Hello Masters, i need advise hopefully to resolve hum issue.
The monoblocks are Cary 805B - they're SET DHT with 6sl7 > 300B > 211.
When they turned on and connected to speakers and disconnected from a PRE - virtually zero hum comes from speakers (klipsch La Scala with 104 db sensitivity).
When RCA cables are connected from the PRE - 50hz hum appears. Not too bad - but it's there.
Hum level lower when only one monoblock is connected to the PRE. When both are connected the hum is louder.
Basic digital voltmeter (not sure if it's True RMS) shows 0.3-0.4 mv AC at speaker outputs.
Nothing connected to the PREamp.
Power cables of the PRE and the monoblocks go to the same AC strip (no filtering).
All equipment in the room is unplugged from AC.
AC receptacles + cables are USA style - so only one way is possible.
Different sets of power cables were tested.
2 sets of RCA cables were tested (all 3m/10ft length).
2 preamps were tested.
With 2 other stereo power amps i don't have this issue - both are tube push-pull, but one chassis stereo, not separate monoblocks...
What do you think?
Is there a hope?
Or having DHT with separate chassis i'm doomed for hum?
Thanks in advance !!!!
The monoblocks are Cary 805B - they're SET DHT with 6sl7 > 300B > 211.
When they turned on and connected to speakers and disconnected from a PRE - virtually zero hum comes from speakers (klipsch La Scala with 104 db sensitivity).
When RCA cables are connected from the PRE - 50hz hum appears. Not too bad - but it's there.
Hum level lower when only one monoblock is connected to the PRE. When both are connected the hum is louder.
Basic digital voltmeter (not sure if it's True RMS) shows 0.3-0.4 mv AC at speaker outputs.
Nothing connected to the PREamp.
Power cables of the PRE and the monoblocks go to the same AC strip (no filtering).
All equipment in the room is unplugged from AC.
AC receptacles + cables are USA style - so only one way is possible.
Different sets of power cables were tested.
2 sets of RCA cables were tested (all 3m/10ft length).
2 preamps were tested.
With 2 other stereo power amps i don't have this issue - both are tube push-pull, but one chassis stereo, not separate monoblocks...
What do you think?
Is there a hope?
Or having DHT with separate chassis i'm doomed for hum?
Thanks in advance !!!!
This sounds like a typical ground loop. Can you measure the resistance between the input RCA ground and the chassis? (With the amplifier turned off, and every cable disconnected).
If it is zero, this is a design issue. Search for "ground loop breaker" on this forum or elsewhere.
If it is zero, this is a design issue. Search for "ground loop breaker" on this forum or elsewhere.
Read this article.
Try to follow the the "analogue ground"-chassis ground point-mains safety ground point routes in preamp and power amp too (if possible).
Use same distro board for pre and power as there suggested.
p.s.
Is RCA cables are shielded ones?
Try to follow the the "analogue ground"-chassis ground point-mains safety ground point routes in preamp and power amp too (if possible).
Use same distro board for pre and power as there suggested.
p.s.
Is RCA cables are shielded ones?
Thank you all for the ideas!
The RCA cables i used are shielded.
Preams tested - Audio Research Ref6 and VTL The Ultimate.
Resistance between the input RCA's ground and the chassis - i assume you mean RCA's "cold side" ie the outer round jack if the RCA.
It measures zero (or next to it).
But same measures on my stereo power amp - which is absolutely quite.
I also tried XLR cable between preamp and monoblocks - the hum is much lower but is still there.
The DVM actually shows 0.00 mv AC at speaker outputs (instead of 0.3). There is still a hum though audible from listening position during quite music sections.
I would use XLR but my good cables are RCA, CARY's xlr input section is not great design and there is still hum...
The RCA cables i used are shielded.
Preams tested - Audio Research Ref6 and VTL The Ultimate.
Resistance between the input RCA's ground and the chassis - i assume you mean RCA's "cold side" ie the outer round jack if the RCA.
It measures zero (or next to it).
But same measures on my stereo power amp - which is absolutely quite.
I also tried XLR cable between preamp and monoblocks - the hum is much lower but is still there.
The DVM actually shows 0.00 mv AC at speaker outputs (instead of 0.3). There is still a hum though audible from listening position during quite music sections.
I would use XLR but my good cables are RCA, CARY's xlr input section is not great design and there is still hum...
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Yes, when i engage feedback the hum level drops.
But sound volume drops too - so maybe that's why.
