Making a passive EQ active

[ianmgull]

Is it as simple as throwing a buffer before/after it?

I found a PCB for a "Fender tonestack 3 wire passive EQ" on the DIY Layout Creator gallery. I want to make this tonestack active so the bypassed volume is the same as the EQ volume. If I put the GGG op-amp buffer before it and change the value of the bias resistors, will this give me the results I want? Or is it far more complicated than this.


Thanks

ian

[John Lyons]

It's more than that...
Active means that you have the ability to boost, to create an amplified boost at a certain frequency.
The fender/vox/marshall/orange etc amp "tone stacks" are all passive circuits.
Technically they are cut only circuits, no boost ability.

Of course if the controls are set mid way up and you turn up the treble it wil get brighter but
technically there is no added frequency, just turned up to "flat" (or as close as the circuit will go to flat)
If you turn them up to 10 they are all close to "flat" but they do not boost.
Active eqs have either an op amp or a transistor to amplifiy the frequency.
It's not so much about level but about how the work.

john

[ianmgull]

Thanks John, that seems pretty obvious now that I hear somebody else say it. So I guess if I kept all of the pots all the way up it would (in theory) be the same as not having the circuit in place at all?

I'm trying to find a 3 band eq to put after a LOT of distortion. Is there some other 3 band that anyone would recommend? Since it's after the distortion I guess just being able to cut frequencies isn't all that bad anyway. Thanks!

[petemoore]

  Duncans Tonestack Calculator...a very highly valued learning and demonstrating tool for tonestacks.
  Notice how the Marshall TS's frequency gain line [average] is higher amplitude [which means less passive losses] than Vox or Fender [just visited it today to show my brother, Jeff, who is visiting here, a bit about it].
  If a passive tone control doesn't reduce signal [in any sense including frequency specific reduction of amplitude]...it is basically more like a wire, and isn't a tone control.
  Active system...if you see a power supply, and active component, it's probablistically very much a booster, either to amplify the signal before the TC [boost] or after the TC [recovery] to bring what's left of the signal back to a certain level, either way the signal is amplified.
  It's common method to add an impedance booster before the TC to drive it [also may help improve function].
  Whether the amount of passive TC losses you experience with it after your boosty distorter are leaving enough output for your needs...?..
  depends on the circuit, the amount of passive losses [see Duncans TSC, if the line is averaging to be up higher on the graph, there is less passive loss [and will sound louder], and what else you run it through after and ... and.

[ianmgull]

Maybe I should refine my question a bit.

I'm not interested in being able to boost a frequency so much. I'm more interested in eliminating the volume drop that is inherent in passive tone stacks. I want an eq that when set flat, will have approximately the same volume as when it is bypassed. It seems that my best option might just be to put a booster after it. This means that my signal would be going through a booster at all times. That being the case, what would you recommend for a "transparent" (sorry for using that word) booster?