Loud buzz with Marantz receiver preout to amp

SOLVED! I had to run a spare speaker wire between the chassis ground on the Marantz to the chassis ground on the Ashly. No more hum! Thanks for the help, everyone.

I am suggesting that perhaps your "RCA cable" does not have long enough positive pins to properly engage the signal through the jack.  Are the pins on your cable as long as shown on that jumper?

Thanks, @toddalin — yes, I have four of those: One for each set of Preamp-to-Mains (front and rear). I removed the top two (front) so that I could plug in the preamp output to the amp. The mains are sitting empty — I tried them with an RCA cable between the rear preamp out and the Front mains in (as the manual suggests so that you can have the Marantz running power to the rear speakers). I don’t use rear speakers, but I tried it just in case, and it didn’t make a difference in the hum.

OK, it is my recollection that Marantz jumper plugs have an extra long pin to engage the internal connection or such. 

I had actually considered this, but it seems like you would end up with no sound, rather than a loud hum..., but who knows?

This is a Marantz Jumper.  Note how long the positive pins are.  You can readily find examples on ebay.  

Making progress: The amp has a clip that connects its “input” and “chassis” grounds. The manual states that while the clip should remain there in most cases, a ground loop can be helped by removing it. I removed it, and while a loud hum remains, I DO finally get sound from the Marantz’s preamp output — the only change I’ve made that’s done that. The hum seems to be improved slightly by reversing the AC plug into the power strip, as you mentioned, @jea48 — thank you. I removed all the other source inputs — no difference.

The Ashly makes a slight hum while it’s on that’s coming from inside its chassis. This is regardless of speaker output — it remains audible even with the gain turned all the way down, not from the speakers, from the amp itself. The hum sounds like the little version of what comes from the speakers when the gain’s turned up. The Marantz is silent when on — no buzz or hum coming from inside its case.

I wonder if getting an RCA-to-XLR box and the associated cables would improve the situation at all. I’ve been looking around the house for a ground cheater plug. May need to run to the HW store…

Thank you for the response — I sent images to Ashly tech support and they didn’t call out a lack of grounding in the connection, but maybe that’s part of the puzzle. Another odd thing is that my Marantz works with the preamp jack engaged — and audio has to go through the preamp circuit into the main front in that case. That would indicate, as I understand it, that the preamp works. I’ve read of a couple other people trying to connect their Marantz units to external amps and getting no audio output, so I wonder if I’m just missing something — or if this is a common fault of vintage Marantz units… Just weird that the preout works perfectly when connected to its own internal amp.

Sounds like you have no ground connection between the preamp and amp and maybe should be using using that/those ground terminals.

Also, some interconnect cables are only grounded on one end, not both, and maybe this is the case here. If you have an ohmmeter, be sure that both the ground and hot leads are coming through the cables (i.e., check continuity).