Trying to understand the Paralooper

[therecordingart]

http://moosapotamus.net/THINGS/paraloop/paralooperSCH.gif

I'm trying to figure this thing out...can you guys assist?

-1M input resistor provides a path to ground for anti-pop
-U1-A, U2-A, and U2-B are all voltage followers (buffers).
-U1-B appears to be a non-inverting amp with a gain of 11, but shouldn't R11 go to ground?
-C1, C4, C6, C7, C8, C9, C10 are DC blocking caps. Are C7 and C8 necessary since C9 keeps DC off U1-B?
-What is C5 doing? Passes hi frequencies to ground? RF?
-Why is R3 necessary?

[PRR]

> -1M input resistor provides a path to ground for anti-pop
> -U1-A, U2-A, and U2-B are all voltage followers (buffers).
> -U1-B appears to be a non-inverting amp with a gain of 11, but shouldn't R11 go to ground?
> -C1, C4, C6, C7, C8, C9, C10 are DC blocking caps. Are C7 and C8 necessary since C9 keeps DC off U1-B?
> -What is C5 doing? Passes hi frequencies to ground? RF?
> -Why is R3 necessary?

Lot of questions.

Answers no particular order:

- yes

- yes

- If R11 went to ground, but R10 went to Bias=4.5V, then U1B would take 4.5V as its DC input, multiply by (yes) 11, and swing its output to +49.5V. Or try.... actually it would be stuck as close to 9V as it could get.

Bias is _AC Ground_.

C7 C8 C9 are somewhat redundant. If opamp DC errors were dead-zero, they could be omitted, also R10. However there will be some mV of opamp DC error, which could cause scratch or thump. It is 50 cents of parts to avoid a dollar's worth of trouble.

A chip-pin can usually take voltage between rails and current to 10mA. Accidents happen. One popular one is loudspeaker level, ~~25V. This is beYOND the chip rails. Current flow could be infinite. With 10K there, 25V/10K= 2.5mA, chip won't die. Some designers favor R4 on the other side of R3; however the difference in this case is 1% and inaudible. R13 has similar function.

[therecordingart]

Thank you very much for taking the time to answer my questions. I'm trying to make it a point to stop "painting by numbers" when I build something. I'm forcing myself to understand the circuit before I even think about picking up a soldering iron. This seems like a pretty useful piece  of gear.

I'm sure there are tons of threads regarding suitable TL072 replacements....not that there is anything wrong with a TL072 in this application. I could be wrong, but I'm fairly certain a "better" opamp probably wouldn't have a huge noticeable benefit in a circuit of this variety.

[earthtonesaudio]

The specs needed for the op-amps in this circuit include:

9V operation
Unity-gain compensated
Audio bandwidth
High input resistance

The vast majority of "general purpose" op-amps have these specs, and as long as a replacement op-amp has these specs or better it will work.  Lower noise vs. power consumption is a typical tradeoff.  Rail-rail ICs are often more expensive, and less tolerant of fault conditions (not as robust) but may offer better headroom at 9V.

I would further address the wrongly-polarized caps, either by converting them to non-polar or adding correct DC bias like this:

[therecordingart]

...and thank you, sir! I did catch the cap issue. Last question...is there a benefit to using two dual opamps vs one quad?

[earthtonesaudio]

Often two duals makes PCB layout easier than a quad.  Electronically equivalent.

[therecordingart]

Often two duals makes PCB layout easier than a quad.  Electronically equivalent.

That's what I though, but wasn't sure if there was a benefit beyond that. The more I look at this schematic I see a bunch of different things I can do. I'll try to post my implementation of this general idea in the next few days.

[therecordingart]

A different implementation of this design is located here:


http://www.diystompboxes.com/smfforum/index.php?topic=95041.0