Baxandall problems

[DUCKFACE]

Whats wrong with this Bax tone controller. When twisting bass/treb nothings changed in the spind picture. 4.1v is 4.5v

[antonis]

Delete R13 and show us what IC3A drives..

P.S.
I presume IC3A & B are powered with +9V single supply..

[DUCKFACE]

Delete R13 and show us what IC3A drives..

P.S.
I presume IC3A & B are powered with +9V single supply..

IC3A & B can be powered by +9v or +-15v for more headroom.
now its powered from +9v

R13 shorted - the same problem - no bass or treble adjustment

[MrStab]

Is C17 shorted, or is IC3A otherwise bypassed somehow?

[antonis]

R21 / R22 / IC3A inverting input junction..??

[ElectricDruid]

R13 is a definite killer, since Baxandall tone controls don't like to be fed from a high impedance. So 10K is out. Not that that that's a big hardship since it does basically nothing.

I have no idea what the power supply to these chips is. There's all sorts of voltages mentioned on this schematic, but we don't get to see which ones are actually connected to the op-amps. 4.1V is very specific but not really related to any of the others! How is this 4.1V being provided?

[MrStab]

My bet is power amp powered from +/-12V and single-supply pre-amp with series diode, and i want a large sum of money or a sandwich if i'm right.

Hang on, schem denotes pins 8 & 4 as being on the dual supply. Okay no sandwich

[ElectricDruid]

No, I think you're on the right lines, Stab. The power supply situation is "unclear" at best, and without a better picture of what's going on there we're just left wondering.

[Rob Strand]

A more blunt question is where does the 4.1V come from.   Whacky power rails will kill the signal.

R13 isn't normal but on rare occasions I have seen circuits with a deliberate "R13" resistor.   It won't kill the signal just make the tone controls behave differently.

[MrStab]

From the information available, I inferred that the signal is still passing, and that neither pot works, so my thought was that the tone stack is being bypassed.

Maybe there's some mojo to be had from an effective +8, 0, -16V supply! Asymmetrical clipping!

[Rob Strand]

There's actually more weirdness on the schematic:
- "A" log taper bass and treble pots when they should be "B" linear tapers for this circuit
- Quite a low valued treble pot.   That will reduce the amount of boost/cut for sure.
   It could also push the treble frequencies up.  Both of those effect will make
   the control very weak.
   The 10k R13 value will make things even worse.

Try simulating the circuit without R13 and perhaps a 50k or 100k treble pot.


A similar circuit of unknown origin:


The Treble pot value being the biggest difference.

[bluelagoon]

looks like a poor mans Boss FA-1 circuit, Here's a better way with the original FA-1 verified working circuit -

might be a better place to start from.


http://byocelectronics.com/fetpreampschematic.pdf

https://aionfx.com/app/files/docs/prism_documentation.pdf

[Rob Strand]

looks like a poor mans Boss FA-1 circuit, Here's a better way with the original FA-1 verified working circuit -

might be a better place to start from.


http://byocelectronics.com/fetpreampschematic.pdf

https://aionfx.com/app/files/docs/prism_documentation.pdf

Quite close, just different Bass caps - and of course all of the fixes: 50k treble pot, B taper pots, no R13.

[bluelagoon]

Yeah, better to stick with the tried and true sometimes.

[DUCKFACE]

My bet is power amp powered from +/-12V and single-supply pre-amp with series diode, and i want a large sum of money or a sandwich if i'm right.

Hang on, schem denotes pins 8 & 4 as being on the dual supply. Okay no sandwich

powered from +- 15 at the moment. and single power supply for 9v  4.1v is actualy 4.5v
where am i suppose to send the sandwich and the beer ?

[antonis]

I think Grant didn't mention anything about beer..
(only sandwitch and a large sum of money..)

[DUCKFACE]

Pot changed to B-50k. situation is the same. should i have to post the hole picture ? you will say that its a mess probably.

[Rob Strand]

Pot changed to B-50k. situation is the same. should i have to post the hole picture ? you will say that its a mess probably.

It's probably just a silly mistake somewhere.

Check all your part values and connections. 

Check the tone control opamp +input pin is actually connected to Vref and that the output pin is also at Vref.

[ElectricDruid]

Rob's right.

If this is a debug thread, I've completely lost where we're at. What's the situation now, and what's still wrong? Honestly, a baxandall can't be *that* hard to get going. We can fix this.

[MrStab]

a large sum of money

Thanks, Antonis - I was too polite to express my *utmost fury*!!!!

If signal is passing, neither pot works, then surely either baxandall is being bypassed or the pot(s) wiper isn't connected properly?