Non-opamp Active Tonestacks

[drew.spriggs]

G'day.

Been noodling through this design and made it up to the tonestack portion. The preceeding stage has a reasonably high output impedance, and I'm running out of board space.

It's a CMOS Muff-ish thing, so I've got 3 preceeding gain stages. A solution I found was using an active BMP tonestack (the tone controls are in the feedback loop) which looked like a great option as does double duty of having a low output impedance, but without vref already on board I'm out of space adding a DIP8 footprint, a voltage divider and filtering/decoupling caps.

Are there any interesting non op-amp tonestacks that you've seen? I know I could just go buffer>tonestack>gain recovery stage, but that would definitely be pushing it for space. I've seen baxandalls running off a single transistor so I am assuming it's possible to do; I just haven't seen many other examples of somebody doing it.

Thanks!

 

[jwin615]

So, you don't have a vref but you have a PCB that is packed?
Is your Z too high to use a jfet as a buffer?
Or maybe put the opamp on the other side of the board?

 

[drew.spriggs]

So, you don't have a vref but you have a PCB that is packed?
Is your Z too high to use a jfet as a buffer?
Or maybe put the opamp on the other side of the board?

No vref as everything else is transistors/CMOS.

I can probably fit a single transistor stage with the tonestack, but can't fit a buffer + gain recovery stage.

Even if I put an op-amp on the other side, I still need half a dozen extra components to use it - and I don't have the room for that.

 

[pingpongcomputer]

You can run a passive Bax tone stack with high input impedance, you just lose a good amount of volume and have to play with values a bit to bring the HF response back up. So you could experiment with putting it in without a buffer and see if you can make up the desired volume with a single transistor recovery stage.
Edit: The opamp in this active stack looks like it could be replaced with an inverting amplifier, and still only use one transistor, but I have no experience with that.

 

[drew.spriggs]

You can run a passive Bax tone stack with high input impedance, you just lose a good amount of volume and have to play with values a bit to bring the HF response back up. So you could experiment with putting it in without a buffer and see if you can make up the desired volume with a single transistor recovery stage.
Edit: The opamp in this active stack looks like it could be replaced with an inverting amplifier, and still only use one transistor, but I have no experience with that.
View attachment 101241

That is exactly what I am trying to avoid...