Dual opamp version of LM308

[likethehotels]

Hi y'all.

I'm considering building a modified version of a rat, with an active bass control, and am wondering if there is a dual opamp version of the LM308? I'm curious to know how the poor slew rate would affect an active tone control.

Also, in this dual opamp package, is each side compensated separately, or do you have to compromise and share the same compensation between sides?

If the dual 308 approach doesn't end up working, what is another externally compensated dual opamp you'd suggest for the application? I've seen a few posts regarding TL072, but am curious to hear what other members have to say about it.

If all ends up well, I'll have a nifty pedal to show you all when I'm through.

[Fancy Lime]

Hi!

I asked myself the same question for exactly the same reason a while back. I could not find any dual opamps with external compensation from my usual sources. They may exist but if so, they are not easy to get, apparently. So I ended up solving the problem differently:

https://www.diystompboxes.com/smfforum/index.php?topic=121671.msg1144561

Hope that helps,
Andy

[Rob Strand]

if there is a dual opamp version of the LM308? I'm curious to know how the poor slew rate would affect an active tone control.

I'm not aware of a dual version.   The slew-rate is only an issue (roughly) if you start clipping the tone control opamp.   The main thing is an LM308 is probably wasted as the tone control opamp.

The NE5534 (single) vs NE5532 (dual) is a good example of single vs dual.  They are basically the same design but the NE5532 doesn't have the compensation or offset pins.   The compensation is fixed IIRC something like an NE5534 with a 22pf compensation cap.

[anotherjim]

The only dual amp that's like a single that I know is LM747, but that's a dual 741 so only has offset null pins, the comp cap is internal as in a single 741. Still, the 741 slew rate is pretty lame too, so you can always try it (it works well enough in the MXR Dist+) -  besides the number 747 is cool -  it will be a Jumbo Rat!

[amz-fx]

The LM1458 dual op amp will get you in the ball park of what you are looking to achieve. No comp caps but it has specs similar to what you need.

regards, Jack

[likethehotels]

Hi!

I asked myself the same question for exactly the same reason a while back. I could not find any dual opamps with external compensation from my usual sources. They may exist but if so, they are not easy to get, apparently. So I ended up solving the problem differently:

https://www.diystompboxes.com/smfforum/index.php?topic=121671.msg1144561

Hope that helps,
Andy

It's good to know I'm not the only one who has thought of this. I'll give it a lookover and see if I can't implement something similar.

Thanks so much for all the responses, guys. I'm trying to build a preset-based pedal, similar to the TC Nova drive, but utilizing a SHO and rat with some other modifications. Glad to have ya'll. Will update as progress goes along, likely in a new post.

-Ian

[PRR]

> a dual opamp version of the LM308?

Not that anybody knows. Use two '308.

In hindsight this is fairly simple. The '308 was high-performance and eXpen$ive. You usually only needed one, at your system front-end. And if you really needed '308 input current, you wanted to keep those inputs AWAY from all else; so singles were natural. The "best" '308s were metal-can (before epoxy was trusted to be good). The metal can packs mostly did not have enough pins for Dual with comp and offset. By the time systems got complex enough to need multiple high-performance chips, JFET-in and Bi-FET opamps could beat '308 for input current (except in boiling heat) and were cheaper and faster (good for most, bad for you).

But look what I found! Ken Shirriff sawed the top off an LM308, took pictures, back-tracked, and found that this specific mask could be finished as either '308 or LM11!
http://www.righto.com/2016/12/inside-lm108-op-amp-superbeta.html
And meditates: "The LM108 is used in distortion pedals; people search out this op amp to get its specific sound quality when overdriven. Since different manufacturers have different internal designs for the LM108, this raises the question of whether people may unexpectedly end up with different op amps that produce different sound effects."

[yanng45]

But look what I found! Ken Shirriff sawed the top off an LM308, took pictures, back-tracked, and found that this specific mask could be finished as either '308 or LM11!
http://www.righto.com/2016/12/inside-lm108-op-amp-superbeta.html
And meditates: "The LM108 is used in distortion pedals; people search out this op amp to get its specific sound quality when overdriven. Since different manufacturers have different internal designs for the LM108, this raises the question of whether people may unexpectedly end up with different op amps that produce different sound effects."

Did he dare to mention that guitarists have a tendency to listen with their eyes ? Heretic! 

That was a very interesting read, thanks for sharing.

[anotherjim]

I'm not seeing an explanation for why the chip might be so good overdriven other than the well-known assumption that its the slew rate.
From the article...

The input clamp diodes appear to be transistor BE junctions Q4 and Q3. These then go on to have some control over the input Q1 and Q2 biasing. That part is all a bit too convoluted for my brain!
Is this showing that the chip has a limiter that will attenuate its inputs if the clamps conduct?