Dual Opamp Clipping Pedals - why transistor buffers, why cap/resistor between st

[Kindly Killer]

I am just getting some momentum going with my electronics studies. I am wanting to design my own simple drive type pedals and I am looking at lots of schematics. I am noticing that LOTS of dual opamp clippers have a couple of elements that are presently puzzling to me:

• Why do so many of them use transistors for input and output buffers? It seems like it would be win from every angle to use a quad opamp chip instead. What advantage is there to discrete transistors as buffers?
• Why is there always a resistor and cap between the gain stages? I assume the resistor knocks down the gain but what is the cap filtering?

[merlinb]

What advantage is there to discrete transistors as buffers? [/li][/list]

They can take up a bit less borad space. They also distort, which may please some people. Apart from that through, and opamp would indeed be a better choice.

• Why is there always a resistor and cap between the gain stages? I assume the resistor knocks down the gain but what is the cap filtering?

The cap is probably being used for some treble or bass-cut, or maybe to keep DC off some additional pots (?). There's nothing to stop you DC coupling the stages together though.

[Hides-His-Eyes]

a) Because the tubescreamer did it this way, partly

b) To filter the signal, yeah. Clipping introduces higher harmonics (for fourier reasons) whilst attenuating high frequency components of the input; generally we want to tone down those harmonics.

[Mark Hammer]

Depending on whether the op-amp is configured as an inverting or noninverting stage, the RC pair may be there to set the stage gain (in inverting stages it is set by feedback resistance over input resistance).  The cap is needed to block any stray DC.

While it is possible for successive stages that use a single-ended supply to "share" a bias voltage, in many instances designers simply elect to bias one op-amp, "reset" via a DC blocking cap in the output, and the re-bias the next op-amp.

[Projectile]

I am just getting some momentum going with my electronics studies. I am wanting to design my own simple drive type pedals and I am looking at lots of schematics. I am noticing that LOTS of dual opamp clippers have a couple of elements that are presently puzzling to me:

• Why do so many of them use transistors for input and output buffers? It seems like it would be win from every angle to use a quad opamp chip instead. What advantage is there to discrete transistors as buffers?
• Why is there always a resistor and cap between the gain stages? I assume the resistor knocks down the gain but what is the cap filtering?

The original tubescreamer had transistor input and output buffers because those they were part of the electronic switching circuit on the pedal. They are completely unnecessary when using true bypass, but most boutique builders choose to leave them on either for authenticity, or because they are hacks that don't know any better. (Actually the output buffer does kind of serve a purpose, but the input buffer is definitely totally unnecessary.) Since most dual opamp clipper pedals are directly derived from modded tubescreamer circuits,  there ends up being hundreds of pedals out there with all of these extra buffers that exist for absolutely no reason. It's kind of silly when you think about it.

The resistor and cap between stages are there because it's just a standard method of coupling gain stages. It's just a design philosophy thing as far as I know. If there is a pot in between the gain stages it's considered better to block DC on either side of it and reference it to ground instead of VR. This is to prevent noise that can sometimes happen for having DC on a pot, but I've never had a problem with it personally. You can often leave these caps and resistors out without a problem. 

[R.G.]

The cap between stages does two things:
1. It blocks some bass. This may or may not be important in the particular circuit or why it's used, but it is used that way in some designs. Many designs simply use a capacitor so big that no guitar bass is blocked.
2. It prevents the buildup of DC offset errors. No opamp is perfect, and they all have some amount of DC offset. For good opamps, this is millivolts. For all opamps, it can be down to tens of microvolts or up to tens of millivolts. This offset is multiplied by the DC gain of the opamp, inside any blocking caps. If the first opamp has an offset of 10mV, and has a DC gain of 100, its output is offset 1V from where the reference voltage puts it. If it's followed by other opamps, they both amplify that offset and add their own offsets to it. Making the gain stages amplify AC only, and forcing a DC gain of one ensures that the offset never builds up.

These are part of what I conceive of as "design hygene", making a circuit play nice with others.

[johngreene]

I think he may be referring to the series resistor and cap to ground ala tubescreamer. It is a simple RC lowpass filter that filters off the harsh highs caused by the clipping done in the previous stage.