1) that is a STRONG low pass filter and is NOT common or needed, at all, in normal SS guitar amps.

2) you do not show where they lead to, but a few amps or preamps or pedalboards, digital or analog, include similar 24dB/octave or stronger low pass filters as "cabinet simulators".
Normal load for a guitar amp is a guitar speaker which has a built-in "mechanical" low pass, usually above 4kHz or so.
If signal is fed into a recording or PA mixer or into a powered cabinet which have way wider response, tweeters or drivers, etc. , sound is *unbearably* buzzy, so such a filter is added to mimic guitar speaker rolloff.
Your circuit is incomplete but I guess it either feeds headphones/line out or somebody complained about buzzy sound and designer included it for good measure.
Again, not "common" by any means, you do not *need* it.

As a real world example, Hughes & Kettner Red Box: a  signal attenuator -direct box - cabinet simulator, including such a filter:
https://elektrotanya.com/hughes-kettner_red_box_classic_sch.pdf/download.html

I´m impressed by the designs and the looks, congratulations.

This is a DREAM opportunity for somebody wanting to build up a small Factory, either a Tech who wants to take the next step or an investor, who wants to multiply his money and hires a Tech to do the soldering and assembly, he takes care of sales (and collecting money ).

The amps can be assembled and sold as-is in those futuristic head-shells or rehoused in more conventional ones, his choice.

The see-through illuminated front panels look breathtaking on any stage (or Album cover ), of course conventional punched and painted (or polished) Aluminum can also be used.

True enough

Under such sheer lack of data, every b it counts.

I suggest OP to post a couple power amp closeups, : PCB showing readable part labels, wiring, power transformer, power supply, etc.

Even if no schematic visible, "counting beans" method may be used: print datasheet schematic and parts list, put good power board pictures by it, go through parts list (as in  R1 ... R2 ... etc), and try to find each part named on PCB, then tick it on both list and picture.

Part numbers will NOT match, but they "must be there" for the amp to work, at least 90% coincidence if not better.

Write on parts list and schematic actual PCB part number.
Say: R1 on schematic is R203 on Trademark amplifier and so on .

Now you have the schematic matching your amp!

You will certainly have a few different/tweaked values but no big deal, you can now repair your amp.

Quote from: phatt on August 17, 2024, 08:42:49 AMI lost my Wife last year And 20 years earlier she lost her son to suicide, she carried on but it left a mark on her outlook on life.
Yes the things that life throws at us can be soul destroying.
So I have some understanding of your great loss.
Phil.

Ouch!
Hard blows for sure but hey, we must keep playing no matter what cards does Life deal us.
What else? 🤷🏻

In the real world, no big deal.
Since forever I used 100k to ground at the input differential leg and 10k as NFB from speaker out to negative keg and my typical offset is around 40-50 mV so I never bothered to correct it.
So you are fine.

Intrigued by this post, searched Heaven and Earth for a Trademark 112 schematic or at least a Power engine 60-112 which seems to be the same cabinet, power amp, power supply, only lacking the multi-emulation multi-effect preamp and sporting only a basic tone control since it's meant to be driven from some multi-effects pedalboard.

Could NOT find anything, these Tech 21 guys are _secretive_ , but one of their designers complained about Yamaha copying their product.

They even use same Celestion 70-80 speaker!

And *that* service manual is available at Elektrotanya. 😉

Different power amp technology (discrete vs chipamp) but the useful point is that power amps have no fancy tricks at all,they are flat and clean (why doesn't that surprise me?), 20X gain (so about 700-1000 mV sensitivity), only slight mod is mixed feedback technology which functionally "changes nothing"

So what's my point?
Now I am *quite* certain your amp literally uses "official datasheet" schematic, so you can use that as a Service Manual.
Not "flying blind" any more. 😊

Hey! KILLER amps and designs, very original, very good looks.
"They sell all by themselves"
I would be a present from the Gods for anybody to be able to get his hands on this and start a new Company or endeavour with so much "start up" job already done for him.
It would save YEARS. And money and teething troubles.
Guess you would also transfer your current customer database, to which they would later add on.
This is similar to buying a working Company, ready to keep earning money  "tomorrow".
Hope some smart guy jumps in to profit from all the groundwork you already did.

 
 

Ok, ONLY NOW can we see the schematic.

1} FORGET current feedback and any fancy stuff.

2) *do* use the output transformer, you *need* it.
Output is bridged and IT primary winding combines out of phase signals.

So connect speaker to proper 4 or 8 ohm tap as needed.

PA amps are weird,this one has 70V taps and also is *designed* to be fed from a 12.6V car battery for street/field use.

Quote from: wound3rd on August 31, 2024, 10:19:12 AM....there's a heat sink pad between the LM3876T chip and the heatsink. In my case the pad was bad - kind of gooey or greasy.

It is *supposed* to be gooey/greasy.
It is smeared on both sides with silicone/thermal grease.

QuoteSo, for a few $ we got some new pad, cut it to size, cleaned the area well, and replaced the pad. Worked like a charm.

It _may_ work fine ... until you play LOUD for an hour or two and it becomes *hot*.

Get a small tube of heatsink grease/compound and apply a thin layer between chipamp and pad, and between pad and heatsink. 

This is a mid power (around 6W RMS into 4 ohm) car radio type amplifier.
You *can* turn it into a current feedback one but I can't read the fuzzy datasheet for instructions 
If available, post the PDF here.
I mean the real PDF,not a link to it.

PS: if bridged, forget about adding current feedback.

Thanks for posting.
We may _guess_ all day long but at the end "the proof lies in the eating"

Or, as we say in Argentina: "en la cancha se ven los pingos" literally meaning "racing horses are proven in the racetrack" 😉

Yes.

By using a Log/Audio pót you have a finer control at lower settings.

Quote from: utas5 on August 19, 2024, 03:59:51 PMMy speaker's impedance is 8 ohm. The reason that I thought to add a attenuator so I can use the whole volume pot, because how I know if the volume pot is only at 25%, then it won't have that good of a tone like at 75%. I know that this is kinda true at valve amps, but I don't know if it's true at SS amps.

*DO* use your volume pot as high as you want, even 100% for quite raunchy sound if you wish, but then attenuate _output_ power to desired level using the Master Volume pot, which is its intended use.
Tube amps with Master Volume do exact same.

@Jazz P Bass: read first post: a Rodd Elliott P27 preamp driving a TPA3118 power board.

Sad about some if your news, happy you made it through your operations and are still with us.
Doubly so because you want to plug the old soldering iron again 😄
Keep us updated.

Yes.
For the full experience, take speaker out signal, which will be about 10V RMS (WAY too high) and attenuate it 100:1 to 100mV using a 2 resistor voltage divider: 100k in series, and 1k to ground, which is perfect to feed it into a mixer Line input (every channel has one), no need to super attenuate it to microphone level (a few mV), nobody does that.

Insert a direct box circuit in the path as suggested earlier.
For example: Hughes and Kettner famous Red Box

Schematic and suggested PCB is widely available.