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

Hi David (hilde45),

First, congratulations on the purchases!

Second, regarding...

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?

... Don’t do that! It would result in the main speakers reproducing mono. And although I doubt that it would happen in this particular case, with some preamps doing that might even result in damage, eventually if not sooner, since you would be shorting the preamp’s outputs for the two channels together.

Manuals for a number of REL subs that I’ve seen in the past suggest that to connect their speaker-level inputs to monoblocks, when one sub is being used, what should be done is to connect the red wire from the sub to a + output of one amp; connect the yellow wire from the sub to a + output of the other amp; and connect the black wire from the sub to a ground point or a chassis screw on the **preamp.** I suspect that is likely to work fine, although depending on the particular designs it is possible that a hum will result. But give it a try.

Also, you don’t necessarily have to connect the red and yellow wires to the same output tap (i.e., the 4 or 8 ohm output tap of the amps) that you are connecting to the main speakers.

Good luck. Best regards,
-- Al

@pjr801 — the reason is because the standard advice for this REL is to connect it to the main speaker terminals on a normal (non-monoblock) amp. I am trying to follow that advice from REL but adapting it to monoblocks.

@almarg Thanks! I feel confident about these purchases.

So, before I ordered the preamp, I had Mike wire the "Tape Out" as a *second* main out. Right now, 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? (I don't know why the main speakers would become mono by doing this, but that's just from lack of general understanding about how these are wired.)

Depending on what you say to the above question, I'll try the monoblock connection approach you suggest — and just listen for a hum. I'll also contact REL. It would be shame if I couldn't use the sub.

Or is there another alternative?

You can connect your preamp  second main out to a 
Stereo to mono signal converter
and connect one of the converter mono output to the REL 328 low level input. This converter utilizes an independent 1:1 ratio audio transformer for each channel, which also function as a ground loop isolator.

https://www.ebay.com/itm/MCM-CUSTOM-AUDIO-50-20795-Stereo-to-Mono-Signal-Converter-/164155969190?_tr...

Thanks imhififan -- I'll definitely keep that in mind if the high level input solution won't work. 

Best video's I've seen that are extremely informative:https://rel.net/how-to-connect-my-rel/