T-Rex Mudhoney - Static/Noise with 9V, OK with <9V

[Juppstein]

I built a Mudhoney some days ago and when using it with a battery ( so with about ~8.6 V) the box seems to work as intended. But as soon as I connect it to a regular 9V power supply all I hear is noise (like static radio noise) and when I disconnect the power cable I can faintly hear something that sounds like a discharge noise.

Someone around that could give me a hint on where I might have to look around on the board to troubleshoot? I am a bit at a loss here. I knifed all the rails (I hope/think/pray) a few times and looked out for obvious solder blobs where they don't belong. Also the issue is happening where the effect is boxed up or not, so I should not be the something around there.

[induction]

There's a decent probability that the noise is coming from the power supply itself, and that the circuit is fine. What kind of power supply are you using? Unless it's regulated and filtered for audio use, it may be noisy. Do you have another power supply you can test it with?

[Juppstein]

Yup I already tested it with multiple PSU (a Boss and an Ibanez one) in different wall sockets. So I don't think it is the PSU or the wall sockets.

[strewel_peterson]

I was going to suggest a 100R resistor in series with the 9v in, which I have found works wonders suppressing PSU noise on some circuits, but then I checked the layout for thr Mudhoney on this site and it already has a 100R in place. Worth checking you haven't missed the cut underneath this resistor, though?

[Ciaran Haslett]

Yip.....agree with all the above.  If it works with battery but noisy with PSU then it really only points to one the...the PSU (assuming your wiring/DC socket is all fine).  Have another look at that cut under the 100r, or maybe add a 100n non polar cap in parallel to the 100u as extra filtering.  Theres plenty of space to the right of Q1 between 9V and GND.

Remember, commercial pedals have tons of filtering built in (they have no idea what kind of supply their customers will be using) so a PSU that works with a commercial pedal may still be noisy with a DIY circuit. And when it comes to making layouts...most designers omit the extra filtering as they assume the builder will use a fully regulated supply AND save space for the layout.

So...a known filtered/regulated PSU is the way to go.  If its still noisy with that, then a build error/layout issue is the most likely culprit.  But seeing that there are many who successfully built that particular layout....I'm happy to say that the layout is fine.

Good luck and please report back with whatever you find.

[Juppstein]

Issue found/solved (somehow) against a recommendation in the comments, the 2N3904 does NOT work with this pedal as a replacement transistor for the BC546B. Just tried a BC548 and that one works, at least until the BC546B arrives in the mail :)

[induction]

The BJT in this circuit is a simple buffer. A 2N3904 will work just fine, but you have to mind the pinout (it's reversed compared to the BC547).


This image says BC547, but it applies to BC546 and BC548 as well.

[Juppstein]

*facepalm* you know and I did even google the pinout before I put in the 3904 and I was so sure I got it right....