Muff farting / gating

[statzern]

I just finished a BMP build and it's sounding great except one thing. When you play any percussive notes it sounds like it's gated, and does this for more than half of the sustain range.

I used stock values for all components, except that I used 1 uF film caps instead of electrolytics.

This sounds like it is not biased properly to me. My question is, is it normal to have to select specific values for biasing resistors even with stock transistors? Also, would it be better to try smaller cap values to let less low freqs through?

Thanks for your help!!

[R.G.]

Gating is nearly always a biasing problem. In a proven design, it's nearly always caused by the usual coterie of DIY deadly sins: wrong component value, wrong orientation/connections of the component, wrong placement of the component, bad soldering, bad wiring, etc.

Read and follow the instructions in "Debugging; what to do when it doesn't work", that stickie post that you had to skip over to get down to the rest of the forum.

[GibsonGM]

Yes, and posting your transistor voltages will help greatly to see if your bias is off, or if it's something else!

[statzern]

Thanks for the replies guys.

Here's transistor voltages:

Q1
b 0.61
c 3.82
e 0.03

Q2
b 0.61
c 3.83
e 0.03

Q3
b 0.61
c 3.81
e 0.03

Q4
b 1.49
c 4.51
e 1.01

I feel like this is a "one of these is not like the others..." situation. I double checked orientation and (at least marked) parts values. Everything appears correct, no wrong parts as far as I can tell. Wondering about Q4 there...

Hope that sheds some light! Thanks again!!

[statzern]

Looking at the GGG values, these aren't that far off at all. My circuit is from the tonepad layout. Also, voltage from battery is 8.63 v at the board

[anotherjim]

Q4 base is starving - something stealing its current from the bias network. Usually it's something to do with C1 failing to block DC (either the cap is leaky or there's a conductive path around it). Checking the voltage with guitar plugged in and twiddling the guitar volume control would show bias dependant on the guitar IF it was C1 faulty.

[duck_arse]

from recent experience, we do have to ask: what transistors do you think you have fitted, and what transistors are actually fitted? and you do need to check both your documents and your board before you answer.

[statzern]

They're 2N5088s. I just checked the board to make certain didn't get mixed up. They are indeed correct.

[bloxstompboxes]

Am I the only one who thinks the title of this thread suggests a completely different topic? I know... I need help.

[statzern]

Lollll... I originally had at the beginning of the thread: "sorry about the title, I couldn't help myself..."

[bluebunny]

I'm pretty sure that "farting" is a bona fide EE technical term.  Just looking it up in my Horowitz & Hill...   

[duck_arse]

-sniff- some of us rise above the obvious, sometimes, blox. we worry more about getting it backwards.


oh, dear, now look what I've done.

[statzern]

Guys - I really appreciate the effort you've put in helping with this little problem. Unfortunately, I am going to be away for a couple days on vacay, so may not work on it for a week.

Jim - I did try your suggestion, turning knobs on the guitar does not alter the voltages measured at all.

I am thinking I might replace one or more of the coupling caps and see what that does, and if no improvement, change them to a smaller value and see what that does. I have some 0.47 UF and some 0.01 UF extras that I can try.

Thanks again, you guys are awesome!

[duck_arse]

if yr going to replace the coupling caps, take them all out, then measure all the stage voltages. if the DC's are right then, you know it was a bad cap, and probably which one it was.

or a bad joint.

[Gus]

statzern

Are you using a DC bypass cap 100uf or larger across the +9 and ground?

Have you bypassed the switch and wired jacks in and out to test just the circuit and not the switching and LED? wiring how do you have the LED wired?

[statzern]

Hey Gus -

Not sure what you mean exactly by the DC bypass cap. There's none in the schematic like that, so I guess I'm not. I see something like this on a different schematic on the big muff page, referring to some of the clones of the Big Muff, but never in BMP circuits themselves. The circuit I'm working off is the tonepad "stock" schematic.

As for the wiring, it is currently not boxed and there is no switch or LED. I just added wires to the board and wires to jacks and pots and connected it all together via jumper wires. So while there might be more noise from longer runs due to the jumpers, it's wired up like you suggest already.

Good news is, I'm heading back home tomorrow and might get time to switch out the caps while I'm there. Will keep you all posted. Thanks!

[statzern]

Sorry to be so long to reply, but I finally got around to switching out the coupling caps in this thing and I have two things to report:

1) the gating and farting is completely gone with C1 3 4 and 7 replaced with 0.1 uf in place of 1 uf.
2) the sound is terrible with these caps - it's all midrange honk and no scooped awesomeness that I was going for.

From here I will try tweaking things one at a time. Maybe going in the middle would've been a better idea (eg, 0.47 uf) or just put the 1 uf ones back one at a time until it's just right.

[statzern]

End of thread: went back and just replaced all the coupling caps to their stock value, 1uf. Sounds perfect now. Guess it was just a bad cap or a crappy solder joint. Quite happy with it. Will post boxed pix over in etched enclosures when it's done.

Thanks all for your help