Adapting the Wampler Basic Overdrive

[timkenyon]

Hey y'all,

Continuing my exploration of pedals and pedal design, I'm doing a lot of schematic searching, redrawing, rebuilding and tinkering.  A few days ago I came across a blog post from Brian Wampler with a basic OpAmp overdrive design - OpAmp, symmetrical diode clipping.

I've added a couple of modifications to include:  SW2.1 - adds asymmetrical clipping to the feedback loop; SW3.1 - adds another diode to the feedback loop for either asymmetrical in the opposite direction to SW2.1, or symmetrical clipping with 2 pairs of diodes; and SW4.1 to change the clipping circuit to a pair of red LEDs.

I'm also considering switching R2 for a B1M pot to alter the bias of the OpAmp on the fly.

Any and all feedback on the options and the circuit would be gratefully received.

Tim

[antonis]

I'm also considering switching R2 for a B1M pot to alter the bias of the OpAmp on the fly.

Better replace R3+ R4 with a pot and get bias voltage via its wiper..

R2 reflects input impedance and forms HPF with C1 (which, by the way, may be lower down to 10nF) so you'll dominate input impedance and simultaneously raise HPF corner frequency..

[Mark Hammer]

Brian is essentially attempting to simplify a Tube Screamer and add some options.  As such he has included a low-end rolloff similar to the TS, at just over 720hz.  That gentle rolloff is selected to provide roughly equal amplitude across the entire fingerboard, given that lower notes on thicker strings are generally higher amplitude.  Nothing basically wrong with it, but retaining more bass not only gives you more bass but also gives a little more oomph and drive to lower strings.  If that's something you would like, make C5A a larger value.  I would suggest 680nf to provide more guts without unintentionally giving any advantage to 60hz hum.  Because bass signal is higher amplitude, you notice a little more distortion on full chords as well.

[iainpunk]

nice

i really like the idea of choosing which side the asymmetry favors, since guitar signal is asymmetric to begin with. i am experimenting with that myself too, but with changing the bias of a CMOS inverter gain stage to get asymmetric clipping, depending on the guitar flipping asymmetry can be quite pronounced or inaudible.

cheers, Iain

[timkenyon]

OK, lemme get this right:

@antonis - replacing R3 and 4 with a pot (I'm thinking 10k or 50k?) and using the wiper (pin 2) to feed back down to the OpAmp.  Should I then consider removing R2 completely, or keep it in place?  Switch out C1 for 10nF.
@Mark - replace C5A with a 680nF will reduce the amount of bass rolled off.

That would look like this?

[PRR]

No, you have the power and bias SWITCHED from the start. Copy carefullyier.

[Mark Hammer]

@Mark - replace C5A with a 680nF will reduce the amount of bass rolled off.

That would look like this?

Yes.

[timkenyon]

@Mark - thank you.

@PRR - I see that now.  Thank you.

[antonis]

Wire Bias pot like below:



P.S.1
R2 needs to be there, or else, signal is AC grounded via C4..
(it sets stage input impedance..)

P.S.2
Redraw TL082 possitive suplly rail like above..
(and place a 10 - 100 nF ceramic cap as close as physically possible between op-amp pin 8 & GND or bring 47nF diode shunting cap there..)

[timkenyon]

@antonis OK, so I hope I have this right based on the description and image you gave me.

I moved the 47uF cap to just above pin 8 and connected it to ground, I thought it logical then just to connect the other side of the pot to ground.

[antonis]

 

You're asleep at the switch, aren't you..??

Get back 47μF to its original place and bring 47nF there..

P.S.
Keep my last schematic as it is and just add 100nF ceramic cap close to pin 8..

[timkenyon]

I am honestly not having the best day in the world.

So, either bring FILTC1 closer to pin 8, or just add a small cap there.  Gotcha.

Thank you for the patience.

[timkenyon]

OK - so I have a couple more questions even though this design is pretty much done now (and in need of breadboarding, which would require actual boards!)

1)  With OpAmps like this (I have another 2 designs I'm working on) - they should always be powered directly from the +9V rail?

2)  The inverting input to the OpAmp (pin 2) should always be the biased input voltage?

3)  The 100nF capacitor next to pin 8 (good practice for all OpAmps I'm guessing) is for smoothing?

[antonis]

1. Yes..

2. %$#@#&& 
(just realize you've mixed inputs..)
Interchange inverting with non-inverting input..!!

3. Actually, is for HF decoupling..
It provides a direct return path for the high frequency load currents from ground to the rail (which are the sources for the load current). Otherwise the output may be distorted or oscillate



For both configurations, R_IN sets input impedance..
You can see bias resistor is omited for Inverting amp 'cause V_S/2 has nothing to do with signal path..

[timkenyon]

I really appreciate the answers and thank you for taking the time.

[Elektrojänis]

I think you have your opamp inputs mixed. You are feeding your signal to the positive input and feedback is connected to negative input. That's positive feedback. It could be interesting, but it will very likely oscillate and possibly at a very high frequency.

[iainpunk]

I think you have your opamp inputs mixed. You are feeding your signal to the positive input and feedback is connected to negative input. That's positive feedback. It could be interesting, but it will very likely oscillate and possibly at a very high frequency.

ooh yes this!!!

i think it would oscillate around 1k to 9kHz depending on the gain control setting.

you should definitely change those around.

cheers

[Elektrojänis]

i think it would oscillate around 1k to 9kHz depending on the gain control setting.

Would it... In the negative feedback loop that 100pF C5 would limit the top end of the gain bandwidth by increasing the negative feedback for higher frequencies. But in the positive feedback loop it would increase the positive feedback in the same way so it might oscillate even higher. It probably also depends a lot of the opamps open loop bandwidth though...

This makes me want to try how it would react, but I don't have an oscillloscope.

[timkenyon]

@Elektrojänis - you're saying the feedback loop from the diode circuit should go through the +ve (pin 3) input and the signal through -ve (pin 2)?

[antonis]

Some times I feel like a useless member..