8mV of noise causing havoc in valve pre amp.

[as_audio]

It has been said before, in 2013 (but not by everyone).

In my experience hum usually comes from poor ground wiring.
Its vital that power supply surging on the peak of the supply voltage causing pulses of current in the ground wire don't get into the audio signal chain.

In valves you have the extra dimension of heaters which can cause pick up if not carefully wired and routed.

I built a valve mixer and it hummed very badly.
Changing to DC heaters made a small difference.
However rerouting the ground wire from transformer to power supply capacitors and then to amplifier made a big difference.

 

[DF96]

I found removing the grid stopper and moving it right up next to the valve grid caused the hum to have much less harmonics in it.

Right at the grid is the right place for a grid stopper. Two reasons:
1. it acts better as a stopper for parasitic oscillation
2. it reduces the size of the high impedance grid connection (grid - grid-stopper) which can pick up hum via stray capacitance - once you are past the grid-stopper most hum will be attenuated by the low impedance of the source

 

[nigelwright7557]

In the end to get the mixer to work I had to reduce the mic input to 1k impedance.
This just about killed off any hum.
However it means I can only use 600 ohm mics.
There is still a little hum if the mic pot is at zero or at 10.
If I put the pot in the middle all hum goes.

The second pcb design has gone off to be made.
Here is the mixer in its case.

An externally hosted image should be here but it was not working when we last tested it.

 

[nigelwright7557]

PCB.

An externally hosted image should be here but it was not working when we last tested it.

 

[DF96]

There is still a little hum if the mic pot is at zero or at 10.
If I put the pot in the middle all hum goes.

That is unusual; the other way round is more common. Maybe you have some hum cancellation going on, or maybe the pot is acting as a grid stopper?

 

[nigelwright7557]

That is unusual; the other way round is more common. Maybe you have some hum cancellation going on, or maybe the pot is acting as a grid stopper?

The input track runs close to a transformer and a 160VAC track so its not surprising it is picking up hum.
Its odd while I was testing the pcb on the bench the hum was the other way around with worst hum with pot at centre position.

 

[rayma]

That is unusual; the other way round is more common. Maybe you have some
hum cancellation going on, or maybe the pot is acting as a grid stopper?

I've found that when there's a ground loop, this is often the case.

 

[nigelwright7557]

The input track runs close to a transformer and a 160VAC track so its not surprising it is picking up hum.
Its odd while I was testing the pcb on the bench the hum was the other way around with worst hum with pot at centre position.

When I put the pcb in a box I had to run the USB audio input past the transformer so maybe what is happening is triode 2 is picking up some hum from that, triode 1 is picking up hum too depending on position of mic pot and due to inverting stages the hums are cancelling out.

 

[nigelwright7557]

Does the noise stop the instant it is unplugged?

Yes.

New layout.

An externally hosted image should be here but it was not working when we last tested it.

 

[hooman]

An externally hosted image should be here but it was not working when we last tested it.

 

[Tedknowlegy]

Sounds exactly like a problem I saw in a Telex portable PA. The toroidal power transformer was close to the circuit board and was inducing current into the ground traces for the volume pot. The hum was at a minimum when the pot was halfway up, and was loudest when the pot was all the way down.

In your case I would verify this by removing the transformers from the circuit board and wiring them into the circuit with 6" or so leads. I would bet the hum is gone.

 

[nigelwright7557]

In your case I would verify this by removing the transformers from the circuit board and wiring them into the circuit with 6" or so leads. I would bet the hum is gone.

I scrapped the original pcb and took out the transformers, caps etc for future use.

The new pcb will be here in a few days so I will see what improvements (I hope) I get.

The new pcb has minimal area for AC to DC conversion and short paths to the smoothing capacitor.
The mic input has been made as short as possible with grid stopper right up against the valve grid pin.
I have kept the heater wires separate from anything else and they dont run over/under any other traces.
I have added copper ground pours on both sides of the pcb.
The transformers are now well away from audio tracks and the valve.

 

[nigelwright7557]

New pcb came through today.
I built it up and hum is now negligible.
Changes made were:
1/ Make mic channel input as short and as close to valve as possible.
2/ Grid stoppers right on to valve pin.
3/ Added copper pours on both layers attached to ground line.
4/ Move valve as far away from transformers as possible.
5/ Make high voltage AC power supply as short as possible.
6/ Rectifiers close to transformer to shorten high VAC lines.
7/ Make first smoothing cap close to rectifiers and have input to it from rectifiers only and output only to valve supplies.

Here is new pcb with ground planes.

An externally hosted image should be here but it was not working when we last tested it.

 

[nigelwright7557]

Also ran heaters around edge of pcb away from audio.
Added trimmer to heater elevation circuit to trim out hum.

 

[Celsius]

You can move the pot to the preamp out, if you have the pot normally at 30% when move to the out preamp the noise will be too 30%, the best is to find the origin .