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Rat Op Amp Chips

joelster

Silver Supporting Member
Messages
3,400
I know that this has been discussed around here quite a bit but I'm just trying to distill it down a bit.

I know that there are several op amp options for the Rat. I also know that some require external compensation caps and some don't. So...

Are these all of the options available or did I miss some? Additionally, are these the correct comp cap values?

Rat Op Amps:
Model -- Comp Cap
OP07 -- None
5534 -- 22pF?
LM741 -- None
TL071 -- None
TL070 -- 180pF
LM308 -- 30pF

And finally, in a more subjective vein, what's your favorite?

Thanks!
 

cj_wattage

Member
Messages
6,716
LM301
CA3130
CA3140

Those require compensating caps. Values between 33pf and 100pf should work fine. Tweak to taste.

Any "standard" 8-pid DIP single op amp package will work. I find the non-compensating models not to sound as good as the good old compensating variety. YMMV
 

joelster

Silver Supporting Member
Messages
3,400
You want a comp cap on the OP07 also. ~30pf to ~100pf to taste
I'm not so sure that you do.

Please correct me if I'm wrong. Data sheets seem to say only that the OP07 may be used in certain sockets "with or without the removal of external compensation or nulling components," which seems to imply that it doesn't matter if it's there or not. I could hear no difference with or without one on mine. Also, when you remove the cap on chips that do require it they go completely nuts. At least the TL070 I tried did. The OP07 was stable.
 

cj_wattage

Member
Messages
6,716
I'm not so sure that you do.
I don't think it is required. The datasheet says:

The OP07 has very low input offset voltage (75 µV max for OP07E) which is obtained by trimming at the wafer stage. These low offset voltages generally eliminate any need for external nulling.

For amp that require it, the data sheet specifically says "compensation" on the diagram for pins 1 and 8. The OP07 has "trim" for those pins.
 

luckett

Senior Member
Messages
407
It's not required IIRC, but you do want a comp cap on an OP07 if you want it to sound like an LM308 rat.

The OP07 has a higher slew rate than the LM308 and if you don't put a comp cap on it the OP07 won't be as fuzzy and will have better clarity than the LM308. The value of the comp cap changes the slew rate so it needs to be higher than the LM308's 30pf to bring the slew rate down to the same as an LM308.

If you can't hear it, remove the diodes so the only thing clipping is the opamp and the difference will be obvious when you change comp cap values.
 

joelster

Silver Supporting Member
Messages
3,400
OK. I think I've wasted enough time this morning. Here's what I've got so far. Thanks for the input, guys!

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Last edited:

cj_wattage

Member
Messages
6,716
If you can't hear it, remove the diodes so the only thing clipping is the opamp and the difference will be obvious when you change comp cap values.
If you can't hear the difference with the standard circuit configuration, then it really isn't all that important (at least to me).

But to each his own.


I'm still not convinced that the pins in question are actually doing the same thing on both chips. Have a looksee:


LM308



OP07
 

luckett

Senior Member
Messages
407
If you can't hear the difference with the standard circuit configuration, then it really isn't all that important (at least to me).
I can easily hear the difference with the clipping diodes in, but it may not be readily apparent if you haven't had the opportunity to play with the circuit on a breadboard to A/B the changes quickly. It's just easier to hear the difference when you isolate the clipping to just the opamp. The cap changes increase the ragged, fuzzy distortion from the opamp that has a different character than the distortion from the diodes. The opamp distortion is easier to hear as you increase the diode clipping threshold from germ to silicon to LED.

I'm no EE so you'll have to explain to me the specific technical difference in what the caps are doing to those opamps. The cap might not be changing the slew rate in the OP07, but it does change the amount of distortion coming from the opamp in a similar way to cap changes in an LM308.
 

AXXA

Silver Supporting Member
Messages
7,109
How does the LM301 sound compared to the LM308? I'm especially interested in the difference between them, with clipping diodes out of the equation.
 

joelster

Silver Supporting Member
Messages
3,400
I found mention of it on some other forum and that guy described it as a good blues chip for the Rat. Less gain but maybe smoother iirc. He compared it to a Zen Drive.
 

fieldsroyal

Member
Messages
2,063
How does the LM301 sound compared to the LM308? I'm especially interested in the difference between them, with clipping diodes out of the equation.
I had a vintage small box RAT with the 301 chip and it sounded fantastic - a little mellower than the 308 (I also had a vintage big box reissue) but honestly no discernible difference at volume.
 

cj_wattage

Member
Messages
6,716
I found mention of it on some other forum and that guy described it as a good blues chip for the Rat. Less gain but maybe smoother iirc. He compared it to a Zen Drive.
I'm not sure I'd say it sounded like a Zen, but the 301 is definitely less "aggressive" than the 308 (if you leave all the other parts on the PCB the same). Same for CA3140. But the CA3130 is a lot more gainy in the mids and highs.
 

AXXA

Silver Supporting Member
Messages
7,109
Thanks for the replies. I actually have a 'You Dirty Rat' modded by CJ, and it has a LM301. I find I prefer it with no clipping diodes, which makes it less saturated and more dynamic IMO. It cleans up great without the diodes too. I was wondering if switching the chip would really change the sound much.
 

sarah1

Member
Messages
1
Hi everyone , actually i have too many diff to deal with the OP-07 ,i'm master student in microandnanoelectronics and my thesis is about it , so if you have any technical document exempt datasheet , i will so intereseted ..
 

Kanalu72

Member
Messages
184
I can easily hear the difference with the clipping diodes in, but it may not be readily apparent if you haven't had the opportunity to play with the circuit on a breadboard to A/B the changes quickly. It's just easier to hear the difference when you isolate the clipping to just the opamp. The cap changes increase the ragged, fuzzy distortion from the opamp that has a different character than the distortion from the diodes. The opamp distortion is easier to hear as you increase the diode clipping threshold from germ to silicon to LED.

I'm no EE so you'll have to explain to me the specific technical difference in what the caps are doing to those opamps. The cap might not be changing the slew rate in the OP07, but it does change the amount of distortion coming from the opamp in a similar way to cap changes in an LM308.
On the math side a larger capacitor will not only make the pedal a little more fuzzy, it’ll also warm up the tone by shelving a little more high end. That’s why the original rats sounded warmer and more aggressive than the op07 models. It shouldn’t be that difficult to compensate for the difference.
 






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