Uni-Vibe "replica-ish"

[Mag]

Hi there,

I've built this Uni-Vibe replica based on the "Forum vibe" R4-F board/version. The vibe effect works fine but I have massive noise, especially when I hit the cancel switch.


I installed a variable power supply (set at about 16-17V) inside the casing, LM7815 voltage regulator for 15V.

Could this power supply be too weak? It's rated for 3A (I'm guessing at lowest setting of 9V). When I measure voltages at various transistors in audio path, they all slightly oscillate.

Should I use shielded wires from input/output jacks, pots and switch to PCB? (Already used one from volume to output)

Thanks,

Marc

[GibsonGM]

Hmmm....what happens to the noise if you move the power supply physically away from the board?

What's your setup for the voltage regulator - do you have a schematic of it as-built? 

2 things come right to mind, if all else is built OK - PS coupling noise into the circuit, and maybe not enough filtering if the PS is not intended for audio use. Might need to isolate the PS and/or add some filtering.

I'd say it HAS to have the ampacity for that circuit, 3A rating.   

If above doesn't reveal anything, can you post voltage pre and post regulator + 'how it's built'?
Might as well post the forum vibe schematic too, if you can.

[Mag]

Here's the regulator schematic:





I'm going to test with the power supply out of the enclosure and report back with the results.

Here's the forum vibe schematic:




Thanks,

Marc

[antonis]

Could you plz post a schematic of the exact way of LFO B+ wiring..??

You should consider that connection the most "dirty" supply so it should be wired before the, further filtered, B+..

[R.G.]

When you say "noise", do you mean hum or hiss?

If hiss, you could be getting it from the power supply or from internal RF oscillation; well, OK, maybe both.

I suggest: (1) temporarily move the power supply out of the box and connect it through several feet of wire; this moves any radiated noise from the presumably switching power supply outside the box. If the noise changes at all, it may be radiated from the power supply.
(2) add a temporary resistor load across the power supply, perhaps 33 to 47 ohms; this adds a minimum load to the power supply; some switching power supplies are quirky at nearly no load, and for a 3A power supply, a univibe circuit is nearly no load.
(3) Remove C4, the 330pf compensation capacitor. Replace it with a 30pF or 33pF capacitor from the collector of Q2 to the base of Q2. The forum vibe board is known to have RF issues with some modern transistors. This change of compensation fixes it as far as I can tell.

In fact, do #3 first. It's useful in any case, and won't need to be un-done. It may stop the noise if it's RF hiss.