What would be the quietest opamp to use in a RAT?

jrc4558 · February 14, 2005, 12:15:13 AM

subj.

Paul Marossy · February 14, 2005, 12:29:22 AM

From what I understand, there's pretty much only one opamp to use in that circuit - the LM308. Compare the LM308 data sheet to a standard opamp, and you'll see some differences. The Rat capitalizes on the unique features of the LM308. Using a different opamp may change how the circuit sounds and it may not sound like a "true Rat"...

jrc4558 · February 14, 2005, 10:13:37 AM

Yeah, that much I know. However, there should be uncompensated op-amps ( and that's what I'm looking for, sorry for late qualification

) with j-fet inputs and low noise figures. Then I just would use a capacitor btw pins 1 and 8 to hammer the slew-rate down.

Paul Marossy · February 14, 2005, 12:08:37 PM

I am not aware of any other uncompensated opamps other than the LM308 that are currently in production, but there may be others. Anyone else?

StephenGiles · February 14, 2005, 01:48:02 PM

TLO70! I have Rats using 709, 748 and TLO72 and they all sound exactly the same as my 308 Rat!
Stephen

Paul Marossy · February 14, 2005, 02:21:10 PM

QuoteTLO70! I have Rats using 709, 748 and TLO72 and they all sound exactly the same as my 308 Rat!

Well, that's good to know! :wink:

aaronkessman · February 14, 2005, 02:48:22 PM

this is an ignorant, but not stupid question

what does "uncompensated" mean with regards to the LM308? Slew Rate???

MartyMart · February 14, 2005, 02:59:53 PM

I just modded my "pro-Co" Rat and it had a TL070 inside !!

Marty.

jrc4558 · February 14, 2005, 03:30:12 PM

Thanx, guys!

Paul Marossy · February 14, 2005, 03:34:25 PM

aaronkessman-

Compensation is:

"Modification of an amplifier's amplitude frequency response; an OpAmp that is said to be "internally compensated," such as the 741, will typically have a capacitor(30 pF for the 741) nested in its second stage(high gain stage) for greater stability at the expense of decreased bandwidth. An OpAmp that is said to be "externally compensated," such as the 301, will have extra pins, labeled respectively, to adjust the value of this capacitor for a desired stability/bandwidth ratio."

From this page: http://www.techsystemsembedded.com/OpAmp.html

The RAT schematic at tonepad.com shows a 30pF cap, so this would essentially be like the 741 or similar opamps. The *possible* drawback with using something other than an LM308 is that you have a fixed cap value, so there isn't really room to experiment with frequency response by changing the cap. I don't know that it would make that much of a difference in the overall sound, but it may make some small difference.

aaronkessman · February 14, 2005, 03:50:29 PM

thanks paul. That's what I thought it was. but I was also under the impression that the LM308's gain for higher frequencies ie, higher order harmonics, was higher than your regular opamp, and this is what gave the circuit its characteristic squeal and "attitude" that we all know and love.

was I wrong?

Benny · February 14, 2005, 03:52:05 PM

I don't mean to hijack the thread but... maybe this is one of those elusive uses for a variable capacitor? This one goes from 10pf-120pf:

http://www.digikey.com/scripts/DkSearch/dksus.dll?Detail?Ref=190628&Row=122182&Site=US

I've never built a RAT so I don't know how much this really affects the sound, but it might be another tunable parameter.

A RAT is on my list of things to build... I had one when I first started playing but I never really liked it. Since then I've heard many good tones that were supposedly made with a RAT, so I think it is worth revisiting!

Benny

Paul Marossy · February 14, 2005, 03:54:24 PM

QuoteI was also under the impression that the LM308's gain for higher frequencies ie, higher order harmonics, was higher than your regular opamp, and this is what gave the circuit its characteristic squeal and "attitude" that we all know and love.

That's a good question. But since there is no "Technology of the Rat" article, I can only speculate. :oops:

Quotemaybe this is one of those elusive uses for a variable capacitor?

That would be an interesting experiment. Pretty cool idea in fact.

aaronkessman · February 15, 2005, 07:56:52 AM

by gum, i think he's got something guv.

amazing. dude, try it out. I cant - my rat was the first pedal i ever built and its a rats nest. i dont want to mess with *anything* in it. one day i'll rebuild it...

j0shua · February 15, 2005, 10:03:07 AM

Don't Forget TL061 but need to remove the C4 ( 30Pf cap ) .... Don't Needed

Gilles C · February 15, 2005, 01:21:26 PM

Quote from: Paul MarossyThat's a good question. But since there is no "Technology of the Rat" article, I can only speculate. :oops:

There's one...
http://www.muzique.com/ebook.htm

But you have to pay for it. But it's a cheap price to pay for that kind of info... :roll:

I learned from it... but I still need to use the info :oops:

Gilles

Paul Marossy · February 15, 2005, 01:33:11 PM

Well, Gilles, I learn somethin' new every day! :wink:

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What would be the quietest opamp to use in a RAT?