Anyway - the sound quality with feedback sucks, so i keep feedback OFF.
Also i've tried to increase distance between amps and kill all the lights in the room (so only pre+powers are consume AC) - didn't help...
But sound volume drops too - so maybe that's why.
Anyway - the sound quality with feedback sucks, so i keep feedback OFF.
Also i've tried to increase distance between amps and kill all the lights in the room (so only pre+powers are consume AC) - didn't help...
Can an approach of following 2 products help?
https://www.synergisticresearch.com/isolation/ground-isolation/grounding-block
https://www.soundonsound.com/reviews/groundology-earth-connection-plug
In basic terms they create a star connection between AC outlet GROUND to all audio devices body earth/ground and/or RCA/XLR's "minus".
https://www.synergisticresearch.com/isolation/ground-isolation/grounding-block
https://www.soundonsound.com/reviews/groundology-earth-connection-plug
In basic terms they create a star connection between AC outlet GROUND to all audio devices body earth/ground and/or RCA/XLR's "minus".
Recently a fellow on another forum was having the same problem with his Cary amps and an Audio Research preamp. He also had a VAC preamp which was totally quiet. Have you tried "cheater" or ground lift plugs on the preamps or amps? Not a good permanent solution but it might tell you something.
According to Stereophile's measurements, the gain at 0dB feedback is 7dB higher than the gain at 10dB feedback!
What stereo amps are they?
This corresponds to what i hear - or am i getting this wrong?
The other power amps are Viva 300P and Harman Cardon Citation 2.
Both are tube push pull stereo power amps, with SS power supply rectification.
Cary also is fully SS rectified.
Try to use the power cable of one amp without the ground connection, central pin of the plug. You can use a cheap pc power cable for test
So you cut a probably loop; the ground is assured by the connection by signal cable.
Or you can put on the receptable on each amp a 10 ohm resistor in series with the ground cable that goes to chassis; it also break the loop
Walter
This is correct because hum only occurs when a preamp is connected, so the higher the gain of the power amplifier, the higher the hum noise level. But that doesn't explain why there's no hum when connected to the Harman Kardon Citation 2 (which has about the same gain as the Cary).
My suggestion is following advice from post #9 and #12, try using a "cheater" plug on the preamp power cord to see if the hum goes away.
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Try this, put the cheaters on the source device(s), leave the monoblocks on 3 prong cords. I had the same issue when I added a Schitt Audio DAC to my system. It had a safety ground, it hummed 60hz. I opened the instructions that came with the DAC, and the mfg instructions were simple, it said, "if you hear hum when using this DAC, plug the DAC into a 3 prong to 2 prong adapter". Hum disappeared.
Safety ground (PE = Protective Earth) is for safety indeed. It connected to the chassis not for fun. Never disconnect the safety ground in an IEC Class I device.
The ground loop should be broken by disconnecting the signal ground (= power supply ground) from the chassis. You have to find the point, wire, screw, whatever where they are connected and break it! In order to not leave the signal ground floating, make an electrical connection with the aforementioned 10 ohms resistor.
This is still not 100% safe. The ultimate solution is to add antiparallel high-current diodes parallel to the 10 ohm Ground Loop Breaker resistor. A 35 A rectifier bridge will do. Picture from one of the million threads on this subject.
Attachments
You can have the safety connection also with 10 ohms and by the cold connection of signal cable
Then after the test you can fix the issue
Walter
Everything you should know about ground loops (and how to eliminate them) is written here:
https://hifisonix.com/technical/ground-loops/
https://hifisonix.com/technical/ground-loops/
"You can have the safety connection also with 10 ohms".
No.
Any failure of this resistor causes "non safety grounded" conditions.
BTW if you have -for example- 16A circuit breaker, it "working" at even 4x current (releasing at about 5x current), so "live"-chassis shorting push even 64A to this 10R resistor, thus dissipation immediately evaporates the resistor and chassis would be at life-threatening potential.
No.
Any failure of this resistor causes "non safety grounded" conditions.
BTW if you have -for example- 16A circuit breaker, it "working" at even 4x current (releasing at about 5x current), so "live"-chassis shorting push even 64A to this 10R resistor, thus dissipation immediately evaporates the resistor and chassis would be at life-threatening potential.
You are always connected to ground by signal cables and 10R
Of course , the electric circut in the house must be fine and the differential switch is in order
To check the the loop this is a good test
Then you can investigate if the problem disappaer and take the right solution.