JO3533, yes by all means try to somehow post or link to the schematic you have for the amp. I’ve tried to find one online and have not been able to.
Jim, a couple of your recent statements have touched upon what I’ve been thinking that led me to suggest putting shorting plugs on the RCA inputs. Without a schematic I’m shooting in the dark to some extent, but here is my speculation:
First, even if there is some commonality between the input circuitry for the RCA inputs and the XLR inputs, given that the input impedance of the RCA inputs is vastly higher than it is for the XLR inputs, and given that per JA’s measurements the gain of the RCA inputs is even significantly higher than for the XLR inputs, I can’t envision a scenario in which putting shorting plugs on the RCA inputs would adversely affect the signals on the XLR inputs. (In contrast to the design approach that is used by many amps providing XLR and RCA inputs, in which the center pin of the RCA connector is connected directly to pin 2 of the XLR connector, with the balanced/unbalanced switch simply grounding pin 3 of the XLR connector when the switch is set to unbalanced).
Second, I would discount as irrelevant all of the findings in which hum occurred when an unterminated XLR cable was connected, and when an XLR cable was connected to the AVR while the AVR was unplugged from AC power (which probably also represents an essentially unterminated condition).
Third, what I’m envisioning relates to Jim’s comment that:
If jo3533 disconnected all ICs between the Marantz and Adcom, powered up both units, and then checked with an AC volt meter for an AC voltage from one chassis to the other chassis he should measure zero AC volts, Correct?
I’m envisioning that the Marantz AVR has significant AC leakage to its chassis and its signal ground (which Jim had indicated earlier are common with each other), the leakage occurring either in its power transformer or perhaps as a result of a leaky line filter capacitor. As long as the leakage is within reason, it won’t cause a problem under **most** circumstances.
Now imagine hypothetically that the amp has an infinite input impedance. Since the AVR has a two-wire power cord, the amp would then see the input signals it receives as "floating" relative to its own safety ground and signal ground. Meaning that the two signals in each balanced signal pair would both be seen as riding up and down over a significant voltage range at a 60 Hz rate. In other words, a significant amount of 60 Hz "common mode" noise would be present. The finite (and relatively low) input impedance of the amp’s XLR inputs will then greatly reduce that range, but I’m envisioning that there may still be enough 60 Hz common mode noise on the signals, relative to safety ground and to the amp’s signal ground (which is probably connected to safety ground through a low impedance) to cause the issue. And conceivably it could be causing the issue by coupling into the very high impedance of the RCA input circuitry (although other pathways are possible as well). Which is why I am suggesting the shorting plugs, assuming I am correct in thinking that they will not affect the signals received via the XLR connector.
To respond to some other questions that have been raised: The polarities of pins 2 and 3 on the XLR connectors of the components have no relevance to the issue. I wouldn’t expect a DC offset in the AVR’s outputs to be relevant either. For relatively low-priced but high quality XLR cables I suggest Mogami Gold Studio, in either a
6 foot length or a
3 foot length. Mogami Gold Studio is the de facto cable of choice in most recording studios and other pro audio applications, and has been used in home audio applications and recommended here by me and many others in the past. Although as is usually the case among audiophiles opinions about it are not unanimous.
Good luck. Regards,
-- Al
