https://www.sushiboxfx.com/
https://aionfx.com/project/beta-preamp/
I'm looking for something that has both good clean and drive sounds with guitar and bass, but has simple controls. Doesn't have to be super eclectic as I mostly play hard rock and punk.
Quote from: J M Fahey on July 03, 2025, 10:06:46 AMMarshall Lead 12 is a KILLER little SS amp.
You can build just the preamp to drive your power amp.
I thought about that, but I just don't know if I trust that schematic. I remember seeing discussions in the past where the components on Marshall Lead 12s didn't match the circuit, but I haven't found any actually successful reverse engineering of the circuit yet.
Funnily enough I was looking for the Bass 12 schematic and couldn't find a readable one. I also can't find any videos except for ONE of it used with bass to know what it sounds like. In fact, I had to make a playlist (https://youtube.com/playlist?list=PL_HhXRFHRmBR1S3IYRRjCdEC_ZBZuOpl0&si=hl2l2dT5KPoMPW67) to have some references for certain bass amps being used with bass, because people seem to only use them with guitar.
Quote from: J M Fahey on July 03, 2025, 10:06:46 AMDon´t you have a pedalboard? Or a Zoom (or similar) pedal?
Those can straight drive your chipamp.
I do have a selection of pedals (Marshall Guv'nor GV-2, FAB Distortion, FAB Flanger, FAB Echo + time mod, etc), a Zoom 505II, and an M-vave Cube Baby Bass, but I just don't like dealing with cables, especially if I made this a portable setup.
I was initially using headphone amps to drive the amp, and they work great, but I fear they'll break in case of a bump or fall.
Which is also why I don't think I'll use tubes, but I do have a good selection of them (I'd be here for a couple days listing them, but basically I have 7-pin, 8-pin, 9-pin, subminiature and "pencil/rod" submini tubes, to give you an idea).
I also have like a dozen or so J201s. In fact, I stayed up late trying to finish a Runoffgroove Eighteen on my protoboard to test, but had to stop mid way cus I was falling asleep doing it haha.
Marshall 12w Bass
Quote from: Tassieviking on July 03, 2025, 05:19:03 PMMarshall 12w Bass
Thank you!!! :tu: this is gonna help a lot.
[Edit]
I just found out I can get symmetrical +/- voltages from those cheap DC-DC buck converter boards:
I'm gonna try this when I get home, because I fried one of my ICL7660 trying the circuit yesterday (max V+ for them is 12v and my PSU is delivering a tad bit more than that).
Also, I redrew the Bass 12 schematic and marked the different component values with the Lead 12, so when I eventually make a layout, I'll do it so you can build either one of them in the same board.
Quote from: blackcorvo on July 03, 2025, 07:01:22 PMI just found out I can get symmetrical +/- voltages from those cheap DC-DC buck converter boards(...)
Update: it works! The negative side is not perfectly symmetrical with the diodes I have on hand (they drop a couple volts in comparison to the positive side), but it works.
Doesn't seem to have the same level of distortion from what I've seen on videos online, but it might be just that on those, the power stage is being pushed and overdriven.
In my case, it pushes it to the point I think some kind of overload protection kicks in and mutes it (I tried powering the PA from the positive side of my XL6009 buck converter board to see if the higher voltage stopped that muting issue, and while it reduces it, it's still happening on volume settings beyond like 50% at full gain).
Might need to add a limiter stage between preamp and PA or something like that.
I've also googled a bit more and found this traced schematic on Freestomboxes
https://www.freestompboxes.org/viewtopic.php?p=286640&sid=d31c63eb87f367e522040034f9bc9f15#p286640
(https://i.postimg.cc/NfsXwcWT/Marshall-Lead-12-3005-5005-MK-1-preamp.png)
And not only does it have different values from the one posted earlier on here, it also has clipping diodes. Thought it'd be interesting to share here.
[Edit]
I also found this video talking about it, but I forgot to add it here before.
