K5881 speaker hum.

[shadders44david]

Hi, I`m looking for some advice on a new problem with a K5881 mk2. The amp has developed a slight hum in its right channel and a 50% less hum than right in its left channel accompanied by slightly more hiss (left channel only) . I`ve owned the amp from new (1997) and can usually rectify issues with it - sometimes requiring the knowledge of the helpful members on this forum - but this problem`s got me stumped. The hum is not affected by volume setting and the amp still sounds as sweet as it ever has. The hum is not loud as it`s only audible at less than 2 feet away from the speakers (Focal 816V`s) but it is annoying as previously the amp was deathly quiet.

I`ve confirmed it`s definitely the amp at fault by trying it in a completely different system and the hum persists. I`ve tried the usual suspects such as; changing all the valves, cleaning pins & connections, checked all the resistor values, checked and rechecked for dry/poor solder joints etc, etc but my search has yielded nothing. All the caps (apart from the Blackgate feedback bypass caps) are relatively new and are of good quality - Jantzen, F&T, Elna Silmic 2 etc. Has anyone come across this problem before or any ideas where to concentrate the search ?

Any guidance will be gratefully received or it looks like for the first time in its 19 years the amp will have to go to an electronics engineer for repair.

Regards, David.

[bikerhifinut]

Originally Posted by shadders44david

Hi, I`m looking for some advice on a new problem with a K5881 mk2. The amp has developed a slight hum in its right channel and a 50% less hum than right in its left channel accompanied by slightly more hiss (left channel only) . I`ve owned the amp from new (1997) and can usually rectify issues with it - sometimes requiring the knowledge of the helpful members on this forum - but this problem`s got me stumped. The hum is not affected by volume setting and the amp still sounds as sweet as it ever has. The hum is not loud as it`s only audible at less than 2 feet away from the speakers (Focal 816V`s) but it is annoying as previously the amp was deathly quiet.

I`ve confirmed it`s definitely the amp at fault by trying it in a completely different system and the hum persists. I`ve tried the usual suspects such as; changing all the valves, cleaning pins & connections, checked all the resistor values, checked and rechecked for dry/poor solder joints etc, etc but my search has yielded nothing. All the caps (apart from the Blackgate feedback bypass caps) are relatively new and are of good quality - Jantzen, F&T, Elna Silmic 2 etc. Has anyone come across this problem before or any ideas where to concentrate the search ?

Any guidance will be gratefully received or it looks like for the first time in its 19 years the amp will have to go to an electronics engineer for repair.

Regards, David.

you may have left a hint by saying all the capacitors are relatively new?
How recently did you change them and could you have left a dry joint somewhere?
Or inadvertently disconnected an earth return?
It looks like you have already done this.
I was thinking you could remove the signal valves and then the phase splitters and see if the fault remains. That might help locate the stage where any fault might lie.
I suspect an earth return maybe. And I wondered if you have disturbed the heater wiring at any point? I found that was critical on a homebrew power amp. very easy to induce hum via the heater wiring, even on DC if the wires could pick up hum from somewhere else.

Andy.

[John Caswell]

Hi David,
Following on from what Andy has said, have you moved any of the wiring about or tied them together for neatness? The 5881 has long grid/anode leads and these can cause instability if close together.
A check of the cathode voltages on the 5881 will give you an idea if any of them are being overdriven by oscillation.

John

[shadders44david]

Hi Andy & John,

Thankyou for your replies and possible suggestions.

The internal wiring has not been moved or altered in any way, the amp`s not had its base removed for the last 18 months up until the present hum problem beginning.

The last work carried out on the amp was to install 4 new cathode bypass caps (C8 & C9). I fitted Elna Silmic II`s to replace a failed Nichicon - this is the 4th set of cbc`s I`ve fitted after 3 previous types have failed. There was no hum audible when these cap`s were failing/failed.

I`m inclined to agree with Andy`s suggestion of a broken/high resistance earth return but after spending considerable time looking and checking the internal wiring etc I can find no sign of this.

I`ve just checked the 4 cathode voltages and they are all 32v.

Regards, David.

