Differential Monoblocks with twin REL subs

Hi, I would appreciate any advice to avoid damaging my amps. I have a pair of Magneplanar MG 2.6R speakers which I use with a Primare 928 pre and a couple of Primare 928 Monoblock power amps. I am now looking for a little more bass depth and so have bought a pair of 2nd hand REL Q50 subwoofers. I now realise that I am not sure how to connect them without risking damage to the monoblocks.

The issue is, I think that the monoblocks are fully balanced (differential) amps, and so unlike most amps, the
-ve terminal is likely to be live, and connecting the REL Neutrick black ground lead as normal to the -ve terminal would short the amp.

So my questions are:

1. Would my best bet be to connect both yellow & red Neutrik leads from my left sub to teh +ve terminal on my left monoblock, nothing attached to the -ve terminal, and teh black ground wire attached to a ground point on the amp's chassis? Same for the right sub & monoblock. Would this be safe?

OR because the amp is deifferential would I be losing out on some of the signal by not using the -ve terminal?

2. In which case should I connect the yellow live Neutrik lead to +ve, the red live Neutrik lead of -ve on the same monoblock, and the black ground lead to a ground point on the chassis? Would this be safe?

3. And finally, if the amp isn't really differential (I don't have the manual, would either of the above options be likely to cause any damage?

I hadn't really thought this through properly before buying the subs, so any help greatly appreciated before I take the plunge and set them up.

Well put questions, and welcome to Audiogon!

Option 2 won't work, because the signals that are sent into the sub on the red and yellow wires (which are normally L & R channel signals) are summed together in the sub. Summing together the two signals in a balanced signal pair, which are inverted relative to each other, would cause them to cancel each other out within the sub, resulting in essentially no sound.

What you want to achieve with respect to the black wire from each sub is to have it connected to the circuit ground of the corresponding amp. Option 1 would work ok provided that chassis and circuit ground are connected directly together within the amps, which may or may not be the case. If you have a multimeter, you could determine that by measuring the resistance between an unpainted metal point on the chassis and either the ground sleeve of an RCA connector, or pin 1 on an XLR connector. If you do that, disconnect the amp from everything else, including AC power, before making the measurement. You would want to be precise in making that measurement, looking for at most a tiny fraction of an ohm, as opposed to say several ohms.

An alternative approach which would certainly work (based on the assumption that the preamp and amps are connected via balanced cables) would be to purchase a simple RCA plug, similar to this one, and solder the black wire from the sub to the ground sleeve of the plug (soldering the wire to the lengthy metal protrusion that appears toward the right side of the photo). Connect nothing to the center pin of the plug. Then insert that plug into an RCA jack on the preamp, and you will have provided the sub with a proper ground connection.

The yellow and red leads would be connected to the + output terminal of the corresponding amp, per your option 1. Losing out on some of the signal is not an issue; the setting of the sub's level control that you settle on will reflect the voltage range you are providing it with.

Re question 3, the approaches I have described can be applied even if the amp is not differential.

Nice equipment, btw! Regards,
-- Al
You might check with the manufacturer. Just because the amplifier is fully differential does not mean that the output connection is floating.
Thanks Al,
Unfortunately the amps are not currently connected via balanced cables at the moment, although I have been considering moving to balanced cables for some time.

However, there is a post marked as 'Ground' on the rear of teh Pre-amp, so I guess I can assume that this would do the trick as well? Can I connect both subs' black wire toteh same pre-amp ground post?

Atmasphere - I would check with the manufacturer, but the current Primare company (Swedish) is not the same as the original company (Danish) that designed my amps. They took over the company and moved to Sweden. I doubt if they would have any detailed knowledge of the Primare 928s.
However, there is a post marked as 'Ground' on the rear of teh Pre-amp, so I guess I can assume that this would do the trick as well? Can I connect both subs' black wire toteh same pre-amp ground post?
Although in some systems that approach will provide reasonable results, I would not recommend doing that in this case, at least without first making some multimeter measurements to see if that ground point is directly connected to the ground sleeve of the preamp's RCA output connectors. And looking at a photo of the rear panel of the preamp, which I found via Google, my guess is that it is not. Depending on the specifics of the designs, the result could conceivably be a big-time hum. Or even if that direct connection exists, you might still get a slight ground loop hum and/or slight sonic degradation.

Personally, I'd get an inexpensive multimeter to eliminate some of the guesswork, and some inexpensive Mogami Gold Studio balanced cables, and proceed as I indicated earlier. I wouldn't be surprised if you were to get better sonic results with those cables than with your present RCA cables, even if the RCA cables are far more expensive.

-- Al
Heck I would just hook it up and see if it works. If it seems OK when you power up the subs, maybe it is. If there is a problem it will tell you right away and you turn it off and see what you can do.

Most subs that I have dealt with run right off the amplifier output terminals and don't interfere even if the output of the amplifier is floating (which ours are- we make fully differential amps).

I would thus not worry about the ground wire unless you encounter a hum.
Well, I have taken your advice and hooked it up. As discussed, I connected up the subs (yellow/red to +ve terminals, nothing to -ve, and black from both subs to the ground post on my pre-amp), and everything seems fine with absolutely no trace of hum at all.

And what a difference the subs make to the scale of the music! I had grown used to the bass limitations of the of the Magnepans, but after correcting my original temptation to krank up the bass (sounded fantastic for a while though), and some experimentation about where to position the frequency roll-off on the subs, the results have by a long way exceeded my expectations. Some more experimentation with positioning and in particular with the frequency roll off on the subs will probably be required over a period of a few weeks, but I am already convinced that the relatively small cost of the upgrade has been well and truly justified, and that the subs are here to stay.

Incidentally, I have also been in touch with a dealer (Hi-Fi of course) close to me who still keeps in touch with the amp's designer, and he has kindly offered to get me a definitive answer as to whether or not the amps truly are a differential design.

Lastly, Almarg, I have taken your advice that I should try a pair of Mogami balanced cables for the amps to replace my existing RCA leads. I can't resist giving this a go as well, so they are now on order. Hopefully my last 'upgrade' for a while.

Many thanks to all those who have helped, especially Almarg, and yes, I am now also the proud owner of a multimeter (thinking about going back to University to figure out how to use it properly though).

The following is a response I received from REL:

Connecting a pair of RELs to a balanced amp in stereo is only a little different form the standard REL connection scheme. Your subs should have come included with the REL High Level cable. That cable consists of three wires, Black, Yellow, and Red. The Red and Yellow are positive leads and eth Black wire is your ground lead. You need to pair the Red and Yellow wires from the first sub to a single positive binding post (right side). The Red and Yellow wires from the second sub will be paired together on the other positive binding post (left side). Both Black wires need to be attached to a dedicated ground post or chassis screw. You cannot connect the Black wires to a negative binding post because there is enough current on the negative side of a balanced amp to potentially damage eth ground circuit on the RELs.       

Paul Greene
Service Department Manager
SUMIKO | 2431 Fifth St | Berkeley, CA 94710
ph 510.843.4500 | fax 510.843.7120