why the reluctance to use an opamp buffer on a rat?

[puretube]

So you got a similar problematic high-value bias-resistor (R4 = 2M2) if the "main" opamp IC1 = NE5532 ...

[antonis]

Antonis, was also going to change that 1M after reading what you said about the 5532 opamp. Maybe make 470k? Or am I understanding totally wrong?

Although anything bigger than 100k might cause misbias, 470k should be OK..
As puretube pointed above, same should stand for R4..

P.S.
If you wish to maintain original HPF corner frequency, you have to raise respective cap value by the same percent of bias resistor lowering..
e.g. 0.022μF -> 100nF for 2M2 -> 470k..

[Fancy Lime]

Another issue is the voltage divider formed by the tone and volume pots. As is is now, lowering the tone will lower the volume. One option to solve this, is to change the tone control from the standard Rat wiring to Mark Hammer's Stupidly Wonderful Tone Control (or one of Jack's variations thereof: http://www.muzique.com/lab/swtc.htm). For best results make sure that the Tone pot is no more then 1/10th the value of the volume pot, lest the maximum output volume will be quite low.

That being said, why not just put the volume pot behind the buffer as in the original Rat? Sure, the output impedance is relatively high and depends on the volume setting in this case but if the pedal goes into anything with a decently high input impedance, that is generally not a problem.

Andy

Edit: also, if you don't want to think about bias currents too much, you could just use a TL072 or some othe JFET input opamp. Works perfectly well in this role in my experience.

[antonis]

if you don't want to think about bias currents too much..

Initially, he didn't..!!
(you can put the blame on me..)

[m_charles]

Another issue is the voltage divider formed by the tone and volume pots. As is is now, lowering the tone will lower the volume. One option to solve this, is to change the tone control from the standard Rat wiring to Mark Hammer's Stupidly Wonderful Tone Control (or one of Jack's variations thereof: http://www.muzique.com/lab/swtc.htm). For best results make sure that the Tone pot is no more then 1/10th the value of the volume pot, lest the maximum output volume will be quite low.

That being said, why not just put the volume pot behind the buffer as in the original Rat? Sure, the output impedance is relatively high and depends on the volume setting in this case but if the pedal goes into anything with a decently high input impedance, that is generally not a problem.

Andy

Edit: also, if you don't want to think about bias currents too much, you could just use a TL072 or some othe JFET input opamp. Works perfectly well in this role in my experience.

thx for the reply. I probably will end up putting it before the volume. At this point the post has kind of turned into an opportunity for me to learn some things, so I've been enjoying the posts/replies. At the end of the day, doing the buffer with the op-amp isn't even that important. As others have said, a regular old BJT transistor could do the job just fine, but my simple question opened up a lot of things I never knew, which is great. For instance, I hadn't thought about the issue with the tone and volume becoming a voltage divider until you pointed it out! haha