Determining pedal input impedence

[thelonious]

A while back I built a simple optical compressor pedal I found on freestompboxes. The only difference between mine and the attached schem is that I used a 6.8n input cap instead of the 15n combination. I like it as a compressor, but it seems to roll off high frequencies quite a lot. Is it possible that it's loading my pickups? I've been researching how to determine input impedance in solid state stuff. It seems pretty straightforward in the case of a pedal that uses a JFET (as in http://www.muzique.com/lab/imp.htm), but determining it with an opamp seems more complex. In this schematic, I don't see where there's a path to ground after the input cap that would determine Zin.

Any kind-hearted folks care to enlighten me? 

Second and third pics are of my build.

[FYL]

Easy: the op-amp is used in inverted gain mode (- is the input), Rin = 220K, the resistance in series making a virtual ground. Rin := Zin as other factors are negligible.

[thelonious]

Many thanks! You're right - that's a lot easier than I thought. I hadn't thought about the virtual ground.

From one of the articles I just read:

"The higher the inductance (the "hotter" the pickups), the lower the frequency at which the loss of highs begins. Likewise, the lower the load resistance, the sooner the loss begins. This is why there may not be a noticeable loss with regular pickups and a 100K resistive load, but with the same load and hot pickups the sound is noticeably dulled." (http://www.howardmickdavis.com/LoadingandCables.htm)

I think it's the combination of the pickups in my ASAT Special, the Zin of this pedal, and the cable back to my amp that is giving me a problem.

How much do you think I could increase the value of that 220k resistor before I start messing up the circuit---or would it be best to buffer the signal before it hits the pedal? I'm thinking of building http://www.muzique.com/schem/eagle.htm as a buffer. I suppose it might be possible even to build it into the compressor pedal so that it is only in my signal chain when the comp is on. Thoughts?

Thanks again for your help.

[thelonious]

I ended up building an AMZ Simple JFET Buffer into the compressor enclosure, since at this point I only need a buffer when the compressor is on. In the future I could always make the buffer separately switchable if needed. I put them together as shown in the attached schematic. Is it correct to say that the 1M resistor at the input of the compressor is not needed now? The three things I'll probably end up changing are to 1) remove that 1M resistor, 2) replace the 100K resistor with a pot so I can control how hard the buffer hits the compressor, and 3) possibly play around with a different value for the 6.8n input cap now that the impedance mismatch is fixed. I might go to 10n or 15n (the original value).

Any thoughts?

Thanks,

T

[Fresh_Start]

thelonius - thanks very much for that link to the "AMZ Pedal Impedance" essay!  I've been struggling to understand how my fuzz box interacts with my guitar's volume and tone controls.  I think I'm beginning to "get" it now!''

Cheers,

Chip

[thelonious]

Sure thing! There's some great info on that site.