Muffrat

[Cubes_Mahone]

Hey,

I'm currently building this pedal, the muffrat: http://www.doitfuzz.com/projects/img/muffrat/muffrat_schematic.gif

C3, C6 and C7 say NP (non polarized). So would I use Mylar film? And the other caps are just ceramics?

How are you supposed to know which caps to use?

Thanks a lot

[vigilante397]

Cap material is largely a point of preference. I'm a big fan of polyester film caps: http://www.taydaelectronics.com/1uf-100v-5-polyester-film-box-type-capacitor.html But I also use multilayer monolithic ceramics from time to time http://www.taydaelectronics.com/1uf-50v-multilayer-ceramic-capacitor.html

These are both available in pretty much any value you could ever need for stompboxes, and are a nice small footprint that are easy to fit onto packed PCBs.

Hope this helps

[Cubes_Mahone]

That's great. So to be clear, you're saying to use those for all capacitors?

Why are those 3 highlighted as NP? Is it because of their large values?

Thanks

[vigilante397]

That's great. So to be clear, you're saying to use those for all capacitors?

Why are those 3 highlighted as NP? Is it because of their large values?

Thanks

I'm saying you can use them for all capacitors in this build, it's your build and I'm not going to tell you that you have to use those.

But yes, those three specifically say NP because of the value. Usually 1uF is when it starts to become common to use polarized capacitors.

[Ben Lyman]

I think this is the link that I learned a lot from:
http://www.coda-effects.com/2015/04/capacitors-which-one-to-choose.html

[Cubes_Mahone]

Really helpful. Thanks a lot!

[ashcat_lt]

Does this actually work?  The opamp stages are not actually biased at all.  Does "signal ground" somehow just float to between the rails?  I mean, there literally is no DC reference point in either of those stages, so I can almost believe it, but why are those two caps there at all then?  Without them, it would bias to around 4.5V anyway.

Edit - No, that's not quite true!  The non-inverting input of the second opamp stage is explicitly tied to ground!  That should clip off the entire bottom of the wave plus the first 0.8V or so of the top part.  Am I missing something?  Have you heard this work?

[Cubes_Mahone]

I have not heard it work, haven't actually built it says it's been breadboarded and that it works...

Anyone else? Don't fancy building it if it's going to be a dud

[PRR]

> (non polarized). So would I use Mylar film?

1uFd is a large costly film cap. The commercial builder will use NP Electrolytic.

The schematic linked "violates" some basic good-practice design, as Ashcat points out.

Assuming someone -did- build this, I can pretend to see ways it "maybe could work". If C3 is Aluminum Electrolytic, it *will* leak enough to bias FET-input U1a to around 4.5V. That's unreliable (what if your part-buyer gets a good deal on surplus Mylar??) and seems to use a part's fault where no part is needed.

That does not answer at U1b where the 1K resistors absorb any cap leakage. Maybe he *likes* the less-than-half-wave output? (It IS supposed to distort.)

There's no obvious reason for C7 to be non-polar, as it is clearly biased left end more positive than right end. Maybe the "designer" had a 10-bag of 1uFd NP caps and sprinkled them anywhere.

Trimming R4 is not an effective way to "get 4.5V". Q1 C biases at Vbe plus VR5 (mostly Vbe) times R6/R3 divider. Almost any value of R4 will give "about 4.5V". Even extreme values of R4 won't land very far off 4.5V. A very screwy transistor (say Ge, or Darlington) suggests changing the R3 value and R6/R3 ratio, not the collector load.

Forum thread:
http://www.doitfuzz.com/forum/diy_pedal_project.php?effect=muffrat_distortion

[Ice-9]

That schematic must be wrong, nothing can bias correctly as drawn.

[Cubes_Mahone]

Might breadboard it anyway and see what it sounds like.

Other than that I'll probably keep looking for something similar. Anyone got any links for a simple distortion with a few different options like that one?

Thanks

[Tony Forestiere]

Start here. Breadboard mandatory.    http://www.generalguitargadgets.com/how-to-build-it/technical-help/articles/design-distortion/

[Cubes_Mahone]

Yikes, you guys are super helpful! Thanks a bunch everyone!

[Tony Forestiere]

This too. All of it.  http://geofex.com Welcome.