2 REL Strata Subs and Mono amps

Connection of the red and yellow wires to the red output terminal of the amp is fine. Connection of the black wire to the black output terminal of the amp is fine if the amp does not have balanced outputs, and is not bridged.

If the amp has balanced outputs or is bridged, the black output terminal will have a full-amplitude audio signal on it, equal but inverted with respect to the signal that is on the red terminal. If a path exists between the circuit ground of the sub and the circuit ground of the amp, the result of connecting to the black terminal in that situation would be that the signal on that terminal will be shorted to ground, possibly damaging the amp. Whether or not such a path will exist depends on the internal grounding configuration of the two components. If both components have their circuit ground and chassis ground connected together, for instance, that path would exist via the AC safety ground wiring in their power cords, and the house wiring (or internal wiring in a power strip or conditioner, if one is being used) that interconnects the respective outlets.

If the amps are balanced or bridged, connecting the black wire to a chassis screw should be safe to try, and may or may not work acceptably depending on the grounding scheme of the amp. If hum or other issues result, and the amp has an unused RCA input connector, an approach that would definitely work is to solder the black wire to the ground sleeve connection point of an RCA plug, leaving the center pin connected to nothing, and inserting that plug into the unused connector. Similarly, if there is an unused XLR input connector, connecting the black wire to XLR pin 1, using a mating XLR connector instead of an RCA plug, would also be a good approach.

Regards,
-- Al

12-29-12: Wolf_garcia
I imagine you only need one positive lead per mono amp, as otherwise a single REL sub combines those signals anyway to utilize both channels.

If the amps are not balanced or bridged, I suspect that results in most cases would be similar or identical regardless of which output terminal the yellow wire is connected to. If it is connected to the black terminal, the internal circuitry of the sub will see a signal level that is 6 db less than if it were connected to the red terminal. The level control on the sub would be increased to compensate for that.

If the amp is particularly low powered, it would suggest the likelihood that it would be preferable to connect both wires to the red terminal, resulting in a higher level after the two inputs are summed together. If the amp is particularly high powered, it would perhaps suggest the possibility that connecting the yellow wire to the black terminal would be preferable. Although it is possible that it would make no difference in both of those cases.

HOWEVER, if the amps are bridged or balanced, connecting the red and yellow wires to opposite colored terminals would result in essentially no sound from the sub, because since the two signals are inverted relative to each other, they would cancel when summed together in the sub.

12-29-12: Sunnyboy1956
For a balanced /bridged amp check with REL. If I remember the G1 manual covers all scenarios. The input impedance of the G1 is in excess of a 1k ohms and amp damage is not likely.

Input impedance has no relevance to the damage possibility I described earlier. Input impedance refers to the impedance seen "looking into" the inputs of the sub from the red and yellow wires (i.e., the signal inputs), and has nothing to do with the impedance between the black (ground) wire and the sub's internal ground, which can be presumed to be a direct (zero ohm) connection. In the scenario I described in my previous post that might result in damage (depending on how the grounds are handled in the specific designs), the signal on the black output terminal of the bridged or balanced amp would be shorted to ground through essentially zero ohms.

12-29-12: Zormi
Huh..

Primare A32 is fully balanced design, as emphasized on their website, so it seems that I will have to look elsewhere. Safety is of course the most important issue, at least for me..

Not sure what you mean by "look elsewhere." My previous post described three possible approaches that are applicable to your situation, starting with trying a connection of the black wire to a chassis screw or other ground point on the amp.

Regards,
-- Al

12-29-12: Zormi
Personally, I have a piece of Primare A32 250Wpc stereo amplifier planned for that task. The main part of my system is consisted of Krell Phantom preamp (with active crossover onboard) and a pair of Krell Evo-400e monoblocks. This combo is supposed to drive the main speakers, while Primere A32 is planned to feed two REL Gibraltar G1, via high level inputs, as REL strongly recommends in user manual.

If the only function the Primare would be serving is to drive the subs, it would be better to simply remove it from the system and connect the subs to the Krells.

The reason REL recommends driving the subs from high level power amp outputs is to enable the subs to see the same signals as the main speakers, with the signals that are seen by the main speakers and the subs both reflecting the sonic effects of the main power amp. That purpose would be defeated by driving the subs from a different power amp than the one driving the main speakers. It would also mean that your investment in the Primare would be pointless.

