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

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

That all sounds good, David (hilde45). And looking at the manual for the amp in addition to its description at the QS website I'm certain that none of the exceptions I cited that would necessitate not connecting the black wire to a negative output terminal apply in this case.

And thanks for the nice words. Enjoy the new purchases!

Best regards,
-- Al
 

@hilde45, re your latest update, what is the black wire from the sub connected to?

Also, re the suggestion from QS of connecting the black wire to the ground of the right channel amp, that is a reasonable suggestion. And since I’m pretty certain that your amp does not have outputs which are balanced or bridged, a suitable ground point would most likely be the negative output terminal of the amp. (The black/ground wire of a sub should not be connected to the negative output terminal of a balanced or bridged amp, since in those cases the negative output terminal provides a full amplitude signal rather than ground. Also, there are a few tube amp designs, such as some of those made by Audio Research, in which circuit ground is connected to the 4 ohm terminal, rather than to the negative/"common" terminal). Whether doing that would provide results that are better than, worse than, or about the same as would occur if you connected the black wire to a ground point or chassis of the preamp figures to be equipment dependent and hard to predict. My guess is that more often than not the results would be similar.

Re the mention by @jl35 of the possibility of not connecting the black wire at all, some other members here have reported doing that with fine results. And in fact I recall at least one such member indicating that REL had suggested trying exactly that. The reason that can work in some and perhaps many systems, considering that for an electric current to exist a "complete circuit" must be present from source to load and back, is that the AC safety ground wiring forms the return path in that situation. Again, though, how well that would work is equipment dependent, especially on how or if circuit ground and chassis ground (which in turn is connected to AC safety ground) are interconnected within the specific equipment.

Regarding your recent question about the device suggested by @imhififan , when I looked at the links he provided what particularly gave me a fair amount of confidence was the statement on the packaging that can be seen in one of the photos, that "it provides mono output while retaining L/R input isolation." I interpreted that to mean that the left input and the right input are isolated from each other.

Finally, regarding your question about the possibility of damage, in this particular case I don’t envision any such possibility for any of the approaches that have been referred to.

Best regards,
-- Al

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

...  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

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