2 REL Strata Subs and Mono amps

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

Primare A32 is (was?) planned solely for feeding the REL woofers.

Removing the Primare from the chain basically imply several possible solutions, as far as I can see (I am far from any qualified tech know-how in electronics):

1) simply connect the REL G1 (via Neutric high level input wires) to the Krell Evo-400e blocks, simultaneously with the main speakers, as Al suggested above. Possibly uncomfortable a bit, since Krell has only one par of output terminals. Plus, above discussed possibility of some damage to monoblock amps..? However, this solution ensures reflecting the sonic nature of the main amp(s).

2) connect the REL G1 via low level input, RCA type, which was my natural, initial idea, until I saw the "REL connectivity" section on their website and their strong advice to use the Neutric high level input for best sonics. I can still use REL's RCA low level input to connect the woofer directly to the Phantom preamp low-end RCA output. In this case, any complications with the power amps should be eliminated, but benefits from "reflecting the nature of the main amp", as REL recommends, will be lost. BTW, in this case, 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..?

3) keeping the option with Primare, but connecting the black wire to the amp chassis, instead of black output terminal. Unconfortable a bit, but safe for amp. Krell power amps' sonics "lost" for the woofers.

4) employing another big stereo Krell (btw fully balanced as well) instead of Primare, which will keep the Krell sonics for woofers. Too pricey solution, out of my reach.

5) employing another stereo power amp, which is not balanced design. Krell's sonic nature definitely lost for woofers, but all other problems eliminated. Good that this one may be relatively modest in power and price - as Al elaborated above, relative lack of power may be compensated by putting the woofer's yellow wire to the red amp's terminal, plus additionally reinforced utilising woofer's own amplifiers.

Great inputs Al, anyway, thank you very very much. You truly deserve all positive reputation from us fellows overhere.

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

Zormi, I would like to know what your main speakers are, because I believe you have a totally different problem to Jdolgin, and it hasn't been addressed: you have an active crossover in your system (Krell Phantom's crossover board option) and RELs aren't designed for this; they are supposed to 'augment' the main speakers from below their natural roll-off frequency, without the use of a crossover like the Phantom's. If this is the case (unless you already purchased the subwoofers), you don't need REL Gibraltars as there are many good and less expensive alternatives, some of them with room correction built-in. On top of that, if your main speakers are full-range floorstanders, you don't need the Phantom crossover to use the RELs; if they aren't full-range and depending on their bass response, you can go either way.

Thanks for all of your input.
My issue deals with the specific nature of my Audio Research VM220 Mono Bloc
amplifiers. They accept BOTH balanced and single ended inputs with a toggle switch selecting the appropriate configuration. So....I'm not clear if this specific amplifier uses a differential circuit when running balanced. Also am I safe using the black speaker binding post if running single ended? Overall, a confusing mess. It's not clear if the chasis screws can provide a safe ground connection.
Of course I called Audio Research a number of times and have not gotten a call back. This is a similar experience other Audio Research owners have had since their sale.

Can any one help?

Thanks