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What is A1? MXR Distortion II

7/28/1998 7:16 PM
Aron Nelson
What is A1? MXR Distortion II
On the off chance someone knows....  
I just got the MXR Distortion II schematic and it has a [] square in the schematic labeled A1. As in [A1]. It has 14 pins? At least the component placement page says it does.  
I'm wondering if anyone has one of these things and could open theirs up and see if there's any markings on the chip.  
You can't miss the chip, it's the biggest IC on there.  
I have heard these things are good.  
7/29/1998 11:02 PM
Mark Hammer
Near as I can tell, the Dist II was basically a Dist+ with more advanced tonal control (tone and resonance controls). To do this wouldn't take much more than a quad op-amp, either a TL074/84, or LM324, or 4136. Find the pinouts to these babies, compare against the schematic, and there is your answer (probably).  
Are they good? Don't know. I *do* know that I made a distortion unit about 10-12 years ago that had similar controls for adjusting the tone (2-pole tunable low-pass filter and resonance control), and I liked what it did to the sound, and the versatility it offered. Chances are that the Dist II has similar flexibility.
7/30/1998 12:44 AM
Aron Nelson

Thanks Mark.  
Well, the schematics clearly show a 4558 dual op amp (Both inputs used). 3 TL072's (5 inputs are used).  
Most disturbing is that they are clearly shown as as an IC (Triangle shaped). Why would this A1 be shown as [A1] if it was an op amp like the others?  
Wish I had a scanner. Maybe I'll go somewhere, scan it and post it online.  
7/30/1998 11:14 AM
Don Symes

If I remember right (this was ten years ago that I last saw the schematic), A1 is a CD4016xx/CD4066xx quad analog switch.  
Does it look like some pins go to the footswitch and others select between effect and bypass? There may be some transistors between chip and switch.  
If I am right (odds <50%), just put in a true bypass instead.
7/30/1998 12:18 PM
Aron Nelson

>If I remember right (this was ten years ago that I last saw the schematic), A1 is a CD4016xx/CD4066xx quad analog switch.  
Hah!!! This makes a lot more sense!  
Yes, there are 4 of those [A1] boxes. Upon further inspection, yes, they seem to be mainly "away" from the main circuit.  
I wish I could post this schematic. Cept that Kinkos wants $10 per page to scan!  
I will have to get my friend to do this. Then I will put it up on my site.  
7/30/1998 1:13 PM
Mark Hammer
Well, Don beat me to the punch. As I read your response to my posting, I thought "That's more than enough op-amps to do the job. It has to be a switching chip."  
MXR was never really shy about using CMOS chips (e.g., the envelope filter ONLY uses CMOS hex inverters and CMOS switches), but they tended to be a little more conscientious than some when it came to true bypass. Chances are it is for switching purposes.  
Want to nail it down? Check the schematic and see if there is a SPST switch anywhere. If yes, then clearly any kind of FX bypassing has to be done through solid-state means.  
BTW: I bought a handheld B/W scanner for $45 Cdn., and it does the job just fine. I imagine they can be had much cheaper these days, now that 30-bit colour scanners have gotten so cheap. Check out the surplus places advertised in Nuts & Volts or Popular Electronics for ads. You'll appreciate having one when you lose a file and only have a hard copy of a draft!
7/30/1998 3:53 PM
Aron Nelson

Thank you Don and Mark!  
What I would really like to do is to post the schematic, then have someone chop out the unecessary parts (bypass circuitry). Maybe the switches are not even needed? If so, then it would make this already complicated circuit much easier. I will have someone scan it for me.  
Yes, I do have a handheld B/W scanner, I got mine on a closeout (SCSI) for $50. But how do you do an 8.5X11 page? Mine doesn't do so well with large pages...  
I will try the scan anyway.  

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