Design Challenge #2.1 - The CMOS Overdrive Deluxe

Chuck D. Bones

Circuit Wizard
A few weeks ago, MattG asked if it would be possible to replace the CD4007 OD's JFET front-end with an opamp. That sent me down the proverbial rabbit hole and this is what (eventually) popped out. First stage is an opamp 18dB clean boost. Second stage is a variable-gain CMOS amplifier. Third stage is a Catalinbread VariOboost, with slighty-altered component values. Fourth and fifth stages are the same variable gain stages used in the original design, with a few component value changes. At the end is a BMP-style tone control followed by a volume control.

Seven knobs o_O
PREAMP sets the 2nd stage gain.
BASS is a variable freq bass rolloff based on the Friedman TIGHT control. It's located near the front of the signal chain, ahead of the distortion stages.
MID & MID FREQ are the familiar VariOboost controls. VR5 is a dual-gang C100K.
TREBLE is a passive high freq cut / boost, using the familiar BMP tone circuit. It's located at the end of the signal chain, after all of the distortion stages.
MASTER sets the 4th & 5th stage gains.
LEVEL is the usual volume control.

With PREAMP & MASTER dialed-down, it's a clean boost. Turning PREAMP up gradually adds dirt before the Mid controls. Turning MASTER up adds dirt after the Mid controls.

So far, it only exists as a Breadboard. I'm pretty happy with it as-is. The most I'd change at this point, if anything, would be a couple of component values. Total gain at the output of the 5th stage is a healthy 72dB with the MID controls set to noon and everything else dimed.

CMOS Overdrive Deluxe sch rev 1.0.png
 
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cooder

Well-known member
Super cool and great to see the varioboost incorporated in it, now let me wrap my brains around that... looking very interesting.
 

cooder

Well-known member
With Chuck's blessings I put a board together for this, he helped me comb over the schematic and all, so big shout out to the man, Sir Chuck!
When I put it in the box I had it going but it didn't quite qo right, the mid pot had an off range and mid freq was off too. I must have had a solder bridge somewhere and after some poking arond and redoing the smd chips, re flowing some joints it's all go now and a rippa!
Very versatile and unique, I really dig the gainier side of things with pre and gain halfway or higher, and of course the ability to shape the sound big time with the vario boost snippet in there. I really like the Varioboost as a stand alone circuit already, having it in here is just sweet.
Having four pots in the lower row meant of course using 9mm pots there to get it in to the 125B.

I present you the 'Mirage' aka Harmonic Overdrive Experience #2.1.

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cooder

Well-known member
Thanks man! Where would we be without the boneyard and this forum!
With both pre and gain dimed there is some noise but considering the gain amount it's not not out of line I would say. My neighbours might disagree, but I think they mainly are concerned about my playing as such... ;)
This CD4007 is from Tayda, I might get myself one from Mouser next order and compare, maybe? Any hints what to shoot for as sources and specs?
 
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MattG

Well-known member
@cooder - that looks absolutely fantastic! Thanks for posting the pics. At first I thought it was a bigger enclosure than 125B, all the more impressive how you made it all fit without looking congested.

Looks like you're using a soft-switch instead of the mechanical type? And presumably you integrated the soft-switch circuitry on the PCB along with the overdrive circuit? Is that soft-switch circuit your own design?
 

cooder

Well-known member
Thanks guys! All Chucks brain power in there, I'm just putti ng it together.
@cooder - that looks absolutely fantastic! Thanks for posting the pics. At first I thought it was a bigger enclosure than 125B, all the more impressive how you made it all fit without looking congested.

Looks like you're using a soft-switch instead of the mechanical type? And presumably you integrated the soft-switch circuitry on the PCB along with the overdrive circuit? Is that soft-switch circuit your own design?
The soft touch switch is based on the coda effects one (I have a slightly changed code, but pretty much similar), you can find it here:
https://www.coda-effects.com/2016/04/relay-bypass-conception-and-relay.html
 

Chuck D. Bones

Circuit Wizard
Thanks guys! All Chucks brain power in there, I'm just putti ng it together.

The soft touch switch is based on the coda effects one (I have a slightly changed code, but pretty much similar), you can find it here:
https://www.coda-effects.com/2016/04/relay-bypass-conception-and-relay.html
You are too modest. It's a beautiful board layout and integrated relay switching is top shelf.
Where did you get that? I checked Mouser and they only seem to have it as smd footprint.
The thru-hole MC14007UB is no longer in production. On Semi is only making them in the SOIC-14 package.
 

Sturdag Lagernathy

Active member
The thru-hole MC14007UB is no longer in production. On Semi is only making them in the SOIC-14 package.
Yeah, my local shop doesn't get much new stock. It was always kind of a pull-a-part type place. Years ago, it had all kinds of radio equipment, industrial machines of all kinds, bought out electronics manufacturing raw parts. It was really incredible. 1.00$ grab bags were an especially fun time. I made a movie accurate proton pack after many long hours digging. It's a shell of itself now. Couldn't get a B100k pot of any kind yesterday.
 
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