Op-amp recovery stage

[caspercody]

Does anyone have a schematic on how to wire up a op-amp recovery stage? I built the Thor, and want to add a tonestack. I replaced the op-amp with a dual op-amp and want tuse he other op-amp as a recovery stage. ut not exactly sure how to wire this up.

[Cliff Schecht]

Do you know what a "recovery" stage is? Sounds like a fancy name for an op amp non-inverting gain stage. Check this link: http://www.play-hookey.com/analog/non-inverting_amplifier.html

Note: You can ignore Rz, it's not necessary in most applications.

[caspercody]

Honestly I do not know what arecovery stage is. I read about doing this on a site somewhere, and I think what is trying to be done is adding the tonestack at the end of the op-amp in the Thor project. Than after the tonestack using the un-used op-amp on the dual op-amp as a booster. Since some of he level would be lost from the tonestack.

[caspercody]

Okay I remember where I read this, it was on Gaussmarkov's site. I built the Thor using runnoffgrove pcb layout, and made the changes to the board to add the dual op-amp. So to add a tonestack (looking at Gaussmarkos schematic):

Come out of pin 1 (before C14) to the in on the tonestack. The out of the tonestack to pin 5 of the op-amp. Out from pin 7 to C14.

Would this work? And my big question is should I leave pin 6 to 7 wired like in the schematic? Or is there other components that need to be added here?

[Ben N]

Honestly I do not know what arecovery stage is. I read about doing this on a site somewhere, and I think what is trying to be done is adding the tonestack at the end of the op-amp in the Thor project. Than after the tonestack using the un-used op-amp on the dual op-amp as a booster. Since some of he level would be lost from the tonestack.

That's it. Doesn't have to be an opamp, any voltage amplifier configured to restore a signal level depleted by a lossy passive circuit like a tone stack fits the bill. (Classic) cases in point: The last transistor stage in a Big Muff or the second triode stage in a blackface Fender preamp, or (extreme case) whatever follows the tank in a spring reverb circuit. Whatever it is, the main thing is to tailor the gain to the need.

[R.G.]

Honestly I do not know what arecovery stage is. I read about doing this on a site somewhere, and I think what is trying to be done is adding the tonestack at the end of the op-amp in the Thor project. Than after the tonestack using the un-used op-amp on the dual op-amp as a booster. Since some of he level would be lost from the tonestack.

A recovery stage is simply an amplifier that "recovers" the signal lost by the tone stack. This usually in the range of 10:1 (-20db). So a 10x gain stage "recovers" the level back.

[caspercody]

Okay, I tried adding a tonestack with the extra op amp stage. It works, but turning the bass control down made the distortion fizzle out. I took out the op amp stage and just left in the tonestack, but what a lose in level output. I do not think I had the op amp wired in correctly, I wired it just as I described earlier. Does anyone have a schematic showing how to wire up a extra op amp in as a amp like RG wrote?

[Cliff Schecht]

Look at the circuit at the bottom of page 8 here: http://www.eng.yale.edu/ee-labs/morse/compo/sloa058.pdf

Eliminating R1 and making R2 large (470k to 1 Meg) will give you an amplifier that prevents your signal from clipping with a single power supply (you can't swing below ground if the op amp is referenced to ground). Adding some negative feedback (resistor from minus pin to output and another resistor from minus pin to ground) will give you an amplifier with gain. The gain formula is the same as any other non-inverting op amp circuit.

R.G. calls this "noiseless" biasing, but I'm not so sure it's any more noiseless then other methods. Of course, the noise is dependent on how you develop your VCC/2 and my favorite method is a voltage divider feeding a unity-gain op amp stage.

[caspercody]

That is pretty much how I wired it up, without Cin, and R2. Maybe without them that is why I got the fizzle out of signal when I turned the pot down. What would be good values to use for these components? Where does the one side of R2 go to? It says vcc/2?

I wired this up with (reference Gaussmarkov Thor schematic) pin 1 going to input of tonestack. Output of tonestack going to pin 5 (TL072). Pin 6 shorted to pin 7. Pin 7 out to C14. I tried this with VR removed from pin 5, and it worked (with fizzle). But when attached VR to pin 5, no sound.

[Cliff Schecht]

You want that blocking cap to be a large as possible, within reason. I usually default to a 1 uF cap if I have any on hand and the input impedance is high enough (which it is). Pretty much anything above .1 uF will work.

For VCC/2, you want to use something like this for the very best results. I'd make the resistor values 10k resistors and bypass them with a .1 uF cap as well as a 10 uF cap (both caps go in parallel where the 4.7uF unit is). Where it says 12 V, you put in 9 V and the output will be exactly half that (if your resistors are matched), 4.5V. This is your VCC/2 point.

[caspercody]

So, to get this straight, I can use this circuit to boost the level that is lost from the passive tonestack?

I would apply the positive of my battery source to the +12v?

Is this also where my output of my tonestack connect? Or at the triangle icon on your drawing?

And the output is at the +6v?

Thanks for all your help on this!!!

[Ben N]

Cliff's drawing is just of the bias supply (Vr, or Vref), and he is assuming a +12v main supply, but it would work the same with 9v, 15v, whatever. Where Cliff has 12v is where you connect this to your battery or main supply AND to the power pin in your opamp. The 6v output goes to your bias connections. Note that this opamp is NOT your recovery stage, just a nice way to bias it up.

Cliff: is this what I have seen referred to as a servo supply, and what are the advantages? Also. can one half of a dual be used to bias the other half?

[petemoore]

  Another way to get a ''stiff'' 1/2v [doesn't move when current is drawn], is to use an LM386, hook the PS across the V_/+ of the opamp, the output is 1/2v.
  Or two batteries.
  ...from which an audio circuit which requires split supply can get power.

 

[WGTP]

http://www.aronnelson.com/gallery/main.php/v/WGTP/Gus.GIF.html

The op amp on the right side is a gain recovery stage suggested by Gus.   

[caspercody]

Thanks WGTP, I think that is what I am looking for.