Sounds like you've already chosen a path, but another good option would be the Sansamp GT2. There are kits, schematics, and even used pedals strewn about the web. They have a multitude of great sounds, are relatively low priced, and run from a single 9V source. I always carry one as a spare/emergency rig.
Quote from: The Dude on July 07, 2025, 07:40:25 PMSounds like you've already chosen a path, but another good option would be the Sansamp GT2.
I appreciate the suggestion, but that's way too complex to my taste, with way too many controls to fiddle with, and from the samples I've heard, I don't like how it sounds on Bass.
Also a bit of an update.
I had ordered a couple battery holders and they arrived yesterday. I drilled a hole in each to run the wires under them into the speaker box, and installed an LED on the original holes for the wires as an indicator for that pack being on.
Potentiometers and jacks for the build will probably arrive by the time I'm back home from work today.
I also did a layout. Yes, I cheated a bit by looking up pics of the original amp boards, but I changed a few things around to try accomodating it in a more compact build. Lastly, I show the changes needed to turn the same layout into the Lead 12 in red on the board.
More pics of the build.
Drilled some holes, routed and soldered some wires, now I'm wainting on another order of 1M Log pots (weirdly weren't available before today) and knobs, coming tomorrow.
The extra jacks are for using the P.A. with my headphone amps (Vox AP2-AC, Joyo JA-03B, Xvive GA-3). They plug on the top P10 jack, Phones out goes into the 3.5mm jack next to it, guitar cable goes on the jack on the back of the cab.
When these are unplugged, they connect back to the internal preamp.
Another thing I wanna do is add a power jack, and also have the original speaker contacts be a utility power out for pedals and such. Curently, it's the power input.
Lastly, I wanna see if this "passive compressor" (https://groupdiy.com/threads/passive-compressor.49275/) on the last pic could help with the P.A. input overload. I'll put it on the protoboard and see how it does.
Getting a bit further.
Tomorrow I'll test the "compressor" circuit to see what it does to the signal.
:dbtu:
Here's a video update showing the passive compressor working, and a second solution I came up with that I feel works better and is simpler.
The simpler solution is an AGC circuit:
(https://upload.wikimedia.org/wikipedia/commons/2/22/Vactrol_AGC_%28Yushin%29.PNG)
But with the clipper using 2 LEDs instead of an LED and a diode, and only a series 100R resistor, no capacitor.
Next I wanna try a rectifier with the LED in the middle (like the diode clipper in a JCM900) and see how that does, reason being that in the 2 LEDs clipper, the LEDs are only on for the half of the signal each clips, while with a rectifier the LED will clip both sides of the signal.
[Edit]
Attachments added for the tone stack curves with the switch at different settings, and the project layout.
Also of note, I changed the capacitor to be wired like a typical Marshall mid control instead of how it originally was (morelike a Fender), which makes it so turning all tone knobs down doesn't cut the signal completely, and gives IMO better isolated control over the mids.
I thought it was killer :tu:
Almost heard a little N.I.B. at 5:27 <3)
It started out like geddy and ended up like geezer :dbtu:
I was perusing this thread on DIY Audio about the chip amp I'm using:
https://www.diyaudio.com/community/threads/fasten-seat-belts-tda8932-pessimistic-review.277130/
And people there mentioned cutting out issues related to overheating and over voltage, so I got adhesive heatsinks to see if it'd help.
Nothing.
I then tried using a variable PSU I have to see if a stable 12v or under would work.
And... For a moment, it did. Even got a bit overdriven in a nice way.
But as soon as I tried recording, it stopped and went right back to cutting out.
So I'm completely clueless to what might be causing this issue.
I think I'll just put a TPA3118 in there and call it a day, as I've had no issues with that amp in the past. I even have one in a repurposed modem case, that I use as a PA for testing pedals and preamps.
For now, I'll give it a couple days to see if anyone else has any idea of what I might be missing.
