PSU umbilical hum

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IDM
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#1 PSU umbilical hum

Post by IDM »

Hi I have built a phonostage that during development behaved beautifully. Having boxed it up I am getting a problem with hum. The layout has Salas HT regulators in the same box as the actual phonostage and the the HT and heater PSU's are housed separately.

What is odd is that if I grip the HT umbilical chord the hum stops. Any ideas on how to cure this? All I can think is to put some caps inside the phonostage box ahead of the regulators. Will this stop the problem?

FYI the phonostage is the design shared by Andrew using a C3G and ECC88. I have altered it too use triode strapped E180F instead of the ECC88 and have used anode chokes. Additionally I split the HT to have two separate HT's for the C3G stage and the E180F. As said during development it was stunningly quiet. Earthing all starred earthed within the phonostage.

Hope someone can help.
Cheers
Ian
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Nick
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#2 Re: PSU umbilical hum

Post by Nick »

Maybe worth trying caps before the regs to see, but I cant see why it would make a difference.

Possible its being induced into the ground line, what happens if you connect a separate link to signal ground instead of the one via the umbilical.

Given the valves you have, its possible the hum is actually due to RF and the extra capacitance on the umbilical to your hand is enough to stop the oscillation. Maybe try some ferrite beads. Do you have a scope that could see RF?
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Mike H
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#3 Re: PSU umbilical hum

Post by Mike H »

Couple of ideas, re the above, are there grid stopper resistors?

Also might need an earth lift resistor?

As a matter of fact I did use a multi-way screened lead for the PSU umbilical of the one I made a few years ago, but can't remember why...
 
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IDM
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#4 Re: PSU umbilical hum

Post by IDM »

Hi Nick,

I am not sure if my aged scope is up to the job. I think you might be onto something with the RF as sometimes after about 30 minutes it just disappears. If it is RF what sort of ferrite beads do I need? I have some tiny ones which could probably go over the HT+. Would these do any good or do I need big ones to wind the whole umbilical round? Also should I have ferrites on the HT at the anode pins?

Cheers
Ian
IDM
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#5 Re: PSU umbilical hum

Post by IDM »

Hi Mike,

I did wonder about a ground lift resistor, but tried it yet as it was silent before. Also there are grid stoppers on both the C3G's and the E180F's.

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Ian
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Mike H
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#6 Re: PSU umbilical hum

Post by Mike H »

E180F (aka posh version of EF180) is a RF pentode, these can be notoriously tricky. So maybe as Nick suggests a ferrite bead on the signal grid after the grid stopper resistor, and maybe a couple on the heater connections also. Unfortunately they do like to try to oscillate if given a chance and can do using just a short bit of wire as the 'coil'.
 
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#7 Re: PSU umbilical hum

Post by IDM »

Hi,

I bought a ferrite clamp and a large a ferrite bead. I threaded the bead over the HT and umbilical and also put the clamp round the umbilical (belt and braces). The ferrites were placed round the cable immediately before the plug into the phonostage. When I switched on I was met with absolute silence (the phonostage was working!!), even turning the volume right up there was just the faintest hiss if I put my ear against the speakers. So I think the issue is sorted.

I am delighted, and have learnt something as I thought it was the grids and heaters of valves that care was required to prevent RF break through. The filtration in this case was before the HT regulators.

Thanks for your help.
Cheers
Ian
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Nick
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#8 Re: PSU umbilical hum

Post by Nick »

Regulation is never perfect, RF is hard to keep out and will pass through most regulators by capacitive coupling. The error amplifier in the regulator doesnt see it. Likewise adding capacitors is not that good a solution, the inductance of most electrolytics means they also dont see the RF. inline inductors and ceramic caps made for the job are the best solution. As you have found.

Ferrite’s on the grids of the valves are to stop oscillation of the valve generating RF, not particularly to prevent RF from outside getting into the valve.
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#9 Re: PSU umbilical hum

Post by IDM »

I have a habit of building my PSU's in separate cases to their associated pre-amp, digital streamer etc. It makes me wonder if I should add ferrite clamps to all the HT cables. Even if the RF doesn't cause highly apparent instability I imagine that it is better to keep it out. Given how cheap the clamps are I think I will try a few experiments.
Cheers
Ian
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