Isolating Digital Noise, need help.

Thanks, jea48.
First of all, that is my amp...Sunfire 300 Load Invariant.

What is odd though, from your measurements, the safety equipment grounding conductor of the power cord is connected to the signal ground. You measured 1 ohm of resistance. Correct? That’s basically "short".

That's correct. The digital scale measured 1.0 ohm +or- a couple units since the probes were hand-held. It never approached zero.

I've never had the top off, but it was serviced 3 years ago and maybe the wire could have come off. I'll have to wait til I have some time to devote to opening it up since the amp is not easily removed from my rack. I built a custom shelf for it.

Check for continuity from the outer contact of one of the RCA input jacks to the metal chassis. It technically should read "short", "000.0", continuity. But from what you have said from the measurements you have taken, you should read "OL" open circuit. The signal ground is not connected to the metal chassis.

I will do this tomorrow. It seems that you and Ralph are right about the amp not being properly grounded.

I assume you have the jumpers installed in XLR female connectors.

I've never heard any mention of using jumpers on this amp; not in the manual or on Carver/Sunfire forums. Can you explain?

I know that this amp is SE and the RCA and XLR are wired in series. The next generation of the 300 amp is supposed to be fully balanced and has specs to indicate this.
My amp only has a single input sensitivity of 1.64V.

Well, I have some test results to report.
I used an AC receptacle across the room as a second line to separate the components from the amp. It is a shared circuit with ground.
The amp was plugged into it’s dedicated line and the Furman strip with preamp (using cheater-plug), and CDP was plugged into this 2nd line. Both components using Audience PC’s.

1. The result was no high-pitched noise from the CDP. This was tried with the Jenson transformer in-line and also removed. Result was the same; a quiet tweeter.
So, I believe this proves that the ARC CDP is introducing noise into the shared dedicated line and needs to be isolated.

2. Ground-loop issue: used a 3-prong PC from the preamp into Furman and into 2nd AC line.
The amp is plugged into dedicated line with 3-prong (usual setup).
The result was a low-level hum thru the speakers and not the loud 60Hz hum.

The amp is now known to have a grounding issue, but is it possible that that this low-level hum could be due to the fact that the second line’s grounding is unknown?
IOW, I think it is worth running another dedicated line. Ideally 3 lines would be best, but cost is an issue.

Your thoughts?

I believe this proves that the ARC CDP is introducing noise into the shared dedicated line and needs to be isolated.

Yes, but per my previous post perhaps the extension cord itself would provide sufficient isolation, when plugged into the outlet that is normally used. Seems worth trying, on both the CDP (the source of the noise), and the preamp (which is probably where the noise is causing the trouble).

The amp is now known to have a grounding issue, but is it possible that that this low-level hum could be due to the fact that the second line’s grounding is unknown?

That is very conceivable. Jim (Jea48) has previously provided us with a link to a paper by Bill Whitlock (the Jensen Transformer person, although he may be retired now) in which it is explained that ground loop hum is usually caused or contributed to by coupling of EMI between the hot & neutral wires and the safety ground wire in the house wiring. Which occurs very minimally with Romex, due to its symmetrical design, but occurs to a much greater degree with wires in conduit. Is there any way to determine what kind of wiring is used between that outlet and the breaker panel?

Also, Jim (Jea48) makes a good point about jumpers. Given that there is no balanced/unbalanced switch on the amp, and that it has an unbalanced internal signal path, I suspect that jumpering pins 1 and 3 together on each XLR connector might be helpful, with respect to hum and/or sonics. Check for continuity between the center pin of each RCA connector and pin 2 of the corresponding XLR connector. Also check for continuity between the ground sleeve of each RCA connector and pin 1 of the corresponding XLR connector. I suspect you’ll measure close to zero ohms in both cases. If so, I would suggest that at some point you obtain some solid core wire of a gauge that would provide a snug fit and use it to jumper pins 1 and 3 on each XLR connector.

BTW, "OL" is most likely derived from either "out-of-limits" or "overload," and when you are measuring resistance signifies that the resistance is greater than the 20 million ohm upper limit of what the particular meter can measure. In other words, an open circuit.

Best regards,
-- Al

thanks Al. Will do tonight.