K5881 hum / buzz

[Tom How]

I made a World Audio K5881 back in the late 90s. There is a long thread here from a few years back when I did some updates and maintenance.

http://www.world-designs.co.uk/forum...ead.php?t=6918

The amp still has a slightly annoying hum/buzz even with the volume turned down/signal unplugged. I couldn't say if it has got worse over time, but I've recently moved house where i am (a) sitting closer (b) listening at typically lower levels, and it has started the annoy me, especially on quieter pieces. As far as a can tell both channels have the same level of hum/buzz.

I've made a video on my phone https://youtu.be/eCzhSAZeZTk it is a bit hard to hear - crank it up!

I think most of the advice in the other thread has been followed, especially the suggestions about fitting isolated phono sockets and "raising" the circuit earth. The case now has an earth post which is connected to mains ground and the input ground on the mains transformer. Everything is star-earthed to another point, connected to the mains/case earth post via a 15R wirewound resistor.

i think the input valves are original but the output valves go replaced during the above thread around 2014. I think all the heaters are on the same phase - although i notice the heater wires aren't twisted.

The sound is fine.

Any suggestions on how to diagnose this? Or perhaps tell me it always does that

[Richard]

Hi Tom, good to hear from you again. First thought is a near-bye flourescent light or dimmer switch, I have an old style low energy flourescent lamp near my amp which does the same buzz but very low level so try switching any lights or other near-bye electronics on/off. That noise is a definite buzz rather than a hum and I'd also suspect a faulty diode, or possibly a smoothing cap, and change those all out.

[Tom How]

The new house does have some alarming clicks and pops on the mains - so scope for investigation there.

I guess UF4007 will do for the diodes? Enough current?

The amp has the power supply with the one huge blue LCR capacitor. Not sure how easy that is to replace.

[Richard]

Yes UF4007 will be ideal and don't look too difficult to change. That big LCR cap is actually 2 electrolytic caps x 50uF 500V, as per the circuit diagram, and should have 3 tags I think. I don't know if you can still get that actual unit but 2 x 47uF 450V electros wired as per the diagram will be available. It's dated 1996 so I'd just change all the electrolytics in the amp now if it was mine.

[Richard]

Here's the LCR unit, https://www.hificollective.co.uk/cat...or-p-9975.html

No stock but an alternative the same physical size is in stock, https://www.hificollective.co.uk/cat...or-p-9395.html

Looking at the data sheet it's rated for 3000 hours at 85C so perhaps 5 years at typical 45C running temp. With switched-off time you could double that but the extrapolation only holds true for so long as the seals start perishing.

Illinois Capacitors have a sheet on life, https://www.illinoiscapacitor.com/pdf/.../Life%20expectancy%20of%20Aluminum%2...

and Rubycon say similar, www.rubycon.co.jp/en/products/alumi/pdf/Life.pdf

[Tom How]

Fort he record, I turned on only the amp at 7am this morning, with every other device in the house off including the central heating and the fridge. As far as I can tell the buzz is unchanged from last night.

[Tom How]

I agree, 20 years is pushing your luck for any electrolytic.

All the other electrolytic caps got replaced during the 2014 revamp, so I'll start with the big PSU cap. Thank you for pointing out a suitable replacement, I was struggling to find something.

[Richard]

Good move, process of elimination. If it is interference it's difficult to know where it's coming from, RFI or mains, your house or next door etc. I had a regular buzz for 20 years from my next door neighbours old hot air heating system kicking in. Sad to say but when they died and the new owner fitted a new heating system it finally went quiet and confirmed what I'd become used to expecting over years of weather patterns and buzzes.

[Tom How]

Naive question I guess, but is it possible to filter this kind of rubbish out of the mains? Answering my own question: I guess that's what the PSU is supposed to do!!

And yes, I did try google and found what looks like a lot of overpriced snake oil.

[Richard]

Yes, 2 suppressor caps can be fitted, they must be marked X2 rated for fitting across 275V mains L to N.

Fit one circa 0.047uF value across the mains L to N after the mains switch and
one smaller 0.01uF value across the 10R ground lift resistor (in parallel with it) if you don't already have one fitted.

These provide a low impedance path to ground for RFI picked up in the mains and signal circuit.

They will be something like this, https://www.ebay.co.uk/itm/0-047uF-4...frcectupt=true