Switching Between LM308 and OP07 in a Rat

[mremic01]

In an attempt to see for myself whether the LM308 sounds any different from the OP07, I built a Rat with various mods, including both op-amps connected to a 4PDT. Pins 2, 3, 6, and 8 are on the switch. I have pin 1 of both ICs on the same node, 4 is ground, 5 doesn't connect anywhere, and 7 is 9v. Now I've found that the OP07 works just fine, but when I switch to the LM308, it just kills my signal. If I pull the OP07, the LM308 works fine. If I swap the LM308 out and put a second OP07 in its place, both OP07s work fine. I actually tried an OP07CP and an OP07DP and they sounded identical. The one thing that seems like it could be an issue is pin 1. I would think that since 1 and 8 both connect to the 30pf cap, I'd only need to switch one side. Is having both pin 1s what is causing this?

I'm also curious if there's a better way to switch between ICs. Would having both connected to everything but ground, and then have their ground pins on a switch effectively turn one IC on and the other off?

[Beo]

I don't really see why it wouldn't work, but why not just socket, and do sound recordings. By the way, I'm very interested in what you find, as I scored a ton of OP07's from my work last year, but haven't had a chance to try them.

[duck_arse]

I've been thinking about something similar to this just recently. I have a "printer select switch-box", cost about 2$ at the junk shop. it comes with either a 2 or 3 way switch, with 5 thousand poles (I can't remember, 25? 37?) all with wires attached. and a knob. what can I do with that switch? switching an op-amp, single or dual, is only 8 poles, you can switch the compensation cap as well. if you had to, you could switch 2 duals, at the same time.

something to think about. switching just ground would not be optimal. the input stages would be in parallel, as would the output stages. odd side effects.

[PRR]

> Is having both pin 1s what is causing this?

Yes.

Pin 1 on both chips goes to a super-sensitive bias point. But not the same bias in each chip.

With both pins 1 tied together, both chips are working screwed-up.

Strapping internal pins together ignorantly, there's half a chance you could fry one or both chips.

> the OP07 works just fine, but when I switch to the LM308, it just kills

I guess the OP07 is less-upset than the LM308. But it still must have some ill effect on the OP07.

[mremic01]

I don't really see why it wouldn't work, but why not just socket, and do sound recordings. By the way, I'm very interested in what you find, as I scored a ton of OP07's from my work last year, but haven't had a chance to try them.

Actually both are socketed. The idea is for this pedal to be something like Beavis Audio's ScreamerLab, but I didn't want to stack the ICs. I figured I could get away with a 4PDT by switching between one half of the comp cap section, but it looks like it needs to have both sides disconnected, so I'd need a 5PDT.

Anyway, it only took a few minutes to drill a new hole and put in an additional SPDT to switch between the first pins and now it works. No difference in tone though. But that's ok, since it was more about proving whether or not they sound different than about getting two different sounds. The 2n7 cap in the Chinese made models seems to have more effect on the way the Rat sounds than the IC, and even that's subtle. I also threw in a switch for the other half of the Ruetz mod, the 560 ohm resistor. Removing from the circuit doesn't make much of an audible difference, but I haven't tried a trimpot in its place.

> Is having both pin 1s what is causing this?

Yes.

Pin 1 on both chips goes to a super-sensitive bias point. But not the same bias in each chip.

With both pins 1 tied together, both chips are working screwed-up.

Strapping internal pins together ignorantly, there's half a chance you could fry one or both chips.

> the OP07 works just fine, but when I switch to the LM308, it just kills

I guess the OP07 is less-upset than the LM308. But it still must have some ill effect on the OP07.

Well, both chips still work just fine. I don't mind losing a chip or two to experimenting. What's interesting is now that I have pin 1 on an SPDT, both ICs function just fine when only the LM308 is connected at pin 1. When the OP07 has its pin 1 connected on the SPDT, but I switch to the LM308 on the 4PDT, the LM308 gets a little sputtery, but doesn't seem to die or sound any worse when I reconnect its pin 1. Is this because of the slew rate? If that's the case, since the OP07 is supposed to be internally compensated, why does ProCo even keep the 30pf cap in the new Rats?