Maestro MFZ-1 copy noise

[fuzz guy]

I recently built a copy of the MFZ-1 from a Fuzz Dog kit. I really like it but it's pretty noisy and the volume knob cuts out below noon (no sound) but the upper half of its range gives me what I need.

Is this an inherently noisy circuit? The volume knob thing doesn't really bother me, I only mentioned it in the event that it's linked to the noise issue.

I'll put a link to the build doc below, I used the stated components and made no changes to the circuit. Any advice on how to possibly quiet it down would be appreciated. Thanks

http://pedalparts.co.uk/docs/MFZ-1.pdf

[iainpunk]

it would be more logical if the gain control had that problem.
the pedal has a fixed gain of 454, so the least amount of noise will be amplified to relatively high levels.
the gain control is basically just a volume control before the gain stage, not an actual gain control, the gain stage always amplifies the noise it receives a lot, especially when there is a high impedance in front of the gain stage.
the schematic on the build doc seems wrong.

cheers, Iain

[Mark Hammer]

the schematic on the build doc seems wrong.

Yup.  The factory schematic on the service manual shows the R8/C5 pair connected to the inverting input of the op-amp (pin 6) and NOT the output pin.

Now, it can also be the case that the drawing does not correspond to the PCB, and the PCB connections are just fine; wouldn't be the first time in history.  But if the schematic drawing reflects the board, then you're going to have to cut a trace or two, and wire up a jumper to make the circuit behave properly.

[antonis]

if the schematic drawing reflects the board

IMHO, it can't be or we have a noisy attenuator..

P.S.
Reduce R2 value down to 100k, R5 down to 220k, R6 down to 470k and R3 & R4 down to 47k (or so)..
Make Vol pot 100k or reduce R7 value down to 2k2..
Let C1, C2, C4 & C6 values as they are for the time being..

[fuzz guy]

Thanks for the replies, I'll open it up and try to check the board. If the PCB is wrong I'll contact the company and see what they'll do for me.

[anotherjim]

Apart from the constant high gain of the design already mentioned, the circuit might misbehave noisily if the polarized capacitors C3 and C5 are fitted the wrong way around.

[iainpunk]

the schematic is off, but the PCB is correct, otherwise, there would be no fuzz going on.

[fuzz guy]

Apart from the constant high gain of the design already mentioned, the circuit might misbehave noisily if the polarized capacitors C3 and C5 are fitted the wrong way around.

For a moment I thought maybe I'd done something stupid, but no, they're in properly.

the schematic is off, but the PCB is correct, otherwise, there would be no fuzz going on.

There's definitely fuzz, and like I said it sounds pretty good, sort of reminds me of a Big Muff but less saturated  with more mids.

So if it's not the caps and it's not the PCB what else could I check?

[iainpunk]

i think its a noisy circuit to begin with!
do you already have an enclosure around the pedal, that generally helps a lot. it should be grounded of course.

yes, it should sound like a low gain, mid boosted big muff, just like the original.

cheers, Iain

[antonis]

If you don't scale down all unecessary high value resistors, like R2 to R6, noise will persist..

[iainpunk]

If you don't scale down all unecessary high value resistors, like R2 to R6, noise will persist..

that's a really good suggestion, every resistor except R1 and R7 can be scaled down 1 to 10.
if you change C5 to 22uF it will sound the same
smaller resistors means less noise

cheers, Iain

[fuzz guy]

Reduce R2 value down to 100k, R5 down to 220k, R6 down to 470k and R3 & R4 down to 47k (or so)..
Make Vol pot 100k or reduce R7 value down to 2k2..
Let C1, C2, C4 & C6 values as they are for the time being..

that's a really good suggestion, every resistor except R1 and R7 can be scaled down 1 to 10.
if you change C5 to 22uF it will sound the same
smaller resistors means less noise

cheers, Iain

So if I change the resistors to antonis' suggestions, change the Vol pot to 100KA and C5 to 22uF I should get a similar sounding circuit with less noise?

The original PCB is all soldered and boxed up, but I might just get a new PCB and make those changes.

[iainpunk]

have you considered getting a breadboard?
its really useful for experimentation like this, and its a great learning tool in general.

it also gives the opportunity to experiment with the values to change them to your liking. you can for instance change this in to a tight distortion if you change a few parts values, but to test out what you like, its a pain in the butt to solder, de-solder and re-solder components jus to check things out.

cheers, Iain

[fuzz guy]

I was thinking the same thing. Up till now I've mostly been a follow the steps kit builder. I'm going to get a bread board and mess around with this circuit and see what I can come up with. Thanks to everyone for all the advice.

Does anyone have a link or copy of the proper schematic for this pedal?

[fuzz guy]

I found this in the schematics section,

https://www.diystompboxes.com/pedals/MFZ_FUZZ.jpg

Is this the correct schematic?

[antonis]

Better than previous one but similarly noisy..

[fuzz guy]

Better than previous one but similarly noisy..

But is the layout correct? It was mentioned above that there were issues with the Fuzz Dog schematic.

If the layout on this one is correct I can use it to breadboard the circuit and try your suggested value changes.

[anotherjim]

Here's a suggestion using the same PCB if it follows the last scheme posted.


On the input amp, remove both 1M resistors and the 100nF cap across the 2M2 resistor.
Replace the feedback 1M with wire.
Join the 10nF input cap from its junction with the removed 1M resistor to the junction of the 2M2 resistors.
The input signal now feeds the +input of the opamp.
You then have the same input impedance and gain without the resistor noise. Same sound too -  the only purpose of the input stage is to buffer the drive pot and bias the gain stage.
You also now have a true polarity non-inverting pedal so at high gain the possibility of feedback squeal unless you have careful wiring layout to avoid stray feedback.