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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.)
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almarg's avatar
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almarg
9,670 posts
 
Ian ( @ieales ), thanks for the references.  The OP's amps have a specified damping factor of 20, which at least in theory corresponds to an output impedance of around 0.4 ohms for their 8 ohm tap.  That impedance of course being well below average for a tube amp, and almost approaching solid state territory.  And the Salk speakers he has or will soon have are specified as 8 ohms nominal, and I suspect don't go much below that value at any audible frequency.

Wouldn't such a pairing significantly ameliorate the effects you refer to in the section entitled "Connection" of your paper shown at the second of the three links you provided?  I recognize, of course, that those effects would still be present to some degree.

Best regards,
-- Al

lalitk's avatar
lalitk
5,561 posts
 
“you should NEVER use the high level output. Especially on a tube amp.”

I am not sure the basis of this statement. I am using a pair of REL subs via its high level input with tube mono amps (8W SET’s). I hear no compromises in bass production or lack there off. 
ieales's avatar
ieales
1,201 posts
 
@daveyf
which system page? please post impulse and phase plots. I had a look. I find the Sonus intolerable. The low end is just awful in every system I’ve heard. [I’ve never owned a ported system]. I didn’t care for Nordost speaker cables in any demo.

@almarg
The QS have low output impedance, but if one looks at response into a real load, it is nowhere near flat.

Additionally, current drives the loudspeaker and voltage drives the sub. For any non purely resistive load I & V differ by an ever changing phase angle. Hence, a sub driven from the LS out can N E V E R be time correct with the mains.

@lalitk
I hear no compromises in bass production or lack there off.
I'm sure there is plenty of level.

I must admit that Time Coherence is my grail and systems without it are uninteresting. IMO, frequency response is, and always has been, overrated.

hman7's avatar
hman7
1 posts
 

I have a REL S/812 and two PrimaLuna EVO 400's as mono blocks.

this is what I received from REL

Thank you for contacting us, and for choosing a REL Acoustics subwoofer for your home audio system. While some older REL designs did allow for a single subwoofer to simultaneously be connected High Level to two monoblock power amplifiers at the same time, with all current REL models including the S/812 a single subwoofer should not be connected High Level to two monoblocks at the same time. Doing so will in most cases result in a significant ground loop hum which can be near impossible to eliminate, and in a worst-case-scenario can result in damage to the subwoofer and/or the monoblock amplifiers. 
 
When using a single REL subwoofer in a system that features monoblock power amplifiers, the best option will be to connect it to open outputs on your preamp using the Low Level input. This will allow the subwoofer to safely draw signal from both the left and right output channels, and since the Low Level input feeds through the same filter circuit as the High Level input you will still have full control over the S/812's Crossover dial for tuning it's output to blend with your main loudspeakers. 
 
In this system it is also possible to connect using the High Level input to one of either the right channel or left channel monoblock amplifier. This will of course result in the subwoofer only receiving either left channel or right channel signal, though since sub-bass content in many popular recordings is centered in the stereo mix, meaning that it is common between the right and left channels, it will still be possible to achieve a seamless blend with your main loudspeakers when connecting in this way. When connecting in this way to the Primaluna EVO400, please refer to the following instructions:

  • Twist together the red and yellow wires of the subwoofer's High Level cable, and connect to the 8 ohm positive output terminal on one of the two monoblock amplifiers.
  • Connect the black wire of the High Level cable to the negative output terminal on the same monoblock amplifier. 

 
I hope that this helps, but feel free to let me know if you have any additional questions or concerns. 

 

 

ieales's avatar
ieales
1,201 posts
 

NEVER use high level outputs to drive a subwoofer. It's a Band-Aid that should be banned.

According to the REL S/812 manual, it has two low level inputs. You can simply Y off the main pre out if there are not two sets of outputs

Ideally, you should roll the lows out the mains with a passive HP filter. Doing so will tighten up the low end and effectively double the apparent power of the PLs

That being said, you should flog the REL and get a pair of subs with real controls. IMO, REL markets bling, not good bass and are not serious about proper system integration due to their dearth of controls making them impossible to integrate properly. e.g. 180° phase inversion is equivalent to moving the sub about 25 feet @ 40Hz. Fixed XO slope will only integrate properly in a tiny subset of rooms and systems. 

Martin-Logan X series and JL Audio are excellent offering full control.

 

 

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