Or does your reference to an active crossover function mean that the Primare is to be driving the low frequency sections of the main speakers, in addition to driving the subs, with the Krells just driving the higher frequency sections of the main speakers?

Regards,
-- Al

Zormi, thanks very much for the nice words.

I would go with no. 1, connecting the subs to the outputs of the Krell amplifiers. I looked through the description and the manual for the 400e at the Krell site, and did some other searching via Google, and I found no references anywhere to the outputs of that amp being balanced or bridged, and I found no statements anywhere cautioning against grounding the black (actually blue?) output terminal.

Yes, the amp provides an XLR input, but that in itself says nothing about whether its internal signal path and its outputs are balanced.

I would suggest contacting Krell to confirm that the outputs are not balanced, and that it is safe to connect the black output terminal to the ground of a subwoofer. And if the outputs are actually balanced, if a chassis screw would be suitable for use as a subwoofer ground.

If it turns out that neither approach is suitable, then I would suggest using the RCA input connector to provide a ground connection, as I described earlier. To do that, you would obtain an RCA plug, such as this one, and solder the black wire to the connection point for the ground sleeve, which is the thing that projects out toward the right side of the photo. Do not connect the center pin to anything. Inserting that plug into the RCA connector on the rear of the amp will provide a proper ground connection.

I would expect that with a little care it should be possible to connect the red and yellow wires to the output terminals of the amps, together with the main speaker cables, without degrading contact integrity. That is probably what most people using REL subs do.

2) connect the REL G1 via low level input, RCA type.... I am a bit confused (since have two REL G1s) whether to put the RCA cable from preamp into the right or into the left RCA low level input of the woofer? Al, which one is the accurate, please..?

Since the two inputs get summed together in the sub, it shouldn't matter which input you connect to. If you use that approach, it might be a good idea to connect a shorting plug, such as this one, to the unused input, to eliminate the possibility of noise being picked up at that input and being summed together with the signal. Although chances are that wouldn't be an issue anyway.

Regarding that approach, also, if the connection between the Krell preamp and amps is via XLR, rather than CAST, connecting the preamp to the subs via RCA could conceivably have a slight adverse impact on the sonics produced by the main speakers. That might occur if the design of the preamp is such that the RCA and XLR outputs are not driven from separate output stages, and the signal that is output via RCA is the same as one of the two signals in the balanced signal pair that is provided to the XLR connector. That is another possible reason why having the preamp provide the inputs to the sub may not be the best approach.

Regards,
-- Al

Thanks, Hifigeek. Jdolgin, descriptions I found of the VM220 indicate pretty clearly that it is fully balanced. However, as Hifigeek's comments make clear, the fact that it is a tube amplifier having an output transformer introduces still more variables and unknowns into the already "confusing mess" which you aptly described.

I've seen some older ARC schematics, such as this one for the fully balanced VT130, showing circuit ground as being connected to the 4 ohm output tap of the amp, rather than the common (black) terminal, and with circuit ground and chassis ground being connected to each other through a 10 ohm resistor.

*If* Hifigeek determines that the design of the VM220 is similar in those respects, I think that you could connect the black wire to the 4 ohm output terminal of the amp, and the red and yellow wires to the 8 ohm terminal.

Alternatively, assuming that is how the design is implemented, you could connect the black wire to the 4 ohm output terminal, and the red and yellow wires to the common (negative) terminal. Doing that would introduce a 180 degree phase reversal between the sound produced by the sub and by the main speakers, which you would correct for with the sub's phase reversal switch, assuming it provides one. Doing it that way would result in the sub receiving a somewhat (about 8 db) greater signal level than if you were to connect between the 4 and 8 ohm terminals.

But we'll see what Hifigeek determines.

Regards,
-- Al

12-30-12: Vladimir
I checked the Gibraltar manual online and on page 12 it explains the way to connect to differential amplifiers such as your Krells, basically to connect the black Speakon wire to a chassis screw as was explained by others here.

Thanks, Vladimir. However, as I indicated in my first post dated today, based on my research it appears that these particular Krell amps are not differential or balanced or bridged. I suggested confirming that with Krell, though.

Regards,
-- Al