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Re: MXR Distortion+

10/28/1999 5:54 PM
Re: MXR Distortion+
I just think that the main effect you're hearing is the difference in the gain-bandwidth product (frequency response). If you don't mind tweaking the design a bit you can add a small cap to lower the frequency response of a "better" opamp and get the same sound, I would dare to say even a better sound.  
There are some applications where you *need* a specific component. Examples: the "strange fuzz" that needs a specific suffixed 4136, the CMOS fuzzes that need unbuffered gates, the Hot Cake that needs an "original" Texas TL071, the Fuzz Face and Tone Bender that needs the soft turn on of Ge transistors, the RAT that needs a LM308N for its overdrive characteristics, etc. I just don't think the Distortion + is such a case - high frequency limiting should do the job. Unless of course you have already tried this - then I have to agree there's something else.
10/28/1999 6:14 PM
JD Sleep

No I haven't tried any tweaking on the Dist+, other than the diodes. I will do some experimenting with this and the DOD 250 soon. Since I am near the bottom of the "Electronics Learning Curve" I think it would be a great learning experience to play around with these simple circuits a little bit. ;-) I'll try a cap with a TL071.  
10/28/1999 5:52 PM
Mark Hammer Chip choices
Glad to see this thread emerge. Many of us often act as if there is ONLY the JRC4558 for the Tube Screamer, and then anything goes in the way of op-amps for all other devices; as if all other chip choices were simply a question of improving noise and high end specs by shelling out a few more ducats.  
In truth, it is quite likely that many designs depend on the inadequacies or idiosyncrasies of their op-amps for the identifiable sonic characteristics, although perhaps not quite as much as many designs using discrete components depend on using certain transistors.  
I seem to recall reading here not too far back about the history of the choice to use an LM308 for the RAT pedal (someone had posted a clip from Art Thompson's Stompbox book), which is also used in the Expandora and Roger Mayer Voodoo pedal. Here we have an instance of a chip used precisely because the way it could be constrained (via the external compensation cap) produced a desirable set of harmonics.  
Conceivably the Dist+ design was predicated on using a 741, perhaps notsomuch because it was sought out (like the 308/RAT case), but because it was cheap and then compensated for in the final design.  
Finally, I'm pleased to find out that the diode bypass produces unpretty sounds in everyone else's unit too. I tell ya, I looked at the schematic over and over thinking to myself "Ok, take out the diodes and all you have is a basic non-inverting stage with adjustable gain and output signal level. Why the hell does it sound like something ELSE then?" I was beginning to question my chops when something so very basic behaved in such an unanticipated fashion.  
What I'm wondering about is "WHY?". Out of curiosity, the DOD250 is ostensibly the same, but the resistor values used to divide the 9v down and set the Vref are noticeably higher values. Is this part of the errant functioning of a "non-clipping" Dist+? i.e., does diode lifting in the DOD250 produce a similarly ugly sound or is it as clean as the schematic suggests?
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