Strange volume dependent hum

might be worth an ohm check from mains to those RCA outer shells. I'm wondering if maybe there is AC being dumped through the chassis for some reason, and the path to ground is flowing through the RCA cables.

may also be worth ohm checking from ground to the outer shell of the RCA on both channels. If this has some mains to chassis short it may have burned out the connection to one RCA jack, but connecting both allows that connection to be completed again through the preamp.
 
gadget73 said:
might be worth an ohm check from mains to those RCA outer shells. I'm wondering if maybe there is AC being dumped through the chassis for some reason, and the path to ground is flowing through the RCA cables.

may also be worth ohm checking from ground to the outer shell of the RCA on both channels. If this has some mains to chassis short it may have burned out the connection to one RCA jack, but connecting both allows that connection to be completed again through the preamp.
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OK let me check this.
I did remove and test both rca sockets and they are fine.

What bothers me most is that the excessive humming is only there when both rca sockets are plugged in. Remove one and it's gone. And this is irrespective of what is connected to the rca Jack's. I've tried at least 3 preamps and assorted audio players.

All of this kinda implies that there is a circuit or a ring that, when completed causes massive hum.

Kind regards, Bert
 
So I traced the ground wires, all point to the chassis. There is a single wire bolted to the chassis as well, but it ends where all earth wires are soldered to the primary power pcb. The resistance to ground is about 4 ohms. That is nothing. Without the chassis wire I will break ground potential.

I also checked for stray ac voltage but there is nothing..

I've done my share of troubleshooting hum but this one has got me. The design is very simple, it is not a complicated amplifier. I don't think there is any problem with the actual amplifiers. They sound fine.

That leaves a cable issue. Or a cable shielding problem.

I need to do some head scratching I'm afraid

Kind regards, Bert
 
I may have missed it, but does it matter which RCA cable you pull for the hum to go away? i.e. have you subconsciously always been unplugging the L or R channel when the hum goes away?

Also, I’ve had units where there’s noticeable hum without a top or bottom cover on the goes away when one or both are reinstalled.
 
My knee jerk reaction is 4 ohms on a ground connection is far from nothing. Edit to add - unless there’s current flowing through it still. And I also wonder if the same hum happens if you put grounding plugs in the input jacks instead of cables. Good luck!
Cheers………………….Todd
 
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jheu02 said:
I may have missed it, but does it matter which RCA cable you pull for the hum to go away? i.e. have you subconsciously always been unplugging the L or R channel when the hum goes away?

Also, I’ve had units where there’s noticeable hum without a top or bottom cover on the goes away when one or both are reinstalled.
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OH I really wish that were the case . But I've tried every combination, changed cables, desoldered the rca sockets and cleaned / tested them. Rewired the left amplifier. Etc.

I opened the amplifier because of the hum. Before I merely exchanged the broken gain potentiometers (100k) and put it back together thinking it was just a routine job.

Then I changed the main filter caps and 1 subsidiary cap. I resoldered the entire power board. This had zero effect.

I will try pushing the lid on to see if there is any change

Kind regards,

Bert
 
TConnelly said:
My knee jerk reaction is 4 ohms on a ground connection is far from nothing. Edit to add - unless there’s current flowing through it still. And I also wonder if the same hum happens if you put grounding plugs in the input jacks instead of cables. Good luck!
Cheers………………….Todd
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Yes you are right, 4 ohms is not nothing. Question is what causes it. I don't see any voltages where they shouldn't be. The new filter caps are nichicon, brand new. The hum did not change after installing them.

Please let me dig up my grounding caps, we can try this. However if no appliance is connected to the rca end there is no hum. As soon as I connect anything the humming appears.

It's kinda mind boggling at this point. I could rewire and disconnect things but I don't like working without some sliver of a plan of attack

Kind regards, Bert
 
Hi all,

I found the culprits. There are two black grounding wires going to the right amplifier board. Moving these wires outside of the chassis makes the hum instantly dissappear. These wires end on the main power board where it forms a ground knot with 7 other wires.

So it appears to be a problem with interference and shielding. As soon as the wires go into the chassis, the hum is present.

At this point I don't have a solution for this issue. Perhaps the transformer is the cause. I don't understand why two ground wires would cause this problem



Kind regards,

Bert
 
must be an induced field sort of thing. Do these look like parts may be moved from original construction? I can't imagine this hummed when new.

Is the transformer grounded to chassis, or is there a shield lead that isn't connected?
 
Bert Beukema said:
I rewired the right side, all wires with new ones. Replaced the potentiometer c. No effect.

I put the Amp on an isolation transformer, no effect. On turning on the Amp it has killed the main breaker twice today. That is not right..

Obviously I'm thinking I made a mistake somewhere but as far as I can deduce all is correct. Really odd.

The hum is only present if both rca cables are inserted in the sockets. Remove one and the hum is gone.

Regards, Bert
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are the RCA inputs made from 2 press fit pieces? I have an Onkyo where the 2 press fit pieces were oxidizing and thus getting poor connection. I scuffed up the 2 parts of the RCA input and soldered them and the hum went away
 
gadget73 said:
must be an induced field sort of thing. Do these look like parts may be moved from original construction? I can't imagine this hummed when new.

Is the transformer grounded to chassis, or is there a shield lead that isn't connected?
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They are the wires as seen on the photo. They were on the bottom of the chassis behind the transformer. They end in a group ground on the main power rectification board.

From that group ground there is a wire that is bolted to the chassis bottom. The bolt and screw are secure.

It is as if these wires are picking up some massive interference. Holding them close to the transformer doesn't change the hum (much). But holding the wires outside of the chassis = 90% hum reduction.

The transformer is grounded to the same group ground.

Kind regards, Bert
 

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curtisw99 said:
are the RCA inputs made from 2 press fit pieces? I have an Onkyo where the 2 press fit pieces were oxidizing and thus getting poor connection. I scuffed up the 2 parts of the RCA input and soldered them and the hum went away
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You are correct, they are press fit. I cleaned and tested the left socket and could not find any issues. But perhaps I can do the same with the right side socket.

But it seems the primary issue is with those wires. That said, that could be part of another issue, a symptom.

Man I really dislike hum problems

Kind regards, Bert
 
Bert Beukema said:
Man I really dislike hum problems

Kind regards, Bert
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I'm with ya there! Hum, source bleed over and channel imbalance are the ones that give me the most problems. I have several units that each have one or more of those issues. Haven't gotten anywhere with any of them. Good luck!
Cheers....................................Todd
 
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