SoulFetish 10/10/2017 11:41 AM
loud hum using voodoo labs switching through Randall RH50T
I have a customer who is running a Voodoo Labs GCX audio switching system to patch his effects through the input and effects send/return of stereo Randall RH50T heads. He is getting excessive hum when going through his effects loop. We've eliminated power supply noise in his effects as the cause. The noise sounds to be in the range of 120Hz, but when plugging straight into his Randalls, the HV supply is quiet and does not seem to indicate bad filtering. The same setup when substituting a Fender Super instead of the Randall head seems to work fine (Although, the Randall is a much higher gain amplifier). At this point I'm thinking there is grounding noise and/or a potential problem in the solid state amplifier driving the loop.
I'd appreciate any thoughts on something I'm missing, or if anyone has any experience with this head or switching system. I've attached some clips, and schematics:
[ATTACH]45258[/ATTACH]
[ATTACH=CONFIG]45257[/ATTACH]
[ATTACH]45259[/ATTACH]
[ATTACH]45262[/ATTACH]
 
Mick Bailey 10/11/2017 8:52 AM
What does the amp sound like with just a patch cable linking the FX send/return? Have you tried setting up the switcher and substituting the pedal loops with a patch cable?
 
SoulFetish 10/18/2017 5:11 PM
Quote Originally Posted by Mick Bailey View Post
What does the amp sound like with just a patch cable linking the FX send/return? Have you tried setting up the switcher and substituting the pedal loops with a patch cable?
Mick, with the effects loop jumpered the noise was loud and clear. Dead quiet with it pulled. What are you thinking it might be? I don’t have it on the bench with me yet, but im leaning towards failing LV filtering.
 
Mick Bailey 10/19/2017 1:32 AM
If the LV filtering was failing the amp would most likely hum all the time - especially as the reverb driver/recovery is opamp based. Check if there's any noise if you output the 'send' to your bench amp. If not then that eliminates everything before there, including the loop buffer Check if the amp hums with a guitar plugged straight into the return. My first though is a broken or bad tip contact, or dirty switching contacts in the return socket. Make sure the loop switch operates correctly.
 
nickb 10/19/2017 1:50 AM
Check the continuity between the Randall return jack sleeve and the safety earth pin on the power plug. It should be connected. Do the same for out of the output of the switcher and its plug. if it i also grounded then there is your problem - a ground loop.
 
Mick Bailey 10/19/2017 3:07 AM
He says it still makes the noise with just jumpering the send/return - no switcher.
 
nickb 10/19/2017 8:55 AM
Quote Originally Posted by Mick Bailey View Post
He says it still makes the noise with just jumpering the send/return - no switcher.
Ah! So he did. I misread it.