Change a James to a Baxandall

[bassmannate]

I'm not even sure if this is practical at all but is it possible to simply change a passive James tone stack to an active Baxandall stack? I was looking at the flipster (http://www.runoffgroove.com/flipster.html) in particular and was wondering if it would just be a matter of lifting the ground on it and then instead of putting it between two stages, feed it from the drain of Q2 back into the gate (much like the Baxandall was originally used on tube circuits going from the plate back into the control grid) Any problems here? How would it affect the circuit? I would assume it would give me bass and treble boost and cut instead of just cut.

[CynicalMan]

I think you could put a James circuit in the feedback loop of an op amp in an inverting amplifier circuit, but I'm not sure. I'd suggest simulating it to see what happens.

[bassmannate]

Oh, ok. You mean something like this then:


I just threw this together. I copied/pasted from my flipster schematic so the cap and resistor numbers are off. I just pulled a op amp out of the libraries. This happened to be the first one so I don't know if I would use it in particular. I also don't know about the placement right in the middle of the circuit since I don't know how much distortion this might add. Maybe at the beginning? I don't know. If I set it up for unity gain, I don't think I would have to worry about hitting the rails, right?

[edvard]

Hmmm...
All passive tone controls with In, Out and Ground (where misbehaving frequencies get kicked off the bus) can be converted to active by placing the tone control in question in front of an inverting amplifier stage with the ground point fed by the inverted output instead of ground.
Your circuit doesn't have that connection, just In and Out in a feedback loop, which might do something, but I'm not sure what.

How it should be connected:
First, op-amp pin 3 to 4.5V (for inverting operation).
From 2nd stage -> junction of R4/C6.
From R5/treble pot junction -> inverting input of op-amp (pin 2).
From R6/C7 junction to op-amp pin 6.

Reference:
http://sound.westhost.com/dwopa2.htm#baxandall (among others...)

NOTE:
The conversion is not 'straight-over' due to impedance considerations internal to the op-amp.
I'd say sim it first, then tweak on the breadboard.

Your drawing has now got me wondering if there is a method for implementing active tone control in a non-inverting situation.
Web search doesn't turn up anything, anybody else know?

[PRR]

> You mean something like this then:

No. That is a unity gain follower with several dollars of useless parts.

> a method for implementing active tone control in a non-inverting situation.

Yes. See Dynaco and Quad preamps of the 1960s. For 5dB cut you can use linear; for significant cut you need REVerse audio taper. (Or, like Quad, hide most of the knob so users can't see they turn backward.)

But with 13 cent chips, you can buffer in and out, hide the low and variable impedance of any tone network, flip polarity as needed..... the old clever-tricks are no longer needed.

> change a passive James tone stack to an active Baxandall

WHY?

> ...lifting the ground on it and then instead of putting it between two stages, feed it from...

Will work backward (clockwise is less) and center will not be flat (James needs 10% audio taper, Bax uses linear).

I prefer James for non-precise musical sweetness, but in practice they are so close I just can't see why you would bother. James plus gain of 10 is the same overall gain as Bax. Bax demands open-loop gain greater than 10 or you won't get near 20dB boosts.

[slacker]

Have a look at the Colorsound Overdriver. I'd agree with PRR though, the James is very nice and works well, I wouldn't bother messing with it.

[bassmannate]

Have a look at the Colorsound Overdriver. I'd agree with PRR though, the James is very nice and works well, I wouldn't bother messing with it.

Good point, guys. I looked at the schematic for the original SVT and it is in fact a James circuit. Just filters to ground. I'll probably keep this original then.