Thesis 96/Overdrive 2 and Compression

[marnold]

In using my Thesis 96 pedal (aka a DVM Overdrive 2) I've noticed that the pedal adds a good amount of compression. I know that this is a natural side-effect of clipping of all kinds and especially in the high-gain territory I know and love so well. I read the information on the Overdrive 2 on BYOC's site and came across this:

And we've come up with a new mod that increases the overall output and distortion of the tube screamer circuit and adds some compression and sag like you'd get from a real tube rectifier making the pedal even more tube like.

What exactly did they change to add the compression and sag? This pedal does seem to compress more than my Behringer TO-800 did. It was particularly noticeable when recording in Audacity because I could see it as well as hear it.

Mind you, I like the effect so I'm not complaining. I just wondered how they accomplished that sans tubes.

 

[duhvoodooman]

That would be the "Louder" trimpot, which controls the output of the second stage of the dual opamp. I can't give you all the theoretical particulars, but apparently this trimpot, which is located in the feedback loop of the second stage of the IC, gives rise to the compression/sag behavior that the product page mentions. I assume it has to do with the added gain giving rise to behavior that mimics, at least to some extent, the saturation behavior of a tube amplification system. The fact that you can hear it with your ears and see it in the Audacity GUI says that it works as advertised.

When the OD2 first came out, a lot of people were running into weird tonal behavior and nasty sounding artifacts due to running that trimpot up too high ('cuz more is always better, right? Uh, not necessarily....). A portion of this behavior involves an interaction with the tone control, though I have no idea how that works. Candidly, that trimpot is W-A-A-A-Y bigger than it needs to be at 50Kohms. The equivalent resistance in the TS808 circuit is provided by a 1K fixed resistor! I now substitute a 10K trimpot in all my OD2 builds, including the ZYS, and I generally dial it in around 2K.

The other thing that helps avoid this problem is to use a smaller rating in the tone pot, which is 25K in the kit. Keith now provides a 10K pot as part of the MOSFET conversion kit, and many of us prefer to go even lower, since the 25K pot has a large "deadband" in the middle of its range. (Keith uses the 25K in the stock kit because it's the closest commercially available size to the 20K pot used in the original TS808's; it's a matter of "vintage correctness".) I was using 10K's at first, and reduced that to 5K, and again to 2K! I find this rating gives the most even pot sweep sensitivity for the tone control. Now I need to go back and see how the "Louder" trimpot reacts to that change, which I haven't done yet....

You may want to experiment some with that trimpot in your Thesis 96, Matt. Mark the current position and then try dialing it up and down (though there isn't much room down on your build, which I'm pretty sure is one of the original 50K pots) and see what suits your fancy. You can even measure the resistance across that trimpot with a multimeter--just touch the probes to the two metal posts that protrude through the pot body along the top side, closest to the input jack.

 

[sunvalleylaw]

Hmm, I just googled a bit to remind myself what your were referencing by naming the pedal Thesis 96. Ok, I remember my Western Civ now. I had forgotten the name of the documents, remembering only Martin Luther's act and the idea of the Reformation. As I recall, they taught what had occurred fairly even and openly, given that it was a Catholic school. Jesuits and all. I guess given that there were 95 Theses, that the 96th compresses things a bit might be a good thing!