active tone circuit?

[bassmannate]

So, in reading a lot of the low voltage amp topics, I've noticed a trend of people talking about passive tone filters lowering the volume of the amp because, obviously, they are subtractive. These are things like using a pot and capacitor to pass certain frequencies to ground. The higher the value of the capacitor, the more lows are passed through (just stating all of this to show that I'm not a COMPLETE noob and that I understand how these circuits work)

What would an active tone filter look like? Are these fairly common in commercially produced amps? I'm assuming that one disadvantage of these is more parts and thus more space is needed for the parts (not to mention cost of these parts) Why haven't I seen more discussion of putting active tone circuits in low voltage amps like the Murder One and such?

Sorry. I've been puzzling over and googling this question for a few days now.

[Ripthorn]

I can tell you that the reason Rick didn't put one in the Murder One is because he wants an all tube signal path, but in general, an active tone stack will involve an opamp with the controls altering mostly the feedback path (I think).  The ROG Tonemender is a good example if I am not mistaken (it's been a long time).  One alternative is to build a simple buffer to drive the tone stack harder and also to decrease the output impedance of the circuit, making the tonestack look like a higher input impedance and thus reducing losses.  AMZ has some really good buffer circuits.  I am building a low wattage amp and using the buffers to drive the tone stacks, but I also have the option of bypassing them for an all tube circuit path. 

I can't speak of what is in commercially produced amps, but most of your typical tonestacks come from Fender and Marshall.  These tonestacks usually use a triode (for tube amps) cathode follower to buffer the signal to drive the tonestack harder.  Solid state amps probably use a single transistor or opamp to do the same thing.  But then again, I haven't opened too many amps up.

[bassmannate]

Thanks for the explanation! I kinda figured it involved amplifying or driving what is basically a passive tone filter. I just wasn't sure. Took a look at the Tonemender and it completed my understanding.

Thanks again!