[bikerhifinut]

Originally Posted by shadders44david

Hi Andy & John,

Thankyou for your replies and possible suggestions.

The internal wiring has not been moved or altered in any way, the amp`s not had its base removed for the last 18 months up until the present hum problem beginning.

The last work carried out on the amp was to install 4 new cathode bypass caps (C8 & C9). I fitted Elna Silmic II`s to replace a failed Nichicon - this is the 4th set of cbc`s I`ve fitted after 3 previous types have failed. There was no hum audible when these cap`s were failing/failed.

I`m inclined to agree with Andy`s suggestion of a broken/high resistance earth return but after spending considerable time looking and checking the internal wiring etc I can find no sign of this.

I`ve just checked the 4 cathode voltages and they are all 32v.

Regards, David.

Reading your initial description again makes me think it's not the output stages at fault.
You can try and see what happens if you unplug the first stage valves. Is this an ECF80 design with a pentode first stage and a concertina phase splitter using the triode section?
If the hum goes away or reduces then that might point to signal stage problems rather than power output stage.
If It remains, then the points to ponder might include the possibility of hum induction via or from the transformers? Would an insulation breakdown in an output transformer cause issues like this? or breakthrough from the primary onto the secondary of the mains transformer?
Here's an oddball thought. Take out all the valves on the channel that is humming and see if the hum reduces on the quieter side. That would at least isolate the issue to one channel.
And although i know you have checked all the connections again and again, i remember a frustrating fault on my KT88 WD amp that turned out to be a dry joint on a coupling capacitor that looked awfully "wet" and over a period of time it had gone O/C.

Andy

[bikerhifinut]

And a REALLY stupid question.
Did you check for this hum with any sources connected? And then disconnect them one by one?
And if you checked for hum with a source disconnected did you use "Shorting" plugs on the input? I am assuming a power amplifier.
It could be a hum loop i was thinking. Or a source or preamp introducing the hum?
Does it have an earth lift resistor, have you checked that? Is the mains earth continuity intact all the way to the mains plug?
I remember no end of fun and games chasing a hum on my Phono3 only to discover the earth lift resistor wasnt in circuit. I'm not suggesting you would do something like that but maybe its gone O/C?
As I said crazy questions and clutching at straws.

A.

[shadders44david]

Hi Andy,

Thankyou for your ideas and suggestions, all are welcome !

If I remove the input valve/s (ECC83`s) the hum becomes a loud buzz on both channels - as it always did. It could be an internal problem with one of the output tx`s, although after comparing their resistance readings they are virtually identical so hopefully they are ok.

The hum is definitely coming from the power amp as I tried it in my second system where each component is different and the noise is still present - albeit not as loud as the speakers in that system are 86db as against my main system Focals at 91.5db.

There is no earth lift resistor fitted, although the idea of installing one is beginning to appeal as a possible cure. The mains lead is solidly bonded to earth and I`ve run an earth fault loop impedance test to confirm a good house earth (I`m an Electrician by trade).

As you correctly say finding dry/poor solder joints can be fun and games and a devil to locate. I`ve again this evening checked/tested every solder joint without finding any possible candidates, but maybe tomorrow I`ll run a soldering iron over each of them.

My turn to clutch at straws, could the amp be picking up outside interference ? It does a fine job of bringing in very low level foreign radio late at night in the summer months ! This possible interference isn`t coming from inside the house as I switched off all the circuit breakers on the distribution board apart from one upstairs ring circuit and with nothing else but a CD player plugged into the amp the hum was still present. Could it be mains pollution ?

As mentioned earlier I`ll resolder all the joints tomorrow and look at fitting an earth lift.

Regards, David.

Regards, David.

[John Caswell]

Hi David,
An earth lift will always help 10R-100R with something like a 0.1µF/160V in parallel.
I am very surprised that you have had so many cathode bypass failures, this usually denotes faulty valves or grid coupling capacitors.

John

[Richard]

A bad diode or choke? I had a similar low level hum, took ages to find, where the choke was going short internally. Easiest to find either of these by substitution.

[Ali Tait]

When did you last replace the valves?