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How to connect monoblocks to a single REL sub with ONE RCA input

I just purchased two Quicksilver 60w Monoblock amplifiers and a LineStage Preamp by QS.
To connect this REL 328, normally one uses connect their Speakon cable (Red, Yellow, Black ends that connect to Right+, Left+, and Ground) or a SINGLE "low-level" RCA in. But this preamp doesn’t have a single sub out.

REL’s advice is to buy a second sub and attach one sub to each monoblock. Well, maybe I’ll do that, but in the meantime, I’d like to be able to hook up what I have.

Should I orient the amps so that the speaker terminals are close enough together to attach the strands of the Speakon cables? I have been warned by REL not to hook up high level to monoblocks because it will create a ground problem.

An alternative is that I get a Y cable that will bring two RCA outputs from my Preamp into the single RCA input on the REL?

Or is there another alternative?

Advice appreciated. (But please try not to tell me to just buy a second sub.)
128x128hilde45
almarg's avatar
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almarg
9,670 posts
 
...  the preamp's MAIN OUT goes to each monoblock and the other (TAPE OUT -- now a second main out) is free. Does that fact change your advice?

No, it doesn't affect my comment.  Presumably the change that was made disconnected the two tape out connectors from the internal circuit points to which they are ordinarily connected, and connected them instead to the main out connectors (or to corresponding internal circuit points).  So if you were to connect a y-adapter to the two tape out connectors, to create a mono signal for the sub by shorting the signals on the two tape out connectors together, you would also be shorting the signals on the main out connectors together.  Thus an identical summed-to-mono signal would be provided to both monoblocks.

@imhififan 's suggestions are almost invariably excellent, and from what I can tell from the somewhat limited descriptions shown at the links he provided it looks like the device he suggested should work fine.

Best regards,
-- Al


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hilde45 OP
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@lak Thanks for the suggestions. It's because I watched all those videos but did not see an answer to my question that I didn't find them helpful enough. 

@almarg Regarding the primary suggestion you made, connecting the REL's speaker-level inputs to both monoblock amps and then grounding it on the PREamp, do you have a suggestion how to ground it? There is no dedicated ground on the Preamp -- e.g. no phono ground (because no phono stage). Here's a photo: http://quicksilveraudio.com/wp-content/uploads/2017/07/12-1024x293.jpg

Just back off on a chassis screw and put the small wire around it? 
almarg's avatar
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almarg
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Just back off on a chassis screw and put the small wire around it?
That would be what to try first.

If a hum results, and you are so inclined, you could then purchase an RCA plug, solder the black wire to the plug's ground sleeve connection (while leaving its center pin unconnected), and insert that plug into an unused RCA connector on the preamp (such as either of the "tape out" connectors). Depending on the design of the specific preamp that could very well provide a better ground connection for the sub than a chassis screw.

Best regards,
-- Al



hilde45's avatar
hilde45 OP
4,974 posts
 
Thanks so much. Sounds like a good plan B.
hilde45's avatar
hilde45 OP
4,974 posts
 
@almarg FYI, I just found old threads where you were discussing exactly this problem. I wrote to Quicksilver, but don’t have a clear answer (yet) about how my preamp is wired (for the low-level RCA stereo to mono fix) or how best to ground a high level connection (the Speakon cable approach). I got a very brief reply, but it didn’t answer the question. I also wrote to REL, to see what they say. I’ll try to relay what they say.

I am a little concerned that the high-level rig-up you suggest might feed back to my amp and cause a problem. It's a new amp and I'd hate to immediately hurt it!

The adapter mentioned above by @imhififan would be attractive if I knew it was not going to collapse the stereo field on the mains. I suppose that’s the point of that solution, but I’d like to know before I waste $50 or so!
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