Power Supply Issues - Hum only on certain settings

[yclepticon]

Hi All,

I'm having some issues with a power supply I'm using for a pedal that I designed/built. There is a nasty hum that occurs only when all the dials (fuzz, tone, volume) are dimmed. If I peel back the tone or volume just a little the noise goes away. This doesn't happen at all when I'm running just on a battery (full 9V).  I've tried using an EH supply and the hum was significantly less.

This generic regulated power supply is rated at 1,000ma 9VDC and the pedal's current draw is 2.2ma. Any suggestions on how to remedy this would be fantastic.

Thanks for looking.  

[DavenPaget]

Hi All,

I'm having some issues with a power supply I'm using for a pedal that I designed/built. There is a nasty hum that occurs only when all the dials (fuzz, tone, volume) are dimmed. If I peel back the tone or volume just a little the noise goes away. This doesn't happen at all when I'm running just on a battery (full 9V).  I've tried using an EH supply and the hum was significantly less.

This generic regulated power supply is rated at 1,000ma 9VDC and the pedal's current draw is 2.2ma. Any suggestions on how to remedy this would be fantastic.

Thanks for looking.  

I believe it's not sufficiently filtered and unregulated , check the output voltage .

[yclepticon]

It is a regulated supply at 9.14V.

[deadastronaut]

http://www.beavisaudio.com/projects/Huminator/index.htm


maybe?...

[Rob Strand]

I can only suggest some things to try which might help get to the bottom of it:
- Maybe confirm the PSU output is clean as assumed, maybe it isn't regulated or has a fault.

- if the mains plug has 3 pins check the PSU output 0V (and even +V) is not connect
  in some way to the mains earth - that will cause a earth loop.  (Usually PSU's are ungrounded,
sometimes there is a cap from the output to the mains earth which may form a weak loop.

- Does your power supply 0V run through the input socket?  If so, try wiring the 0V directly
to the effect PCB.

- Try wiring running the input 'hot' wire as close as possible to the chassis.  You could also
try wiring the input with COAX.

- Try the last two and also unbolt the input socket from the chassis.

- Try putting a big cap 100uF to 1000uF across the PSU terminals; at the effect PSU input or on the PCB.

[DavenPaget]

http://www.beavisaudio.com/projects/Huminator/index.htm


maybe?...

Yes ! It works magnificently .

[Paul Marossy]

If it's a really high gain circuit, you may not be able to get rid of that hum because it's picking up tiny amounts of EMI from the environment and amplifying the heck out of them. That will show up when you have all the controls at max but not otherwise. If it were a power supply thing you should have that noise no matter what the settings are. That has been my experience anyway.

[yclepticon]

If it's a really high gain circuit, you may not be able to get rid of that hum because it's picking up tiny amounts of EMI from the environment and amplifying the heck out of them. That will show up when you have all the controls at max but not otherwise. If it were a power supply thing you should have that noise no matter what the settings are. That has been my experience anyway.

Thanks. Yes, it's a high gain circuit. But, I'm unsure why this doesn't happen (or is greatly reduced) with another regulated power source - and it's completely fine when running on batteries.

I can only suggest some things to try which might help get to the bottom of it:
- Maybe confirm the PSU output is clean as assumed, maybe it isn't regulated or has a fault.

- if the mains plug has 3 pins check the PSU output 0V (and even +V) is not connect
 in some way to the mains earth - that will cause a earth loop.  (Usually PSU's are ungrounded,
sometimes there is a cap from the output to the mains earth which may form a weak loop.

- Does your power supply 0V run through the input socket?  If so, try wiring the 0V directly
to the effect PCB.

- Try wiring running the input 'hot' wire as close as possible to the chassis.  You could also
try wiring the input with COAX.

- Try the last two and also unbolt the input socket from the chassis.

- Try putting a big cap 100uF to 1000uF across the PSU terminals; at the effect PSU input or on the PCB.

Thanks, Rob.  I do have negative running through the input switch, however it is important to keep it that way as to not kill the battery when unplugged from the pedal.
I'm going to try putting a big cap across the PSU terminals.

http://www.beavisaudio.com/projects/Huminator/index.htm


maybe?...

I was actually looking at this last night. And, this was going to be my course of action! Seems like a beefed up version of Rob's suggestion. Hopefully it works out.

[Paul Marossy]

Thanks. Yes, it's a high gain circuit. But, I'm unsure why this doesn't happen (or is greatly reduced) with another regulated power source - and it's completely fine when running on batteries.

Might be taking some small amount of hum from an unregulated but relatively well filtered wall wart and amplifying it. Or maybe it's a minor ground loop that doesn't get offensive until you max all the controls.

Could even be the lead dress inside your pedal. Have you looked into that possibility? Sometimes just shielding the input wire from input jack and to circuit can get rid of that problem.