Ground loop problems

[BikerBod]

Applogise if this is a question which has been asked before but I've scoured this forum, tried all of the suggestions put forward and still have problems. I recently built my 6n6p tube DAC into a steel chassis. Prior to this I had it built in a plastic storage box as a prototype. Since boxing it up I've suffered many problems with hum caused by various ground loops. Initially I made the big mistake of using the chassis as the main signal ground but I have since lifted the signal ground from the earthed chassis via back to back diodes and a parallel resistor/capacitor. I have also decoupled the signal grounds at the RCA output sockets to chassis using 0.1uf discs. I have physically isolated the transformers from the chassis using paxolin board and earthed their metal parts to the chassis. The heaters are smoothed DC and hum remains when their power is removed.

These modifications changed the characteristics of the hum but have not eliminated it. I can however completely stop all hum by disconnecting the screen of either one of the RCA cables.

Can anyone offer any advice based on previous experience of this kind of ground loop.?

Any advice would be appreciated as I'm at that stage where I feel like throwing the damn thing in the bin!

 

[Jerms]

Reads like you've tried lots more than I have. I ended up using a £6 ground loop isolator box from Amazon. Prior to that, my only work around was to power my source/DAC separately from my amp.

Do you lose signal from the RCA with the disconnected ground? Without lifting the screen, can you merge the grounds without causing other problems?

 

[DF96]

If disconnecting one RCA screen cures the hum then maybe the stereo pair of screens act as a ground loop? Having a steel chassis provides good scope for coupling magnetic fields from here to there.

 

[Jeffs]

I'm reading your statement as the original plastic box had no problems, then putting it in metal is when you have the issues. I originally tried wood, and didn't have noise then went to aluminum where it all went downhill.
I've been fighting similar issues, haven't tried as many solutions. When running input from a television I get a bad ground loop. It goes away if I (just to test... ok, I watched one movie) use plug adapter which removes the safety ground.
The last comment - one RCA removed cures the hum and stereo pair acting as ground loop. Had similar issues but it was grounding one channel to the chassis would cause the other channel to become quiet but not both ways. I was using an aluminum chassis originally, then removed the RCA jacks and let the lie over the edge of the chassis. Putting the input and output cables in steel tube I had laying around, about 150mm each with the steel tube resting on the edge of the aluminum chassis, the RCA jacks dangling off the ends - it was ridiculous but quieted it down. Mainly 60hz and some 120 hz hum.
I lifted the grounds from the chassis / safety ground with a thermistor and it helped some.
Maybe there is a market for a nice non-flammable plastic and wood chassis out there for those going insane.

 

[rayma]

I can however completely stop all hum by disconnecting the screen of either one of the RCA cables.

Try a 10 Ohm resistor from each input RCA socket ground lug to the circuit ground, instead of a direct connection.
It sounds like you may have those input ground lugs connected together.

 

[BikerBod]

Try a 10 Ohm resistor from each input RCA socket ground lug to the circuit ground, instead of a direct connection.
It sounds like you may have those input ground lugs connected together.

I'll give this a try next time I'm in the workshop. Thanks. It did cross my mind to try this but I figured that It's a sticking plaster type solution rather than getting to the root of the issue. But I guess the steel chassis is causing hum induction into the circuit and the only real solution is to go to a non-ferrous enclosure.

 

[BikerBod]

It sounds like you may have those input ground lugs connected together.

I forgot to mention, the valve stage takes its inputs internally from an ak4396 DAC board. The RCAs are the outputs from the valve stage. And yes their ground tags are connected together with a single thick wire joining this to the star ground.

 

[DF96]

Socket ground tags should be wired alongside the signal conductor, not separately. This is because the 'ground' is actually the signal return. By sending the signal return currents via your 'single thick wire' you have introduced a big loop in the output wiring, which may pick up hum magnetic fields.

The sockets should normally be isolated from the chassis, otherwise you get another ground loop, but I think you did this.

 

[BikerBod]

Thanks DF96, I had never thought of this. It never occurred to me to separate the returns from both sockets because the run from the RCA to the valve is a matter of about 10cm. I'll let you know how I get on.

 

[BikerBod]

Thanks for your advice guys. I didn't have any 10R resistors in the box so tried 100R Instead for now. I also ran separate ground returns to the star from each RCA socket and voila, ground loop has almost completely gone. All that remains is a very low level 60hz hum when my amplifier is turned up to unsociable levels with no music playing. I would still like to completely eliminate this background hum , maybe 10R Resistors would improve this? Thoughts?

 

[rayma]

I would still like to completely eliminate this background hum , maybe 10R Resistors would improve this?

The 10 Ohm values would be best, but those may not eliminate the hum, if there are other problems.
Can you post a clear photo of the wiring and board?

 

[BikerBod]

Thanks for all the advice on this issue. I decided on a design change in the end. I've done away with the SRPP stage which had too much gain and was prone to all kinds of noise, RFI and hum. Instead I have used the same 6n6p tubes but configured them as a broskie cathode follower which accepts balanced signals from the ak4393 DAC.

Just sat listening now. No hum whatsoever, very black background. I'm liking it a lot better than the SRPP. Sounding very good.