Controlling two gain stages with a single pot ADDED distortion example

[bioroids]

Hi!

I had this idea and works pretty well.
You can control the gain of two consecutive opamp stages with a single pot if you hook it's wiper to the output of the first stage and use it to vary simultaneously the feedback resistance of the first stage and the input resistance of the second. The second stage should be an inverting one.

I posted more details and a schem at www.dedalofx.com/bioroids click on "Double Control"
Also posted an interesting sound sample of the 16-step LFO I had posted a few months ago.

Hope it's usefull to someone! I don't think I've seen it before, but this is simple enough to have been used at some point, I'm sure.

Luck

Miguel

Edit: title

[DDD]

It seems I've seen something like that inside M@rhall Gu@nor. Good idea...

[A.S.P.]

http://www.diystompboxes.com/smfforum/index.php?topic=47736.0

[bioroids]

doesn`t work too well with CMOS inverters, because of their high output-Z (1st stage)
feeding into a virtual ground (2nd stage) being loaded too much to keep the gain of 1st stage high enough...
(yes, I tried, a while ago).

I have not tried it, but I was planning too. Should I desist or it can be made to work to some extent?

Luck

Miguel

[A.S.P.]

http://www.diystompboxes.com/smfforum/index.php?topic=47736.0

[Mark Hammer]

If the purpose is to achieve large composite gains, I'm not sure why one needs to control two stages at once.  After all, the gain is multiplicative.  If stage 1 goes from a x1 to an x10 gain, and stage 2 has a fixed gain of x10, then the two in series go from x10 to x100 using a single pot to control the gain of stage 1. 

If the first stage is inverting, you can get gains less than 1.  So, if stage 1 goes from a gain of x0.5 to a gain of x10, and stage 2 has a fixed gain of 20, then varying stage 1 gets you a gain range of x10 to x200, which is pretty wide.

I'm not saying the idea that started the thread is not clever and has no use (e.g., it may be a VERY useful way to determine the relative gain of two stages that have different sonic characteristics or contributions, once tone quality or filtering is factored in).  Rather, if the primary goal is just to get more gain from two cascaded sections, it doesn't have to be more complicated than varying the gain of one of them.

[Doug_H]

Check the marshall bluesbreaker for an example of this. It may be for getting more headroom at lower gain levels IIRC.

Doug

[bioroids]

In fact the idea I'm working on is a simple distortion box that controls the gain (and clipping) at the same time that controls the filtering. So you can get flat eq with low gain settings, and a good low pass with the high gain settings. I'll post this schem tomorrow if I get it done.

I thought that the single pot idea was usefull in a more general way. I'm sure it can have other applications besides the clipping-filter stuff.

I'll have to check those Marshall schems, are they linked in the schematics section?

Luck!

Miguel

[bioroids]

Hi!

If you want to check it now, I added a distortion/filter example for the use of this trick. I think it can be neat for simple Drive and Level boxes ala MXR Distortion +

http://www.dedalofx.com/bioroids click on Double Control v2

Luck

Miguel

PS: I checked the Marshal schems, pretty cool (I haven't seen them before). But I don't understand exactly what is the purpose of using this trick on those circuits. Can it be for headroom control as someone mentioned?

[bioroids]

Check this out:

http://www.diystompboxes.com/pedals/nobelmod.bmp

Someone had already used this trick lol  not exactly for the same purpose though. I wonder what part of that circuit are the mods.

Regards

Miguel