Hum Reduction

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SimonC
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#1 Hum Reduction

Post by SimonC »

Hi everyone,
I'm now the proud owner of a 'WAD' 5881 PSE, there's not much left of the old WAD circuit as its got an akido driver and reworked output stage -just the iron and case. It sounds great, but has one small problem in my system - hum. Not loads, but enough on my speakers to be intrusive. (It sounds like mains hum, not motor boating/squealing or anything exciting like that).
As I like to be systematic in the way I approach things, and to understand more about how hum is coupling to the output of the amp, my plan of attack is to do these and see what affects things:

De-couple the circuits 0v from the mains/chassis earth with a 10R/cap filter (I know I need to isolate the input phono's from the case for this to have any effect).
Temporarily power the heaters by DC.
Fit a humbucker pot.
Float the heater supply to around 50-60v.

Is there anything else I should add into the mix?

Thanks
Simon C
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pre65
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#2 Re: Hum Reduction

Post by pre65 »

Decoupling the 0v would be a good 1st step. :)
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IslandPink
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#3 Re: Hum Reduction

Post by IslandPink »

I don't know if you did the following steps Simon, but make sure it is in the power amp first -
remove interconnects
short the input sockets to ground with clip-leads.
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pre65
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#4 Re: Hum Reduction

Post by pre65 »

pre65 wrote:Decoupling the 0v would be a good 1st step. :)
I know that you know this Simon,

The original amp circuit does show an earth lift resistor and cap.

Perhaps (as the amp is not standard) you could check that all grounds go to a central point ?
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Mike H
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#5 Re: Hum Reduction

Post by Mike H »

SimonC wrote:I know I need to isolate the input phono's from the case for this to have any effect).
I always do this if it's a metal case and is mains earthed, which it usually is. Always use an earth lift resistor too.
 
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SimonC
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#6 Re: Hum Reduction

Post by SimonC »

@Mark: Yes, I've got a pair of shorting plugs made from a give away set of phono leads and a couple of 1k resistors. I find it easier and quicker that clip leads (I can never get them to stay on when I want them to :) )

@Phil: The entire amp had the earth lift applied, mains earth as well. I've drilled and bolted a new earth bond direct to the chassis, fitted some insulated phono sockets and traced the 0v back through with a meter to make sure I've lifted it via the 10R, plus star earthed the driver stages back to the main smoothing cap. So far no change that I can hear.

Onwards...
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cressy
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#7 Re: Hum Reduction

Post by cressy »

Is the heater wiring a twisted pair or is it 2 straight pieces? And is it routed anywhere near the inputs? Shouldn't be too much of a problem with idh valves but as you know can cause issues if it's near something sensitive
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Mike H
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#8 Re: Hum Reduction

Post by Mike H »

Here's a mad crazy idea - pull all the valves out, turn it on, does it hum?
 
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pre65
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#9 Re: Hum Reduction

Post by pre65 »

According to the original circuit diagram the choke in the power supply is 1H.

Seems a bit small ?
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SimonC
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#10 Re: Hum Reduction

Post by SimonC »

Found a bit of time over the weekend to investigate a bit further, it looks to be a combination of lots of little factors.

The heater wiring isn't perfect, there are some loops around the output valves that could do with tidying. Its also shown up a grounding 'feature' in my pre-amp that a croc lead back to the main amp has fixed.

The biggest part it seems to be some form of instability though. It can be almost silent until it gets an input signal, then once the hum starts it stays. The only way to quiet it down is to ground the inputs for a couple of seconds. It then stays quiet until it gets more music to play. It feels like its hovering on the edge of instability, time to trace the circuit and dig out my ancient scope (I don't have an AM radio anymore to do a quick check to see if its pushing out HF mush, the wonders of living in a digital age).

@Phil, I think thats a typo, I seem to remember that the normal WAD power supply filtering was a 10H choke on pretty much every power amp they designed.

Onwards :)
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pre65
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#11 Re: Hum Reduction

Post by pre65 »

As the input circuit is non WAD, does it have grid stopper resistors ?

Well done on your results so far. :)
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Mike H
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#12 Re: Hum Reduction

Post by Mike H »

Could be low freq oscillation, but I would have thought if that were the case it would be full tilt, not just a little bit, and your speakers will be flapping in and out. (I've had some of this! See my EL84 amp blurbs earlier in 'Nothing In Particular'.)

Alternatively could be RF oscillation, inaudible obviously but can manifest as an audible hum. Sort of as a by-product. Grounding the inputs kills it, until as you say, set it off again by putting a signal in.

What valves are used? Using ones that were designed for RF usage will encourage RF instability and RF oscillation if there's nothing to stop them operating in that band. even power valves can do it due to the higher than average (for a small signal valve) anode current hence the use of grid stopper resistors is common for o/p stages.
 
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SimonC
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#13 Re: Hum Reduction

Post by SimonC »

Hi Mike,

Its using ECC83 / 6n6p configured akido style. I'll have a poke around and trace the circuit as all the resistors are soldered direct to the sockets. I need to figure out which are part of the akido and which are purely grid stoppers (or not). The outputs are paralleled 5881, but re-biased and with no UL tap connected.

My gut feel is that its HF instability with a bit of mechanical microphonics thrown in, but until I can get it hooked up to see/hear whats happening that is just guesswork.

Onwards :)
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pre65
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#14 Re: Hum Reduction

Post by pre65 »

Simon, have you got an oscilloscope ?
The only thing necessary for the triumph of evil is for good men to do nothing.

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SimonC
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#15 Re: Hum Reduction

Post by SimonC »

I've got this one back on the slab again. After a bit of fettling and playing around I can get hum free inky blackness under some conditions, but not others.

I'm using a HI-Fi berry DAC on a raspberry pi as the source. Direct into the amp and software volume control via the app. I get the same behaviour with other sources (Squeezebox classic etc), and with or without a passive pre, and with different interconnects, so I'm sticking with the pi for testing as its small and easy to fit on the bench.

Shorting both inputs to ground = silence
Shorting one input to ground and connecting the other to Pi - slight hum on the channel connected to the pi. Nothing major, just enough to be audible if the room is quiet. The shorted channel is silent.
Connecting both inputs to the pi - large hum on both channels. Music plays but the hum is always very noticeable.
Disconnecting one channel from the pi and leaving it floating - silence on both channels. Music from the connected channel is hum free and clear. Swapping from one channel to the other has the same effect.

It's behaving like there a ground loop happening when I connect both inputs to a source, but there is only one connection to ground (at the amp, via a 10R and a cap) so I'm struggling to get my head round where this loop is coming from...(The answer is probably staring me in the face when I find it)

All suggestions gratefully received etc :)

Simon C
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