Power supply noise when the effect is switched off

[Rob Strand]

Other observations:  If I turn on the amp and there is a cable plugged into the input of the amp with nothing connected on the other end (pedal, guitar, etc.) then there is a buzz.  To my ear it is the same as the buzz I'm hearing with this issue. 

That's pretty normal.  The buzz actually comes from the mains supply in your house.    It actually couples onto the exposed tip on the connector. 

When I plug the cable into the output of the pedal, the noise decreases considerably.  It is still audible, but barely.  There is no change when I plug the guitar cable into the input of the pedal (with the guitar on the other end of the cable). 

OK that's normal too.    The enclosure of the pedal shields the tip/center core from the external noise.

When I add the DC supply to the pedal, the buzz gets louder, as though it is the same noise amplified.  I can't be sure, of course, and don't have a scope to measure.

It's still not clear why that's happening.

So my understanding so far is,
- When the pedal is powered from battery alone, the buzz does not increase in bypass mode.   The buzz may increase a bit
  in effects mode because the pedal is a booster but overall the buzz is acceptable.
- When the pedal is powered from the external DC supply, the buzz increases in bypass mode and get loader again when in effects mode (due to the boosting action of the pedal).

Next,
- After performing the test where the pedal is powered from battery but the -ve lead of the DC power supply is connected to the pedal  ground (the positive lead of the DC power supply left unconnected).   The buzz does not increase.  It pretty much behaves as if it was powered from battery.

So the only "bad" case is when the pedal is actually getting its power from the supply itself.   The pedal has filters on the DC rail so we would expect any noise on the DC supply to be filtered out.

The biggest clue is that the noise is present even in bypass mode.

At this point I'm thinking something is wrong with the wiring.   What we think is OK from looking at the schematic isn't actually like that in reality.

From your schematic it looks like the pedal is a germanium transistor.  I'm assuming the transistor is NPN and that the circuit is a negative ground circuit.     Can you confirm that?   If it's a PNP circuit with a positive ground it's going get all mixed up and we will see weird stuff like we are seeing.     Is the 1M resistor on the base actually going to ground?  (That's why I think it's a germanium transistor.)

duck_arse - The original schematic called for the 500K pot.  I'll work on getting a snap of the layout/internals for you all to see.  Thanks!

So that's probably best thing to do.  Maybe someone here can spot the issue causing the noise.   To me it's looking like a wiring issue.   Highly like to be way you have done the footswitch wiring.   Wiring the wrong terminals on the input/output sockets will cause some havoc as well.

[duck_arse]

on your posted schem, can you perhaps measure your voltages and mark them on the diagram, then repost? measure the voltage on both sides of your supply filter 100R [on battery and external supply, if they differ], and on the "transistor" legs. most important, mark the "transistor" part number on the schem!

one other Q, do you get usable BOOST from the circuit, battery or external supply, and does the trimpot vary the sound at all? thank you.

[VibratoLLC]

I have a few suggestions and hope they help:
On the 10k Pot connected to the Transistor, it appears that one leg is unconnected - I'd connect the open leg to the center wiper.
In the past, I had a circuit act glitchy and found that the potentiometer metal case needed to be grounded!
I've also discovered that circuits on my workbench have a lot of noise because of my florescent work light AND my soldering iron is unshielded. Try turning off nearby electronics.
You circuit, as is, may function perfectly, if you put it in a metal enclosure (emi/rfi shielding).
One final suggestion is to put a 1uf or 10uf electrolytic capacitor across the 1K ohm resistor at the emitter.

-Louis

[antonis]

From another point of view, BJT Base looks like exhibiting very high impedance (due to "unorthodox" bias configuration - almost Class C (*)..) resulting into amplifying any interference of even the slightest current ability (noise)..

(*) Take voltage measuremets on transistor legs, as already suggested by Stephen..

P.S.
Just another point of view, supported by "some" glasses of red dry wine..

[pak420]

Thanks for the fantastic feedback thus far from all who have chimed in. 

I have attached photos of the insides as best I could.  You'll notice my attempt at shielding the LED supply line as one of my early efforts to see if I could find the cause of the noise by wrapping it with a lead connected to ground on one end.  It did not make a difference, but I have not bothered to remove it as yet.  I have already reflowed solder for the grounding wires.

I had initially used some old insulated jacks that I took from some other project.  I thought there might have been and issue with those, so I went to the old standard mono jacks.  I did not directly connect their grounds since they ground to the case. Again, no change in the noise/buzz issue.

The FET is a 2N5457.  The drain has a hair over 3V, .77 volts on the source, and the gate has .02 mV.  The supply coming in is 9.07V, and after the 100R it is 8.99V.

[Rob Strand]

It's a long shot but try this,
- Remove the ground wire between the DC jack and the center pin of the switch.
- Wire the ground of the DC Jack to the ground of the input socket.

You might want to check the switch contacts are working in all connections.

[pak420]

Thanks, Rob!  I'll test the switch out and give that a go. I'm also going to triple check for any solder shorts that may have occurred.

Results to follow.

[pak420]

I got an opportunity to move the DC jack ground to the "star" ground location. I was in a hurry and didn't get to play through, but when I plugged in the DC, the noise was still there in bypass, but not when the effect was turned on. 

I have a connection to a pedal company here in town with a tech that is going to look at it for me.  If he comes up with a solution, I will relay it here so that there is a record.

Thanks, again, to all who have chimed in.  What a fantastic community!!!

[DIY Bass]

I recently had a pedal that was noisy.  In this case the external controls were not properly grounded.  The case was grounded OK, but some of the pots and footswitches were not well grounded to the case, especially the footswitches - I had accidentally made the plastic washer that they come with the thing closest to the case, so connection was dodgy

[Rob Strand]

I got an opportunity to move the DC jack ground to the "star" ground location. I was in a hurry and didn't get to play through, but when I plugged in the DC, the noise was still there in bypass, but not when the effect was turned on.

I have a connection to a pedal company here in town with a tech that is going to look at it for me.  If he comes up with a solution, I will relay it here so that there is a record.

Well that's some progress and perhaps a hint the supply wiring has something to do with it.

It's looking like the power supply for sure.    At this point, it would be a big sanity check to try a different power supply.  It could depend not only on the PSU but you amp and your house.     


It would be good to get someone help to look over it.   I look at your pics and see no obvious problems.    The problem seems to be related the DC adaptor itself.   Perhaps a deeper nastier problem is at play.

The only idea I have left would be to move the filter parts to the DC socket,  pretty much removing any common connections between the DC in and the audio.  I'd even try adding a 10 ohm resistor between the ground of DC inlet socket (ie. the negative DC in wire) and the filter ground and circuit ground.   Try the filter ground on either side of the 10 ohm.    It's not normal to do this for a pedal.

The weird thing is the PSU doesn't produce a problem on your other pedals.   It could be your other pedals have JFET switching not true bypass.

Anyway good luck.

[bartimaeus]

the photos show the output jack is grounded via the enclosure. are you sure it's making a solid connection to ground? and same for the input jack? sometimes you need to rough up the inside of the enclosure with sand paper.

[iainpunk]

the photos show the output jack is grounded via the enclosure. are you sure it's making a solid connection to ground? and same for the input jack? sometimes you need to rough up the inside of the enclosure with sand paper.

wow, i totally overlooked that when looking at the pictures.

its good practice to solder in those ground lugs, even if you think its grounded through the enclosure!!!

cheers, Iain