D*A*M: FZ-676

So this looks like one of the D*A*M one-offs from back in the day - a MKI style fuzz, using a variation on a Zonk layout.  There are some departures from stock values, some of which are not entirely clear.

Here's the thread from an old FSB post.  Here's the thread on the D*A*M forum.

D*A*M  FZ-676  VERO LAYOUT

There are a few components that are not visible, but if you have any build experience, you can make some reasonable assumptions.  Pop it on a breadboard...  I know that's what I'm about to do.

One thing that throws me is this - the resistor next to the OC45 (the Q2 collector), is it brown green red gold, (1k5) or purple green red gold (7k5)?  I think 7k5, the schematic on FSB says 1k5.    

On the breadboard, 7k5 works just fine, so I think I will stick with what I see and hear.  

JHS: Zonk Machine

Quick layout to suit an enclosure that I just picked up from MW Pedal Parts - pretty much one row difference in terms of where the mounting holes are drilled compared to old layout that I have been using, and I decided to move a couple of components around while I was at it.  

MW are great by the way - no hesitation in recommending them based on my experience.  

ZONK MACHINE

DOD: 250

So by this stage, I think we can all assume that I'll be the only one making this.  I have a bunch of tin can LM741 opamps that I forgot about, so I thought why not...  

DOD 250 / DISTORTION + LAYOUT

DOD / MXR: 250, Distortion + component values table

This is probably where it all started in terms of opamp-driven distortion -  in one side of 741 opamp, out the other with some hard clippers.  

DOD 250 / MXR DISTORTION + SCHEMATIC

Direct link to Google Doc

741 SPECS

Numbers vary slightly by manufacturer and grade, but these headline specs are what give the 741 its “personality” in effects land:

• Supply voltage: works on dual or single supplies (it was designed with ± supplies in mind, but can be run single-supply with proper biasing).

• Gain-bandwidth product: around ~1 MHz class.

• Slew rate: about ~0.5 V/µs typical.

• Input bias current: tens of nA typical (bipolar input).

• Not rail-to-rail: inputs/outputs can’t swing anywhere near the rails, especially on a 9V battery. 

What that means in practice: a 741 can sound/feel a bit rounder and more easily pushed in simple dirt circuits, partly because it’s relatively slow and because it doesn’t have huge clean output swing on a 9V single supply.

LM741 PINOUTS

Note the little tab on the side of the opamp is pin 8.

OFFSET NULL

Offset null is a pair of pins on the 741 that lets you trim out its small DC offset so the output sits where you want it (often 0V on a dual-rail supply). 

In pedals it’s usually ignored because stompboxes are typically single-supply with a virtual ground and AC-coupled stages, so capacitors block DC and the tiny offset isn’t audible.

While the pins aren't used, be sure to isolate them anyway.

GEC: Germanium Signal Transistors (1971 Semicondcutor Handbook)

From the General Electric semiconductor handbook, 1971

BE: Fuzz Buffer

Not that anyone is likely to, but please don't rush out to build this - it's a concept that should work, but I have not tested it yet.  Might need some fine-tuning.

It's a combination of the AMZ pickup simulator and a Pete Cornish buffer - so basically a fuzz buffer to drop in front of anything that does not like being behind other pedals.  Typically a fuzz face.

out from previous pedal  >  buffer  >  into a transformer (pickup coil)  >  regular guitar volume and tone control > into the fuzz pedal.  

I have a few transformer options to experiment with, so see how that works out.  Switching might be interesting, as I think a loop would be good here.  i.e.  switch the fuzz on/off via the buffer.

A PNP buffer can be done by inverting the power supply and using a PNP transistor (works on LTspice).  This might be handy for building into PNP fuzz, with needing an inverter.

This is similar in function to the Aion FX Proteus, which uses a 42TL019 transformer.