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Vintage Audio (record players, hi-fi etc) Amplifiers, speakers, gramophones and other audio equipment.

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Old 24th Sep 2023, 5:50 pm   #1
Gabe001
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Default Champion 835 with hum

Hi folks

Just a quick questions regarding a champion 835 receiver I'm working on. To be honest it didn't need much to get it going. I'm using it with a 75ohm ribbon aerial and it receives lots of stations.

Note that this is the receiver (not radio) with e series valves, and no output transformer or speaker. This is the model but the radiomuseum description is wrong.

https://www.radiomuseum.org/r/champion_835.html

The problem I've got is that when I hook it up to my amps (all of them) it is really "hummy". The hum starts instantly upon switch on, and is present without the valves in the set, so I think that this is magnetic coupling from the mains. It varies to some extent according to the Champion's volume control setting. I strongly suspect that the problem is the switched volume pot. It's a 500k log pot that switches both mains live and neutral, it measures well and doesn't crackle, but it's the only location where the unshielded signal is in close proximity to the mains. See pictures

In your experience, is this issue resolvable with a new pot, or should I simply route mains live away from the signal and only switch mains neutral (potential safety issue?)?

Thanks
Gabriel
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Old 25th Sep 2023, 12:43 am   #2
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Default Re: Champion 835 with hum

The wires going from the mains switch to the mains transformer should ideally be twisted and routed as far as possible from the low level audio wiring.

The output coax from the volume control to the audio output socket needs to be grounded with great care.

The audio signal going to the 'top' of the volume control should be in shielded cable.
Again, grounding is critical.

The audio ground should be kept well away from the chassis connection of the reservoir capacitor.
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Old 25th Sep 2023, 1:57 am   #3
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Default Re: Champion 835 with hum

Is the Discriminator capacitor grounded at the same tag as the heater and HT windings?
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Old 25th Sep 2023, 6:44 am   #4
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Default Re: Champion 835 with hum

Quote:
In your experience, is this issue resolvable with a new pot
, Yes. Had the same issue with a WEM guitar amp. A combined vol pot and mains on/off SW is a daft idea.

Andy.
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Old 25th Sep 2023, 1:24 pm   #5
Gabe001
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Default Re: Champion 835 with hum

Thanks for the replies. It's clearly "radio-fi" not "hi-fi" and the mullard 3-3 with 89db sensitive speakers I've connected it to takes no prisoners. The grounding scheme isn't ideal I agree and the wires should have been twisted but it came like this from the factory. It's never going to be completely quiet without major surgery.

Andy, did you split up the on-off switch and volume control, or did a new switched pot improve things?

Last edited by Gabe001; 25th Sep 2023 at 1:46 pm.
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Old 25th Sep 2023, 1:58 pm   #6
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Default Re: Champion 835 with hum

Unfortunately the use of such high value pots in valved gear, for obvious reasons, and especially directly at the output of such a unit makes it very difficult to nigh on impossible to get a hum free output as there's simply not enough "back termination" for the hum, the coupling of which is itself high impedance in cases like this. This hum mechanism causes the hum to increase with vol control setting up till at least its electrical midpoint and then if the driving circuitry is "only" say 50K with a 500K pot it can reduce again towards the top end of travel as there is greater back termination from the lower impedance of the units output.... this assumes completely hum free operation of the unit itself of course so with older and not especially hi in fi gear this aspect can often be masked by the poor S/N ratio hum wise of the unit itself.
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Old 25th Sep 2023, 4:04 pm   #7
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Default Re: Champion 835 with hum

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Andy, did you split up the on-off switch and volume control, or did a new switched pot improve things?
Yes I did. I replaced the On/Off SW and moved it to the back of the chassis leaving the pot as is on the front. It reduced the hum significantly.

As a trial unsolder the mains wiring from the SW on the back of the pot and tack together, then move them away from the pot to see if it helps. I always twist my L & N up to a SW then same twisted pair back to the tfmr.

Andy.
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