New guy builds an okish fuzz

[cherler]

Hey guys, so I'm new to the forum and to DIY pedals but I've been having a ton of fun so far. I lurked for a bit, actually learned a good deal and decided to contribute some! So here's a demo of a somewhat mediocre bazz fuss I built recently. I kinda rambled a lot because it felt weird to not say anything, so feel free to ignore all that. And all the string noise....

[youtube]http://www.youtube.com/watch?v=exnmFPwsiIw[/youtube]

So the circuit is basically a bazz fuss sandwiched between to J201's. The first J201 is set up as a boost and the second is just an output buffer. I'm getting a little bit of noise when I'm not playing which could probably be solved with just a capacitor of some value between the input and ground or the output from the boost to ground I would imagine? I'm not really sure on that.

Also I wired up the gain pot in a really weird way for....reasons..... Basically increasing the gain increases resistance between the collector and the base of the darlington pair for the fuzz. I know since this is a single transistor gain stage the signal at these two points will be out of phase, so I think what's going on is that I'm canceling out more of the input signal as I decrease that resistance. You guys think that's what's happening here?

Finally, what are some of your favorite low pass filter circuits? I really want to upgrade the filter in this thing from the rinky dink little passive first order low pass filter I'm using now.

Thanks everyone!

[crochambeau]

Watch out for planting a cap between a signal line and ground, the only thing it's likely to resolve is elimination of your high end.

Does that buzzy rumble persist with other power supplies?

Regarding further troubleshooting advice I can't wrap my mind around your circuit without a schematic, and being the lazy asshole I am (plus you state there's a darlington in circuit, which didn't pop up on my single stop bazz fuss circuit search) so......... if you're at liberty to provide such, a schematic would be handy for head scratching.

Welcome! (I'm bit of a newcomer 'round these parts too)

[lordgalvar]

Welcome to 100 crochambeau!

[cherler]

Sorry I didn't put the circuit diagram up with the post. I knew I should but I posted this from work. Here it is now though. So the reason I used a darlington pair is I saw a few circuits using MPSA13's which are darlington transistors. I didn't have any lying around so I just threw 2 2N5088's in a darlington configuration. That could be part of my problem. So I think some of the noise was the power supply, and some of it is also the weird feedback loop thing I did and called a gain control.

[crochambeau]

Wow, I love it. Please note: I'm not trying to sound mean here, I might just have malformed social skills...

Your clipping diodes at the darlington are oriented to PASS the Vcc from collector to base, this might be tanking your gain control (I see you've planted a resistive return path with the DC blocking 47nf and 5-105KR, I'm ASSuming that is the control you're referring to). As it stands, you're looking at the transistor base is being no more than 1.5 volts under the collector which is going to limit the sort of lifting this stage can do. The diode orientation is also going to block half of your signal duty cycle, regardless of whether it's coming back through the variable resistance or through the diode clipping stage, so you are in a sense shorting out that branch with your clipping (minus the drop at D2).

As an experiment, I would plant the cap on the diode clipping leg and allow a diode free path for your resistance, which may in fact be WAY too small since signal coming off the collector of the darlington/BJT is going to be 180 degrees out of phase with your incoming and will essentially be turning everything DOWN.

Subtractive feedback is a huge tool, but applying it on a hobbled stage can have the same result as trying to push start a car that happens to be in reverse. Given the piggybacked BJTs, the engine you've planted in that car can be capable indeed.

All that said, I'm not sure your darlington stage is the source of your trouble.

The output section has a parallel pair of 100nf ground referencing the gate, which, under low-z operating conditions might not nuke your signal, but consider that FET gates are inherently high impedance and the DC coupling cap feeding this stage is HALF the size of your shunt. This is like icing the wheelchair ramp.

Furthermore, you call it a buffer (my mind settles on low impedance non inverting output when that word is used), but it's clearly an inverting gain stage.

I would look into segregating your tone stack (ASSuming that's the parallel 100 nf pair) or knocking it down by a couple orders of magnitude (200 pf is probably discernible here) and then maybe doing some reassignment surgery on Q4 to implement a source follower (if buffer is your intent, nothing inherently wrong with following everything up with another gain stage).

Finally, some observations on stability in general: I find active stages become a lost easier to manage if all emitters are lifted a little from the ground plane, and all collectors are padded a bit from Vcc... UNLESS I have a reason that I need to be directly tapping the source. This is coming from my fucked up end of the universe in which there are numerous iterated stages that sometimes like to get out of line or run off to the bushes for a quick shag. 100 ohms is usually enough (1K usually doesn't hurt anything), if you need the AC gain slap a bypass resistor (edit, <--- that should say capacitor) in there. So, in the event none of the ideas above help, it might be worth a try to give each stage a little resistive platform on which to stand.

All this is just coffee induced speculation, I haven't built one to try and follow you down the rabbit hole, nor will I be doing so, unless things take a huge turn.

GOOD LUCK!

Welcome to 100 crochambeau!

Thanks! Once I get my introductory run out of the way I'm going to offer myself up as chum. Here's hoping for the best, haw haw haw

[cherler]

Ok thank you so much crochambeau, this is the exact reason I decided to joint he forum. All I want is feedback and to learn stuff. I'll try adding the cap to the diode path, that makes a lot of sense. Also I do realize that the output FET is a gain stage, I was just thinking of it as a buffer since my main goal for putting it there was to isolate my janky tone stack. I may actually replace both of these components with an active low pass filter with some resonance control, which seems like a lot of fun. I still have a version on breadboard, in my time in the engineering world I've learned not to take apart prototypes until I absolutely have to, so I'll try giving the trannies a little resistance to stand on as well. Thanks again man!

Btw I love your analogies, very vivid.