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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
hilde45's avatar
hilde45 OP
4,978 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!
jl35
4,021 posts
 
I have a single REL sub connected to balanced mono-blocks as Al suggests and it works fine, I don't even need to connect the ground. While REL recommends against a single sub with mono blocks, due to hum issues, maybe I just got lucky...there is lots of information on the REL help websites about all kinds of connection issues...2 or 4 subs may be ideal but in some systems and rooms you can get great results with a single sub with careful placement and adjustments...I do use the upgrade REL cable
hilde45's avatar
hilde45 OP
4,978 posts
 
REL's response: "We would recommend connecting low level from your preamp to the R328 as the safest connection for your current setup. As long as it's safe to use a Y splitter out of the QS preamp, this will be fine for going into the single input on the sub."

I will comment that I tried an RCA out from my Streamer and while I could hear bass, it was definitely inferior to the high level connection.

@jl35 When you say "I do use the upgrade REL cable" you mean the "Speakon" cable that clicks into the sub and has 3 wires on the other end? I assume so but these subs can differ. 

Also JL35 (and Almarg)  -- are you at all worried about some kind of damage happening to the amps? @decooney mentioned this happened to him with a Cary amp years ago.

jl35
4,021 posts
 
REL makes an upgraded version of the Speakon cable that is included with the sub...I've had this setup about a year with no issues...but I am not technically knowledgeable so can't advise
imhififan's avatar
imhififan
2,205 posts
 
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?
Unless there’s a resistor in series to the preamp outputs, using a ’Y’ cable that bring two RCA outputs from your Preamp into the single RCA input on the REL is a big no no!
almarg already explained to you:
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.
Don’t do that!!!
REL’s response: "We would recommend connecting low level from your preamp to the R328 as the safest connection for your current setup. As long as it’s safe to use a Y splitter out of the QS preamp, this will be fine for going into the single input on the sub."
I can’t understand why REL gave you such advise.
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