Trace Elliot acoustic amp with DC on pots

[lespaul924]

Hello,

I have a Trace Elliot acoustic amp TDA100 that has 16v DC on the pots, they crackle when a signal is present.
Every pin on every pot has it except the master volume.
The 16v is also present on the EQ faders and every opamp pin of every opamp besides the correct single sided +30 pin 8 and ground on pin 4 and also most preamp components aound the stated areas. The source seems to be the voltage divider at R80/83. the schematic has this going "to circuit" this amp does not have the reverb, so the R73,74 C45,46 and further components are not there.

The R80/83 and two c48/49 220uf caps are all fine. The 16v goes into the graphic EQ section and then from there ends up all over the place. I have replaced the opamps those seem fine. I don't really understand how the voltage is so widely distributed and I'm not finding any bad components. what is the 16v supposed to be doing in the non reverb version? the opamps are all +30 single supply. The only thing I'm seeing is the +15v TR12 mute in the upper right that this amp doesn't have.

Any suggestions or does anyone have a similar amp to compare to?
Thanks for the help

[oc disorder]

Hi
Couple of suggestions ..first thing that springs to mind is a leaky coupling cap
but can't see why the entire circuit should be riddled with it.
If you lift one end of C11 4.7uF, is the voltage still present on the tone pots ?
It hasn't got one of those green "Fluo" panels has it ?
The fet at the begining has 30 volts on it.
Can you measure any voltage at R13 after the 1 uF cap ?
I cant see how this could pass through the whole circuit though.
I would try isolating various parts of the circuit by lifting one end of the coupling caps.

[J M Fahey]

Main problem lies on crazy Trace Elliott designers bizarre grounding scheme.
You have two "ground nets" there, which lie at about 15V from each other.
Trying to measure anything there is confusing (to say the least) unless you are aware of that.
There is a PSU Ground, drawn with a sort of "triangle" made of decreasing horizontal lines, that being the most popular style, and a Preamp Ground, drawn as a downwards pointing triangle.
So:
all op amps have pin 4 tied to PSU Ground, which for them is -15V; all DC grounds tied to +15V, which for them is "0"V, and all pin 8 tied to +30, which for them is +15V.
That shifted powering is generated by the circuit around Tr(11?/14? ... unreadable) , on the left between the Low Z input and the back pcb XLR out. , which takes the +60V and generates +30V and +15V , labelled as "(Ground) to circuit".
Most confusing and downright stupid, what were they thinking?
So in a nutshell: having +16V everywhere in your preamp is "normal" (well, sort of)
Measure voltage not referred to ground but across pot pins, any DC there means scratchiness.
Check what I have just written and post your new quastions, which now will come from a new angle.
Also post which pots scratch heavily and have DC across them (even 100/200 mV are too much)
Good luck.
NOTE: if

they crackle when a signal is present.

only, they are just dirty.

[lespaul924]

Thanks for the quick replies,

no matter where I try to ground I come up with the 16v, but testing just for dc across the pots shows no voltage. So i agree that it is probably just part of the grounding scheme.

Also, since opening the chassis up I've been testing the amp without the plastic knobs and just turning the bare shafts. It seems it only snap crackle pops when the pots are not firmly bolted against the chasis and then turning or torquing the bare shafts. something in the scheme that could cause this?
I'll put the amp fully back together and see if the problems persist. Maybe a simple problem with a simple solution that got side tracked by a goofy design.


Thanks again for all the responses

[Alex R]

Yes that crackling when the pots aren't grounded is normal. Even if they are grounded you get it from touching the metal shafts sometimes. It really sounds like scratchy pots but it isn't.

Trace used AC grounding (as I call it) a lot - caps coupled circuit ground to chassis, with a resistor to keep things a bit stable. It can sometimes be converted to good old straightforward chassis grounding. But let's not go there if your amp works fine with the pots tight and the knobs on.

[J M Fahey]

something in the scheme that could cause this?

Sure :

goofy design.

The only way I would justify something like that is that they had previously ordered 10000 boards to be used with an earlier, smaller, single-supply power amp.maybe even battery powered, and later had to adapt it (sort of), to a split supply power amp, with very high rails to boot (+/- 60V), which probably was pulled from some quite high power bass amp (their specialty).
Having 9900 boards paid for and unused sure makes you want to kludge something using them.