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 AM

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.

Marshall Lead 12 is a KILLER little SS amp.
You can build just the preamp to drive your power amp.

Full schematic

You cannot view this attachment.
Preamp only:

You cannot view this attachment.

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 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!!!  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



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" 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.