Question about a Rat build with different op amp...

[Andrummist]

I built the Tonepad Rat without any substitutions, but unfortunately, I think my lm308 chip is bad. My audio probe had audio into the chip but nothing coming out. To test my theory, I purchased an lm741 from Radio Shack for about $1.50 and replaced the lm308. (I do plan on ordering a new lm308.) Voila, the pedal works perfectly. But with one minor issue. The distortion sounds at 100% though I've only turned the knob to about 50%. It seems that the distortion range is greatly reduced. Is this due to the different op amp? And if so, why?
I am hoping the range will be correct with a working lm308.

[tubelectron]

I am hoping the range will be correct with a working lm308.

It wont, sorry - I tried it. So there is probably a bug in your circuit or you don't have the right value or taper for the distortion pot. Nonetheless, the LM308N is better in the RAT (not to say the best), but the 741 is correct. Forget absolutely OP07DP, TL072 : they're simply from dull to awful compared to the LM308N...

A+!

[Mark Hammer]

1) I hope you included the 30pf compensation cap between pins 1 and 8 with the LM308.  It is necessary for proper functioning.  It is NOT necessary for the LM741, though it does not appear to have any impact on performance if you leave it in when using a 741.

2) The characteristic sound of the Rat comes from 2 principal sources:
- the use of a 308 with that much gain applied (in the thousands)
- the use of differential gain for the mids/highs and lows
The gain applied in the top end vastly exceeds the gain/bandwidth capabilities of the chip and, according to its developer, is responsible for the ghost tone in the background, when it is working properly.

3) Are you sure the orientation of the various caps is correct?

4) The LM741 is severely limited in how much gain it can produce before running out of headroom, particularly with a 9v supply.  That maximum is easily run up against with the gain control well below 12:00.

[DavenPaget]

I am hoping the range will be correct with a working lm308.

It wont, sorry - I tried it. So there is probably a bug in your circuit or you don't have the right value or taper for the distortion pot. Nonetheless, the LM308N is better in the RAT (not to say the best), but the 741 is correct. Forget absolutely OP07DP, TL072 : they're simply from dull to awful compared to the LM308N...

A+!

They are not worser , they are better opamps but LM308 suits perfectly .

[Andrummist]

@Mark
I had the 30pf cap while trying to make the lm308 work but removed it after the 741 was installed. As you implied, the 741 did not sound different with the cap installed or removed.
I spent a lot of time debugging the circuit before trying 741 op amp. I am very positive that all caps are in correctly. If a new lm308 has issues, I'll recheck all once again. ugh. But I'll burn that bridge when I get to it.

@tubelectron
I bought 3 of the same components, 100k audio pots, as the schematic calls for. The other two seem to work as intended. I know that I shouldn't assume anything (I've already ran into a bad ic and diode in this build) but until I get a new lm308, I'll trust that the pot is correct.
You said that you tried. Can you elaborate on this? What was your issue and what was the solution?

@DavenPaget
The lm741 is a cheap, quick solution to replace/test for a bad op amp and I was very happy that it worked at all. I would suggest to anyone debugging one of these builds to spend the $1.50 at the Shack to have a spare op amp. And it does sound pretty Ratty. So at least I have something to noodle with for the time being.

Thank you all for you comments!

[zombiwoof]

I am hoping the range will be correct with a working lm308.

It wont, sorry - I tried it. So there is probably a bug in your circuit or you don't have the right value or taper for the distortion pot. Nonetheless, the LM308N is better in the RAT (not to say the best), but the 741 is correct. Forget absolutely OP07DP, TL072 : they're simply from dull to awful compared to the LM308N...

A+!

"...but the 741 is correct"?  The 741 was never used in the RAT pedals, the LM308 was the classic opamp in RATs until around the year 2000 or so when they changed to the OP07 (which reportedly doesn't sound as good).  The 741 WAS used in the vintage DOD OD250, and I believe the related MXR Distortion+.

Al

[DavenPaget]

@DavenPaget
The lm741 is a cheap, quick solution to replace/test for a bad op amp and I was very happy that it worked at all. I would suggest to anyone debugging one of these builds to spend the $1.50 at the Shack to have a spare op amp. And it does sound pretty Ratty. So at least I have something to noodle with for the time being.

Thank you all for you comments!

You can SAY that . But trust me , i've spent less then that on spare opamps , ( btw i consider 4558 as SPARE )

[tubelectron]

@tubelectron
I bought 3 of the same components, 100k audio pots, as the schematic calls for. The other two seem to work as intended. I know that I shouldn't assume anything (I've already ran into a bad ic and diode in this build) but until I get a new lm308, I'll trust that the pot is correct.
You said that you tried. Can you elaborate on this? What was your issue and what was the solution?

The issue is that the original OP07DP I had in my Vintage RAT RI sounded "bad, dull, ordinary" compared to the LM308N. I made an A/B breadboard comparison to hear it, and it was clear : the LM308N sounded superior, so I understood where the RAT earned its valuable reputation.

So I changed my Vintage RAT RI  to return to the 1981 specs, the main change and the most important being the use of the LM308N. After, using again my breadboarded RAT, I tested in A/B comparison several ICs versus my now modified Vintage RAT RI  : again, the result was clear. The TL071 was the worst, sounding harsh, the OP07D sounded fizzy, it was better for the 4558, the 741 being the most correct substitution approach for the LM308N, in case of short supply. I do not remember the complete list, but none of them that I tested were matching the LM308N. But of course, it is a matter of taste !

Going further, I built a DOD250 with a LM308N and found that the same distortion qualities were there again. I described this release in this forum (search the CRC Overdrive Preamp topic).

"...but the 741 is correct"?  The 741 was never used in the RAT pedals, the LM308 was the classic opamp in RATs until around the year 2000 or so when they changed to the OP07 (which reportedly doesn't sound as good).  The 741 WAS used in the vintage DOD OD250, and I believe the related MXR Distortion+.

zombiwoof  : everybody already knows that...

A+!

[azrael]

The 741 also can't perform in the same way as a 308, in terms of slew rate. The slew rate distortion of the 308 is part of the RAT sound, no?

[tubelectron]

The 741 also can't perform in the same way as a 308, in terms of slew rate. The slew rate distortion of the 308 is part of the RAT sound, no?

I didn't made measurements to clear it, but the lower slew-rate of the LM308N is probably one of the major factors. It would be worth to try on breadboard some very low slew-rate opamps to check the difference. Nonetheless, TL071 and OP07DP have higher S-R than the 741 and of course the 308N.

I am thinking about trying an LM308N (or two) without clipping diodes to see if a good overdrive can be achieved...

A+!

[Andrummist]

I put in the lm308 (and replaced a bunch of capacitors) and now the pedal sounds great. But the distortion range issue is still there so I went back and looked at the purchase order... seems like I have myself 3 linear taper pots, not audio taper as I thought. Ugh. New ones on the way.