Working on my Univibe pedal.

[DrNomis_44]

Hey Everyone,


I'm currently doing some work on my Univibe pedal to resolve a hum issue whenever I try to place it in my Marshall amp's FX loop, I'm suspecting that I inadvertently set-up an earth/ground-loop when I originally installed all the circuitry in it's current housing, so, I'm going to go and re-do all the ground wiring and hopefully remove the suspected earth/ground-loop and therefore make it quieter so it's useable in a live gigging situation.

[Simon Barden]

It's not going to be a ground loop, but it may just be some bad grounding or lack of full shielding from the case.

It may also be that it doesn't like the hotter signal level in the FX loop. It's a nominal -10dBV, so consumer line level. Guitar signal levels are around -20dBu (note the different units), but vary depending on how hot the pickup is. But it also depends on how much headroom the pedal has.

Worth spraying some contact cleaner into the FX return and send sockets. As it's a series FX loop, they are switched contacts and if not regularly used, they can corrode over time which can affect the signal and make things like hum worse.

[DrNomis_44]

It's not going to be a ground loop, but it may just be some bad grounding or lack of full shielding from the case.

It may also be that it doesn't like the hotter signal level in the FX loop. It's a nominal -10dBV, so consumer line level. Guitar signal levels are around -20dBu (note the different units), but vary depending on how hot the pickup is. But it also depends on how much headroom the pedal has.

Worth spraying some contact cleaner into the FX return and send sockets. As it's a series FX loop, they are switched contacts and if not regularly used, they can corrode over time which can affect the signal and make things like hum worse.

Cheers, I'll definitely give the contact cleaner a go next time I have some cause I'm out of it at the moment.


The main parts of the Univibe's circuitry is running off a regulated +15V DC supply, and I think I did some measuring with an oscilloscope and signal generator once and found that the signal-level can get to about 3V or so before it clips, might be plenty of headroom, but don't know for sure, the LFO and Lamp Driver circuitry are running off a raw un-regulated +23V DC supply, I'm considering modding it to have all the circuitry running off a regulated +15V DC supply which might reduce the hum a bit, the power transformer has heaps of output-current capability, about 1.25A.

[DrNomis_44]

Update:

I think I've figured out where the hum in my Univibe pedal is coming from, it's my guess that the hum is coming from the un-regulated +23V supply that's powering the LFO and Lamp Driver circuits in my Univibe, what I think is happening is that since the +23V supply is un-regulated, it still has a residual 50Hz component superimposed on it, that 50Hz component is modulating the brightness of the lamp, and it is then making it's way into the preamp and phaser sections of the Univibe circuitry, the power supply circuitry has a 15V 3-terminal regulator that is supplying a regulated +15V to the preamp and phaser section, so, if I re-arrange it so that the entire Univibe circuit is powered from a regulated +15V supply, that should get rid of the majority of the hum.

[DrNomis_44]

Update:

Finished working on the Univibe yesterday and it seems to be quieter now.