Preamp mod with tone control - comment please

[cl1238]

I'm inserting a tone control into a 12ax7 preamp.
Can anyone tell me would this work?
Or should I remove the feedback from v4 and keep v1 with a cathode 1K2 to ground and a 1M gate ressitor to ground.
Thanks for your help.

 

[Miles Prower]

That design is terrabad for at least two reasons: 12AX7s don't make very good followers: the g_m is quite low, and the current sourcing capability is way too limited.

Secondly, you have your Bax circuit inside the gNFB loop. That means that your NFB will try to "undo" what the tone control is trying to accomplish. Lastly, do you really need that much open loop gain? If you're trying to make a hollow. state op-amp, there are better ways to accomplish that

It also looks like this is set up for positive, not negative, feedback.

 

[Koonw]

"It also looks like this is set up for positive, not negative, feedback."

Yes it's positive feedback. There will be a boost in gain in addition to tone control (?).

 

[DF96]

I assume the 27k bias resistor for V2 should be 2k7? Anyway, as others have said, there are lots of reasons why that circuit won't work.

 

[cl1238]

Thanks everyone, I'll scrap this circuit.

 

[Miles Prower]

"It also looks like this is set up for positive, not negative, feedback."

Yes it's positive feedback. There will be a boost in gain in addition to tone control (?).

Positive feedback will make for a very sharp audio frequency notch filter, or a BPF with a very narrow passband. Either can be very useful in communications xcvrs (to reject a herterodyne note from a nearby AM carrier, or to pull a CW transmission out of the noise). Neither characteristic is useful for a reproduction amp.

I don't think the OP intended PFB.