Converting Tubescreamer opamp to inverting config

[AmpliFuzz]

Is there a quick way to convert the TS clipping circuit to an inverting configuration while keeping everything else the same (diodes in the feedback loop, etc)? I still like a tweaked TS, but I'd like to hear it without the forced clean blend.

[samhay]

Define quick.

With reference to RG's 'The Technology of the Tube Screamer':
http://www.geofex.com/article_folders/tstech/tsxtech.htm

And specifically the clipping section:


Lift the ground side of the 4k7 resistor.
Move the 'from input buffer' point to this (now floating) end of the 4k7 resistor.
You can then remove the 1/50 cap and Ra if you like.

If you want to add a clean blend option back in, while maintaining the inverting configuration, add a 4k7 resistor in series with the clipping diodes.

[thermionix]

In a factory TS (or accurate clone) the "ground" end of the 4k7 is connected to Vb, but that's beside the point.  Virtual ground, whatever.  But if you "move" the "from input buffer" to that end of the 4k7, how will signal get in?  Do you mean "also connect" instead of "move"?  Sorry, I'm just a bit confused.

[slacker]

Yeah he means connect it, so instead of the signal from the buffer going through the 1uF cap into the non-inverting input you disconnect the 4k7 resistor from ground/vref and connect the signal from the buffer to there instead.

Here's an idea I came up with that lets you use a pot to sweep between inverting and non-inverting http://www.diystompboxes.com/smfforum/index.php?topic=72404.msg588038#msg588038 might be of interest here.

[thermionix]

Oh, so move the Ra side of the 1uF.  Makes sense now.  Oh, no, NOW I get it.  Derp.  Please ignore my posts here.  Carry on.  As you were.  Good day.

[AmpliFuzz]

Thanks all.

If you want to add a clean blend option back in, while maintaining the inverting configuration, add a 4k7 resistor in series with the clipping diodes.

err ... connecting where? the + input?

[Transmogrifox]

Thanks all.

If you want to add a clean blend option back in, while maintaining the inverting configuration, add a 4k7 resistor in series with the clipping diodes.

err ... connecting where? the + input?

I personally would lift the diodes on the op amp output side and put the 4k7 (or a 5k to 10k pot) between the floating ends of the diodes and the opamp output (pin 1 in RG's schematic posted above).  If using a fixed resistor install an SPST switch to short it out when you want to switch to the hard-clipping mode.

The reason is the "-" input side is the noise injection end and the diodes will help block noise picked up on wires (especially if using a pot or connecting a switch to switch a resistor in and out).

The 4k7 series with diodes forms a unity gain feedback with the 4k7 on the inverting input.  The end effect is equivalent to the clipping behavior when using the noninverting input; that is, the diodes switch the feedback loop between high gain and unity gain when above the diode turn-on voltage.

If you want to add a clean blend option back in, while maintaining the inverting configuration, add a 4k7 resistor in series with the clipping diodes.

If you weren't planning to hack in a switch or a pot, then you would just undo the modifications to go back to the stock circuit.   The added resistor would only make your board more hackish just to get back to the stock sound . 

Maybe I'm missing some kind of a subtle interaction with feedback from the amp to the guitar but I don't think a phase inversion does anything for you nor against you unless you're resting the guitar's headstock on the amp.  You would probably want a separate pedal to perform phase reversal function if that's what you are after.

My understanding of the OP's objective is for getting the hard clipping -- a phase inversion is just a benign side-effect.

[AmpliFuzz]

Maybe I'm missing some kind of a subtle interaction with feedback from the amp to the guitar but I don't think a phase inversion does anything for you nor against you unless you're resting the guitar's headstock on the amp.  You would probably want a separate pedal to perform phase reversal function if that's what you are after.

My understanding of the OP's objective is for getting the hard clipping -- a phase inversion is just a benign side-effect.

Phase has nothing to do with it, except that it might make the pedal work better in certain paralooper style setups.

It's all about the clean blend. It sounds great for some stuff and with some guitars, horrible with others. I noticed it as some point without knowing the TS circuit was supposed to do that, and I thought something was broken. Once you hear it you can't un-hear it, and it's bugging me

[samhay]

>Phase has nothing to do.

If you make this modification, you will invert the phase of the signal coming out of the circuit (relative to stock).
Unless you run parallel effects, this is probably not going to be a problem.

[AmpliFuzz]

The ability to invert the phase with a toggle is actually pretty cool, the TS circuit sounds great on bass with full-range (done externally with a blend pedal, not the bandpassed internal clean of the normal TS) clean blended in.