TL071 vs. 741

[Jim Jones]

Hey guys,

I've been mucking around lately with the good ol' Dist. +/DOD250/Coron 10/Memphis Distortion...you get the idea...

Just wondering what you've found in the wonderful world of 741's.  Are there particular ones that are less noisy? How much improvement (if any) do you notice with TL071's?  Does the simple op amp/diode clipper benefit from a 741-type op amp rather than say half a 4558 or 5532 or TL072?

Just wondering out loud...

Thanks!

Jim

[Mark Hammer]

A quick scan of the datasheets indicates that with a +/-15v supply the TL071 can swing to +/-13.5v, or within 1.5v of each rail.  An NE5532 can do +/-13v with a +/-15v supply.  The datasheet for the NJM/JRC4558 indicates it will swing to within a volt of the rails under similar conditions.  With a +/-20v supply, the LM741 can swing to +/-16v with the same load (10k).

What this indicates is that the 741 has considerably less headroom with a 9v supply than a TL071 or 4558 does.  Even before it hits the diodes, it is clipping, and those of us who had the smart idea to lift the clipping diodes and make ourselves a "clean" boost for nothing were sorely disappointed to find the damn thing never ever really got clean even when the diodes were factored out.  At least part of the "sound" of a Dist+ comes from feeding the clipping diodes a signal that is already distorted due to headroom limitations of that chip.  The BiFet chips or others may well provide lower noise spec and are certainly much better choices for use as general purpose audio devices (e.g., in mixing boards or audio amps), but authentic replication of the Dist+ likely involves use of a 741 with all its inherent weaknesses.  It is by no means a "magic" chip, and I would scarcely recommend for "improving" other pedals.  It just happens to be part of why this pedal sounds the way it does.  Not the critical part, but part.

[analogguru]

Another important parameter is the slew-rate, a 741 is very slow compared to the TL stuff.  Harris and Rayethon produced a low-noise quad 741 which Neve used in his consoles.  I don´t remeber the number yet (I believe 4156), but this could be a possible try (use only one or build something around with the rest   ).  But with noise will not be much improvement.
That you have full benefit from a low-noise opamp the feedback resistor should not be more than 10k for a NE5532, otherwise the resistor produces more noise than the opamp.

That means in my schematic for a TL071:
R6 = 47k, R5 = 220E, P1 = 25k neg. log., and C4 1µF film cap.

greetings, analogguru

[Manolo Dudes]

Is there a dual version of the 741?... I'm using Tonepad's layout, which uses a dual op-amp, and want to be close to the original as much as possible.

MC1458 maybe?

[analogguru]

LM 1458 (National Semiconductor) is the dual version, LM 324 the quad.
Motorola is not so near with their chips.

analogguru

[Jim Jones]

Thanks for the posts guys!

For the record I didn't have a cap in the feedback loop at first, and a small value ceramic (I used 15p) does cut down on the noise a bit.

Jim

[WGTP]

I have been trying over a dozen op amps in a modded muff fuzz.  The NE5532 is on of the most quiet of the bunch.  It also has a nice clear treble sound and tight bass.  They all have slightly different sounds, but it's a matter of preference.  Might as well use a quiet one.

Since this is a distortion, the worst chip might be the one with the sound you like best, however.

Experimenting with the diodes will have a more pronounced effect.

See "Dave Barber" post about paralleling op amps.