05-04-2020, 03:24 PM
Hi all.
A friend of mine and I have been working on an amp that we built from the London Power F Pre Fender kit. The power supply is the PSU Pre power supply kit for tube preamp.
We finally got sound to come out of the set up and it does sound pretty good except we have some kind of hum. My friend is MUCH more knowledgeable about this stuff and this is what he wrote. I have attached photos as well. If it is any help, I can record a sound file to upload. but here goes. Thanks for your help:
"[font=Helvetica]Hoping you’re doing well in this time of CoronaPanic. We’ve built one of your kits, and we’re having a hum problem we can't diagnose or fix. [/font]
[font=Helvetica]We’ve combined the preamp and its power supply on a chassis along with a 50 wpc class D power amp and its power supply to make a hybrid.[/font]
[font=Helvetica]Here’s the specifics:[/font]
[font=Helvetica]– Hum is definitely on the preamp side. Power amp, when fed with different source, is dead quiet./[/font]
[font=Helvetica]– Everything is on a star ground except …[/font]
[font=Helvetica]– Hum is at constant medium level and doesn’t change when volume is changed. [/font]
[font=Helvetica]– When playing loud, you can barely hear it, but at low volumes it’s pretty prominent.[/font]
[font=Helvetica]– Hum is not pure 60 hz sine wave, but kind of dirty.[/font]
[font=Helvetica]– We experimented moving wires away from transformers, but no change[/font]
[font=Helvetica]– 12AX7; tried a new tube, but no change[/font]
[font=Helvetica]– Same with iron plate added above transformer as EMS shielding.[/font]
[font=Helvetica]– Only thing we added to preamp is a pot on the output as a level control so we can turn the input up and overdrive the pre at modest sound levels.[/font]
[font=Helvetica]– We bypassed the AC IEC filter, but no change[/font]
[font=Helvetica]– Measured voltages (let us know if we should measure anything else)[/font]
[font=Helvetica]- Heater: 17.4 vdc[/font]
[font=Helvetica]- Hi voltage, 367 vdc[/font]
[font=Helvetica]We’ve fiddled and twiddled, but we’re out of ideas. We’ve put a lot of work into this and would like to get it perfect. Hoping you have some suggestions for things to try. Pictures attached. Let us know if you want other shots."[/font]
[font=Helvetica]- - - Jonathan Levin[/font]
A friend of mine and I have been working on an amp that we built from the London Power F Pre Fender kit. The power supply is the PSU Pre power supply kit for tube preamp.
We finally got sound to come out of the set up and it does sound pretty good except we have some kind of hum. My friend is MUCH more knowledgeable about this stuff and this is what he wrote. I have attached photos as well. If it is any help, I can record a sound file to upload. but here goes. Thanks for your help:
"[font=Helvetica]Hoping you’re doing well in this time of CoronaPanic. We’ve built one of your kits, and we’re having a hum problem we can't diagnose or fix. [/font]
[font=Helvetica]We’ve combined the preamp and its power supply on a chassis along with a 50 wpc class D power amp and its power supply to make a hybrid.[/font]
[font=Helvetica]Here’s the specifics:[/font]
[font=Helvetica]– Hum is definitely on the preamp side. Power amp, when fed with different source, is dead quiet./[/font]
[font=Helvetica]– Everything is on a star ground except …[/font]
[font=Helvetica]– Hum is at constant medium level and doesn’t change when volume is changed. [/font]
[font=Helvetica]– When playing loud, you can barely hear it, but at low volumes it’s pretty prominent.[/font]
[font=Helvetica]– Hum is not pure 60 hz sine wave, but kind of dirty.[/font]
[font=Helvetica]– We experimented moving wires away from transformers, but no change[/font]
[font=Helvetica]– 12AX7; tried a new tube, but no change[/font]
[font=Helvetica]– Same with iron plate added above transformer as EMS shielding.[/font]
[font=Helvetica]– Only thing we added to preamp is a pot on the output as a level control so we can turn the input up and overdrive the pre at modest sound levels.[/font]
[font=Helvetica]– We bypassed the AC IEC filter, but no change[/font]
[font=Helvetica]– Measured voltages (let us know if we should measure anything else)[/font]
[font=Helvetica]- Heater: 17.4 vdc[/font]
[font=Helvetica]- Hi voltage, 367 vdc[/font]
[font=Helvetica]We’ve fiddled and twiddled, but we’re out of ideas. We’ve put a lot of work into this and would like to get it perfect. Hoping you have some suggestions for things to try. Pictures attached. Let us know if you want other shots."[/font]
[font=Helvetica]- - - Jonathan Levin[/font]