Baxandall + opamp, more gain

[Lino22]

Guys, i want to use baxandall in opamp's feedback. The signal that enters is about 6 Vp-p and i would like to have 7 Vp-p at the output. I get just 6 Vp-p at he output.

If I understand well, the level the inverting opamp amplifies depends on the total impedance in the feedback divided by the serial impedance to the inverting input (by the way is that the output impedance of the previous stage?).

When i add R6 = 4.7k, the output raises to 7Vp-p, but R6 will have impact on the tone stack function, right?

[ElectricDruid]

If I understand well, the level the inverting opamp amplifies depends on the total impedance in the feedback divided by the serial impedance to the inverting input (by the way is that the output impedance of the previous stage?).

The serial impedance to the inverting input would *include* any output impedance of the previous stage, but generally that should be insignificant against the inpedance of the input resistor(s) of the inverting stge itself. In the case of the baxndall, that's the stuff to the left of the wipers.

When i add R6 = 4.7k, the output raises to 7Vp-p, but R6 will have impact on the tone stack function, right?

Yes.
The actual output level is going depend so heavily on the control position that the only thing you can really say is that you've got 0dB gain at the flat position.
If you want 7V/6V = x1.16 gain somewhere, I'd be inclined to try and put it in a different stage rather than squeeze it in here. The other consideration is that with a 9V supply, 7Vpp is going to be hitting your maximum headroom with many common op-amps, and could easily cause distortion when the bax controls are moved.

HTH

[Lino22]

I may go for 18V then, and use the other half of the opamp to amplify.

[PRR]

When i add R6 = 4.7k

You are wrong, but halfway there. Do a low impedance voltage diver.

This HAS been done in "fine hi-fi", Dynaco and I think McIntosh.

[m4268588]

[Lino22]

When i add R6 = 4.7k

You are wrong, but halfway there. Do a low impedance voltage diver.

This HAS been done in "fine hi-fi", Dynaco and I think McIntosh.

Excellent, thank you Paul.

[Lino22]

Guys i implemented the divider, but it moved the DC voltage on the first opamp output up a volt or so, so i adjusted the DC voltage divider 10k/10k to 10k/8k2 to push it back to 4.5V so the signal; can have a full swing.

Is that a good idea?

[Rob Strand]

Guys i implemented the divider, but it moved the DC voltage on the first opamp output up a volt or so, so i adjusted the DC voltage divider 10k/10k to 10k/8k2 to push it back to 4.5V so the signal; can have a full swing.

Is that a good idea

Not really. Better off putting a cap in series with R11.

[m4268588]

[Lino22]

@m4268588 works like a charm

[m4268588]

whichever

"B" may be slightly better in many cases.

[Lino22]

Thank you m4268588, that is what Rob suggested and it makes great sense. I will try it indeed.

[Ben N]

This has been done in guitar, too.

[Elektrojänis]

I may go for 18V then, and use the other half of the opamp to amplify.

18 volts may seem like a lot of headroom, but it might not be enough. If the opamp can get around one volt from the rails you have maximum signal of 16 volts. That's only about 7.1 dB from your 7 volt flat signal. So... If you boost 12 dB, you will be clipping. Many Baxandal tone controls can even boost 20 dB.

[Lino22]

Guys i implemented the divider, but it moved the DC voltage on the first opamp output up a volt or so, so i adjusted the DC voltage divider 10k/10k to 10k/8k2 to push it back to 4.5V so the signal; can have a full swing.

Is that a good idea

Not really. Better off putting a cap in series with R11.

This one works the best (through a cap to GND). The other variant (to the reference) creates a weird slow oscillation (tak-tak-tak-tak) when the bass is all the way up.