Wampler warm 741 overdrive

[madstayen]

So I'm trying to build this. Would love to here the difference between the jfet O/D's and these ones. I wired it up perfectly (breadboard of course), But can't seem to get sound from it. All I can think of is that I'm not doing something that I should, for example, maybe leg 1 and 5 should be bridged, I don't know. I'm too afraid to do it, don't want to fry the chip   And what about leg 8? Should it be connected?




This is the pin-out. The green block is how I associated the pin-out numbers with the physical chip. Is this correct? Maybe the voltage isn't enough, because from the voltage settings page of the data sheet, it looks like it doesn't work with 9V. Should I up the voltages to 12V?
The only nonconformity to the drawing is R2. I don't have a 470R resistor, so I put a 390R and 100R in series, giving me a total of 490R, which I think is close enough!

[nick d]

                 That schematic looks off - you would have positive feedback from pin 6 to pin 3 , so the circuit
                   would oscillate , likely at a high frequency .

[induction]

The 741 is a garden-variety (if lower-performance) bjt-input single op-amp. 9V should be fine, and your numbering is correct. You can leave pins 1, 5 and 8 unconnected.

Maybe your chip is cooked, otherwise, you've probably got a bad connection somewhere.

Edit: Nick is right. Try swapping pins 2 and 3.

[Pojo]

Which Wampler pedal is this supposed to be a schematic of?

[madstayen]

Thanx. Swopped 2 and 3. Now I get sound through, but its roughly 10 times softer than the clean signal (guitar to amp)

[madstayen]

@ Pojo - It's one of the wampler books, edition one. Lots of overdrive and disto builds. I guess it's either his, or a derivative from a seventies gray 250.

[samhay]

There is nothing particularly unusual about this design, except that it is drawn incorrectly - its a Tube Screamer type circuit with a relatively flat frequency response.
As noted, the input should be going to pin 3. You also need to bias the input to half the supply voltage.

[madstayen]

You also need to bias the input to half the supply voltage

How do I do that?

[GibsonGM]

Go here: http://www.geofex.com/article_folders/tstech/tsxtech.htm

Scroll all the way down to the last schematic.  On the left side, you will see a bunch of resistors and the battery...that is a bias network.  Where it says "4.5", you connect that point to the + input of your 741, just the way it is done in that schematic....thru a 470k resistor.  This is called "biasing" the opamp - setting the point around which it operates when no signal is applied....that's called "quiessence" (quiet state, if you will).   

This will move that input up to lie at 4.5V, half of your power supply....so that when your input signal moves from pos to neg and back again, the opamp can respond and amplify both sides of the signal.    The way it is now, you can only amplify the positive side, maybe why it's so quiet....

[madstayen]

Ah thanks. Will do so.

[madstayen]

This is how I have it now. What I don't understand is the Voltages on both sides of the 470k resistor. It seems to be getting a + reading from leg 3, and from the resistor side.

[nocentelli]

As noted, the input should be going to pin 3.

[madstayen]

Okay, so then I swop pin 2 and 3 around, and put the 4V5 on pin 3 as well?

[induction]

Correct

[chi_boy]

So final copy like this?

[Phorhas]

470k resistor from the inverting input in series with the 10uF capacitor should be 470Ohms

Cheers

[chi_boy]

Cool, thanks Dan!

[candidate]

This schematic was actually sold for money at some point?

Anybody who could figure out the errors probably didn't need the thing in the first place.

[Mark Hammer]

This schematic was actually sold for money at some point?

Anybody who could figure out the errors probably didn't need the thing in the first place.

  Brian needed an editor in the early days.  His quality control is much higher now.    You should see the "schematics" in the Dave Hunter FX handbook.  MUCH much worse.

The corrected schematic that I am looking at presently seems to check out.  T'wer I, I'd stick a 1k5 resistor in series with the 100k gain pot.  I can't see anyone having much use for the circuit as a unity gain stage (gain set to 7:00 and volume set to 5:00), so make the minimum gain something useful enough that you'd have to at least think about where you needed the volume set.

With that added resistor, minimum gain is a little over 4x, and will likely not result in anything more than very brief diode clipping on one half cycle, if that.  A relatively clean minimum gain, with the volume turned up, will let you push an amp.

The 25pf feedback cap is there simply for "stability" and provides no audible tone shaping at the top end.  If you find you'd like to "sand off the edges" a little, consider upping it to 220pf or even 330pf or 470pf.  Wth 220pf, it rolls off above 7.2khz at max gain, 4.7khz with 330pf, and 3.3khz with 470pf.  Note that these numbers are for max gain.  At less than max, the rolloff point is moved higher up.

[madstayen]

@ chi_boy - Thanks for the schem!!!! Am busy sorting it out now!!