[Edit]
One last thing I might try is adding soft clipping at the PA's input.
https://sound-au.com/articles/soft-clip.htm
Quote from: blackcorvo on July 17, 2025, 07:24:07 PMI was perusing this thread on DIY Audio about the chip amp I'm using:
https://www.diyaudio.com/community/threads/fasten-seat-belts-tda8932-pessimistic-review.277130/
And people there mentioned cutting out issues related to overheating and over voltage, so I got adhesive heatsinks to see if it'd help.
Nothing.
I then tried using a variable PSU I have to see if a stable 12v or under would work.
And... For a moment, it did. Even got a bit overdriven in a nice way.
But as soon as I tried recording, it stopped and went right back to cutting out.
So I'm completely clueless to what might be causing this issue.
I think I'll just put a TPA3118 in there and call it a day, as I've had no issues with that amp in the past. I even have one in a repurposed modem case, that I use as a PA for testing pedals and preamps.
For now, I'll give it a couple days to see if anyone else has any idea of what I might be missing.
[Edit]
One last thing I might try is adding soft clipping at the PA's input.
https://sound-au.com/articles/soft-clip.htm
How did you try to add a heatsink to that amp ?
Many of the modern low power D class amps use the PCB as a heatsink, the IC has a heat pad on the bottom of the IC that touches the PCB.
Often there are a hole bunch of vias or sleeved holes under the IC to transfer the heat to the bottom surface of the PCB which is flood filled with copper to provide a heatsink, in that case it might be best to place a heatsink under the PCB with some insulating double sided tape.
It might seem strange that you don't place a heatsink on the IC itself but if the heat is being directed down and through the PCB itself then the heatsink has to live on the back.
I don't know if your amp needs the heatsink on the IC or on the back of the PCB but it is something to think about, feel the IC and the back of the PCB as the amp is heating up.
Cheers
Mick
Quote from: Tassieviking on July 18, 2025, 11:02:52 AMHow did you try to add a heatsink to that amp ?
Many of the modern low power D class amps use the PCB as a heatsink, the IC has a heat pad on the bottom of the IC that touches the PCB.
I got 3 packs of the RPi heatsinks, 1x 14mmx14mmx5mm and 2x 9mmx9mmx4mm per pack, and put 2x of the 14mm under the board right under the IC, and 2x of the 9mm on the chip itself (just to be safe), but nothing seems to have changed.
I'm just grasping at straws here tbh.
Just to see where the chips fall, I would test once more the chipamp, driving it straight from generator or at most a 1 Op Amp low gain stage, say 3X to 10X tops if beyond what the generator can supply, and from a simple power supply, not boosted batteries or regulated supplies but transformer - diodes - caps.
Nominal 12VAC will give you some 15-16V DC
The point being to confirm the amp itself fails and not Preamp or supply throw the towel.
Did you scope the +V rail to see if it drops?
That click click click may come from amp protection but also from supply protecting itself.
*In theory* TDA8932 should work fine for that load and supply ..... in practice, who knows?
But if so, stop using it, period.
TPA3118 has a reasonably proven record so far.
I was using the batteries directly - unprotected batteries, too - for my tests, so power was very clean, and saw no voltage drops with signal. And I tried the preamp on a separate power supply, both with the PA on it's own PSU and on batteries - same cuttting out result.
Also, just as an "insanity test", I went reading up on the DIY Audio thread again, and it seems like TDA8932T has presented issues for other people too, and they had better luck using the BT variant of the chip instead. Some mentioned having better performance with it running from 12 to 14v, and the one time it stopped doing the cutting out thing for me yesterday was when it was being fed around 11v. Then after a power cycle, it did it again.
I'm starting to think this chip or board has some manufacturing defect...
Maybe.
To make certain, get a TPA3118 board and use it the same way you are using 8932 now.
No need to order from China and wait, just get one at Mercadolivre, even if slightly more expensive.
What city are you in?
