[SOLVED] Ground loop between grounded Hypex NC400 DIY amp and ungrounded Denon 3700X

[andy497]

That’s interesting that you mention that. I will try that later this week. Just haven’t got to it yet.

I bought an Ifi gnd defender and the hum went away completely on my center but only lowered the hum level on my fronts.

I still wonder why both Hypex and Purifi (and others) consider disconnecting the shield on RCA side to be the incorrect way of fixing hum.

Also are you connecting an unbalanced source to a balanced input? Actually a RCA-XLR cable? Not the opposite? XLR-RCA.

Yes I'm connecting unbalanced (Denon AVR as source) into balanced (a bunch of pro audio amps with XLR in). I don't know why they say disconnecting rca shield is bad and/or bridging 1 and 3 pins at XLR side (as seen in minidsp documentation) is good, but you see that everywhere.

Consider this minidsp diagram:

I made that jumper from S to Negative for the input. When I touch it to the terminal with all components powered up and idle I'm blasted with maybe 80 db of 60hz+harmonics background hum. It's better with the jumper off but by no means silent. I've been living with this for years but finally achieved silence today with the above-mentioned cable. I plan to do my LCR next and have the monoprice cables on order, but I think I might actually need plain XLRs + solder on my own RCA ends. If the monoprice cables don't have a twisted pair *inside* the shield that then gets grounded to the XLR side I think it might defeat the purpose.

For further reading, this paper by Bill Whitlock from Jensen is great and they show proper transformer-based solutions, but they are super expensive. Do I really need to buy four Jensen boxes to filter 8 channels at ~$300 a piece???

https://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.305.873
sample:

 

[peng]

Yes I'm connecting unbalanced (Denon AVR as source) into balanced (a bunch of pro audio amps with XLR in). I don't know why they say disconnecting rca shield is bad and/or bridging 1 and 3 pins at XLR side (as seen in minidsp documentation) is good, but you see that everywhere.

I did not disconnect the shield at the RCA end and it works well, barely audible noise with volume at +18, virtually silent at 0, and I am very sensitive to hiss and hum. There seems to be no reason to disconnect the shield at the RCA end but I guess if it only works for you with the shield disconnected then so be it.

Monoprice cable connection diagram:

It looks to me it is the same as per Hypex's recommendation.

then

 

[MASKINEN]

Yes I'm connecting unbalanced (Denon AVR as source) into balanced (a bunch of pro audio amps with XLR in). I don't know why they say disconnecting rca shield is bad and/or bridging 1 and 3 pins at XLR side (as seen in minidsp documentation) is good, but you see that everywhere.

Consider this minidsp diagram:

I made that jumper from S to Negative for the input. When I touch it to the terminal with all components powered up and idle I'm blasted with maybe 80 db of 60hz+harmonics background hum. It's better with the jumper off but by no means silent. I've been living with this for years but finally achieved silence today with the above-mentioned cable. I plan to do my LCR next and have the monoprice cables on order, but I think I might actually need plain XLRs + solder on my own RCA ends. If the monoprice cables don't have a twisted pair *inside* the shield that then gets grounded to the XLR side I think it might defeat the purpose.

For further reading, this paper by Bill Whitlock from Jensen is great and they show proper transformer-based solutions, but they are super expensive. Do I really need to buy four Jensen boxes to filter 8 channels at ~$300 a piece???

https://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.305.873
sample:
View attachment 175192

I only get a small amount of hum with shield connected at RCA but I must try with it disconnected as well. It’s worth a try at least.

 

[andy497]

I only get a small amount of hum with shield connected at RCA but I must try with it disconnected as well. It’s worth a try at least.

Well my monoprice cables arrived today and I can report a major improvement. Probably lowered my noise floor by 20 db. I wish I captured some RTA in REW before/after, but oh well. That's probably good enough for now, but I'll reevaluate soon when the hypex amps I ordered arrive.

 

[MASKINEN]

Well my monoprice cables arrived today and I can report a major improvement. Probably lowered my noise floor by 20 db. I wish I captured some RTA in REW before/after, but oh well. That's probably good enough for now, but I'll reevaluate soon when the hypex amps I ordered arrive.

That’s great to hear!
Did you modify the Monoprice RCA-XLR cable or kept it original?

 

[andy497]

Nope, no modifications. I peeked under the metal connector ends to try to get an idea of their construction, but they have an additional heat-shrink insulation layer so you can't see exactly how they're soldered. Nice cables though.

 

[peng]

Nope, no modifications. I peeked under the metal connector ends to try to get an idea of their construction, but they have an additional heat-shrink insulation layer so you can't see exactly how they're soldered. Nice cables though.

You can see that in the diagram I attached in post#82.

 

[Jdunk54nl]

Another thing for people to try is running a wire from a ground location on their avr’s (phono ground works well for this) to a case screw on their amp (make sure it can touch bare metal).

I had a weird hum coming through and tried all the variations of rca to xlr cables. The best was rca side shield and cold connected and then all pins on xlr separate. Even then still had noise. Connected that jumper ground cable between avr and amp case and noise 100% went away.

 

[MASKINEN]

Nope, no modifications. I peeked under the metal connector ends to try to get an idea of their construction, but they have an additional heat-shrink insulation layer so you can't see exactly how they're soldered. Nice cables though.

Thanks for the information. Monoprice cables are soldering like this. I actually opened up my own Monoprice cables and made them shorter. I also remove the bridge between pin 1 and XLR shell. No difference.