Minimising buzz in Washburn VGA15 combo

[mikeydee77]

Nothing wrong with this amp in fact it sounds great but has an annoying nnnnngggg buzz at 100hz with no or grounded input which I measure as a 5mV sine wave peak on the output at 50Hz. None of the controls have any bearing on the noisy output output. I would like to reduce this noise as the stock speaker / cab sems to be tuned to really emphasise this. The amp is based around a TDA2030 chip and the Washburn circuit seems close to the data sheet as well.

Things I have tried

• replacing the filter caps with same value and doubling up. No impact
• added 100uF on the PCB just where the jumper cable runs to the chassis mounted TDA2030. No impact
• added 100nF on to the TDA2030 directly No impact.
• loosened the transformer and moved it around and found no impoact.
• braided up the secondary windings connection wiring. no impact

So my conslusion is that rectification/smoothing is not the culprit here but now thinking more like a ground loop or radiant pick up. In this amp there is a ground plane wiith everything connected to that and that goes to the chassis at one point and via the pot chassis althogh they are not connected electronically to anything.

What steps should I use to investigate further?

I Was wondering about separating grounds but not confident on where to start with that. I know its not worth it economically but as academic exercise maybe some value.


Any suggestions very welcomed?

[Jon Snell]

If you remove C121, does the hum stop ... if it does, check the volume pot grounding,also check R140, they don't like no HF load.
Failing that and as you say the rails are smooth, replace the TDA2030.

[mikeydee77]

H Jon, thanks for taking a look...

R140 measures ok.
Removing C121 and the output hum stops... but there isn't a noisy signal coming from the cap. Do you think this means a ground loop?

Mike

[Jon Snell]

As the hum stops with C121 lifted, the hum comes from the vol pot. Does the hum change with the pot setting, eg; halfway the hum is minimum and increases when fully clockwise and fully counter clockwise.

[mikeydee77]

With C121 in circuit the buzz level is fixed. None of the controls effect the buzz... they just add the normal mix of hiss and noise which is normal level.

Also if I ground the pots with a croc clip wire there is no impact on the output signal.

Also should have said earlier
I changed the tda2030A as I shorted out some pins accidentally. Removing the chip amp from the chassis and altering its orientation makes no impact.

I will add an oscilloscope snapshot in a minute as the shape is curious and may help with ideas.

M

[mikeydee77]

Here is the output measured on the scope 50mV / div x1 probe

[Jon Snell]

Foot switch ... remove one side of D101has it gone?
Are the power supply rails smooth? Could be noisy diode in the bridge rectifier, it has had a surge from the tda shorting after all.
Straws are being clutched here ...

[mikeydee77]

Lifting D101 has no impact at all...
Power supply does have ripple on but it looks even on both sides.

I think we are out of straws now...

Given that disconnecting the pre amp stops the noise and that it is not coming from the pre amp as such I am inclined to give up now with the conclusion that it is ground loop / layout related.

[mikeydee77]

Tried lifting D101 to no difference.
Also as last straw replaced all the rectifiers... no difference.

I think time to call it a day on this.

Thanks for your help and on this Jon. It's good to sounds ideas/theories with someone else. In this ones case since the noise is there regardless of what the pre amp is doing I am inclined to believe that there is layout / ground loop problem causing this.