If in SP, Multicomercial and shops in Sta Ifigênia have anything you need.
Just an insanity check: remember speaker leads must *both* be floating, no speaker terminal can touch ground.
If you use a speaker jack in a metallic chassis or cabinet (many put those amps inside a Hammond type pedal box) then you NEED a plastic jack (Cliff/Marshall/Vox type), not a metallic one (old Fender type).
Measure DC to ground, no signal, at each speaker out lead, both (+ and -) must measure 1/2 +V
Recheck: with amp OFF, measure DCR to ground at each speaker terminal, when connected to amp.
So, new development: could be a grounding issue.
I removed the limiters and went back to the base circuit for the preamp, and discovered that once I grounded the circuit to mains ground (and added a 68k series resistor between preamp out and PA input), I could go to full volume without it cutting out.
This is really weird. An amp that doesn't like not being grounded to an outlet.
Don´t you have a scope?
To distinguish between bursts of oscillation, protection chopping audio, DC jumps, whatever.
Ear alone is not enough.
I had a DSO138, but it broke ages ago and I never got around to get a new one.
But now that you mention it, I looked and found a DSO152 I could get (30 bucks). I'll see if I have enough in my account haha.
Also I forgot to mention, but I had already ordered an extra TPA3118 alongside those heatsinks.
Update: the DSO152 gets here on Monday.
Also one last thing I'll try is adding a pair of diodes to isolate the preamp from the PA (both anodes to battery V+, each cathode to the V+ of each circuit).
Project chugging along, doing a few more tweaks to the build: added BMS boards for each battery pack, and soldered the isolation diodes, gonna try to finish it today after work, so I hadn't had time to test it with the 'scope - which did arrive, and works great from what I tested so far with the function generator app on my phone.
Also ended up finding the old one, and sure enough it's busted, not showing a wave regardless of what signal I inject into it. No idea the cause.
Last but not least, one of the buck converter boards I was trying out let out the magic smoke. Shorts happen.
Last week was a bit hectic, and I got a throat bug on the weekend, but I managed to do this quick test just now to show exactly when it's cutting out, and I think my main issue is the batteries. I wish I could use them but it seems they just can't handle the load.
Got plenty of power from a power supply.
BTW I'm using a XL6009 buck converter to get 25v for the P.A. here.
FWIW I never ever use a buck converter to go *up* in voltage, the stress on the low voltage side is huge.
I DO feed amps higher voltage rails from 12V 7A lead acid batteries but:
* do not "buck" anything but use heavier duty voltage inverters.
More complex/heavy/expensive but the real thing.
What car power amps use.
* lead acid batteries have HUGE current capacity, that´s why they are used to start car engines.
Yeah I'm not interested in getting completely new batteries for this build. I'm already pretty much outta funds as is haha! Also I got a whole drawer full of 18650s I wanna use. I'll probably just drop the integrated battery idea and instead build an external pack that plugs into the power input plug instead.
At the moment I am designing a new line of amps based around TPA3118, which will be fed from generic/surplus Laptop bricks or LED lighting SMPS, anything between 18 and 24V DC, simply because they are cheap and plentiful.
So I am designing preamps, effects loops, distortion, etc. fed from +24V single supply instead of classic +/-15V or +9V so together they become full amps, both pedal sized and small combos.
Basically what you are trying to do ;)
In September there is a Musical Expo (think mini NAMM / Expo Música / Frankfurt Messe) in Buenos Aires where I will show these new products, so I must hurry.
I'll love to see what you come up with!
I've also gotten one of these:
https://www.alldatasheet.com/datasheet-pdf/view/1542047/REICU/WRA1215S-2W.html
Specifically, A1215SDL-2W, Which is a DC to isolated dual rail converter. If it works well, I'll use it to power the preamp - it's smaller than any of my other DC converters anyways.
For now I'll try to figure out at what signal level the amp cuts out, and try to keep my volume under that level.