Ayre amplifier to Subwoofer connections

Drew -- A further thought: Try connecting as described in my previous post, but with the sub black terminal connected to preamp chassis, rather than amp chassis.

Regards,
-- Al

I believe it will work ok either way, but connecting sub negative to amplifier chassis ground is probably preferable in that it would avoid possible problems or damage if at some point in the future the sub ground inadvertently became common. The minor downside of connecting that way is a 6db reduction in the signal amplitude going into the sub, which most likely just necessitates a corresponding increase in its level setting.

Basically, a balanced output has signals on its two output connectors (or connector pins, in the case of line-level xlr connectors) that are equal in amplitude but inverted in polarity, relative to each other. Components with single-ended outputs (or inputs) have their return connection grounded to chassis. Components with 3-prong power plugs have the ac safety ground prong also tied to chassis.

So if you were to connect the two sides of a balanced output into a single-ended input, and both components had 3-prong ac plugs, you would be grounding one of the two balanced signal outputs. Depending on the design of the output circuit, that could conceivably cause damage.

In this case, that situation is avoided because the sub amp's signal ground is not common with ac safety ground. However, as I say connecting it's ground to amp black would create the risk of damage if the sub chassis inadvertently came into contact (either directly or via a cable) with the chassis of any other component that had a 3-prong plug.

By connecting sub negative to amp chassis ground, the sub input will only see half the amplitude that it would see if it were connected between the two equal but opposite polarity signals that are present on red and black. Hence the need to increase the sub's level setting by 6db, which won't be a problem unless there is a gain or sensitivity mismatch somewhere in the system.

Re your final question, the sub leads should ideally be connected at the amp end of the speaker cables, not at the speaker end. That way the current drawn through those cables by the main speakers, and any consequent voltage drops that might result from interaction of that current with cable impedance, will not affect the signals to the sub.

Regards,
-- Al

So if you were to connect the two sides of a balanced output into a single-ended input, and both components had 3-prong ac plugs, you would be grounding one of the two balanced signal outputs. Depending on the design of the output circuit, that could conceivably cause damage.

To clarify my statement a little further, if both amp & powered sub had 3-prong power cords, the path by which the negative (black) output of the amp would be grounded is: Amp black to sub chassis ground to sub ac safety ground to safety ground pin on sub power plug, then via ac wiring to ac safety ground pin on amp power plug, then to amp chassis ground.

Also, I'll add to my previous comments that another way in which sub signal ground could conceivably become common with ac safety ground is if an internal short were to occur within the sub amp. That is very unlikely if it is well designed, but if a short were somehow to develope between its signal ground and the neutral prong of its 2-prong ac power plug, that would create such a situation since ac neutral and ac safety ground are tied together back at your ac service panel.

Regards,
-- Al

Hi Drew,

Not sure what you mean by "with either input pulled from the amp, I still got sound from both speakers." Are there two subs, or one? I assumed there is only one sub, and its amplifier has inputs for both the left and right channel outputs of the main power amp, which it sums together to feed a mono signal into the sub's driver. The left and right main speakers are separately connected to the left and right amp outputs, and I assume are unaffected by whether or how the sub is connected. Correct me if I'm misunderstanding the situation.

If my understanding is correct, my suggestion would be to try connecting sub left red to amp left red; sub right red to amp right red; and one and only one of the two sub amp black inputs to amp chassis (leave the other sub black input unconnected).

Regards,
-- Al

Ouch! Yes, by all means what was meant was sub black to amp chassis, not vice versa. Keep in mind that with a fully balanced amp, both the red output and the black output of the amp have signals on them (in fact, the same signal except with opposite polarity, as you now appear to realize). When you connected both of the amp's black outputs to sub chassis, you were connecting two different amp output signals together, a definite no no!

I don't quite follow the two alternatives you listed at the end of your post, but let me re-state how I envision the connections should be made:

-- Left main speaker red to Ayre left channel red.
-- Left main speaker black to Ayre left channel black.
-- Right main speaker red to Ayre right channel red.
-- Right main speaker black to Ayre right channel black.
-- Sub left channel red to Ayre left channel red.
-- Sub right channel red to Ayre right channel red.
-- One or both of the two black terminals on the sub to the chassis of the Ayre. I suspect that it won't make any difference whether you connect both of them, or only one of them, since they are most probably connected directly together within the sub's amp.

Regards,
-- Al

I connected my REL to it the way I normally would, with the one black lead connected to one channel's "-" terminal.

Drubin -- What model REL is it? And does it have a 3-prong power cord? If so, you definitely don't want to do that. For the reasons I explained above, and also note the cautions on page 5 of the AX7 manual:

http://www.ayre.com/pdf/Ayre_AX7e_Manual.pdf

Regards,
-- Al

Drubin,

The R-205 manual indicates in multiple places that for a fully balanced amplifier the black wire of the high level input should go to amp chassis.

Not sure why you didn't hear a difference when you lifted the ground. Perhaps its signal ground and ac safety ground are not common, the way they normally are with amps and preamps. If you have a multimeter, it would be interesting to see if there is continuity between the black wire and the safety ground pin of the power plug.

In any event, even if things are somehow isolated so that you avoid the risk of damage to the amp, I don't think you will get the right sound with the black wire going to the negative terminal of one of the amp output channels.

If we denote the instantaneous voltage between the amp's left positive terminal and the amp's ground as Lvolts, and the instantaneous voltage between the amp's right positive terminal and the amp's ground as Rvolts, the corresponding voltage that the sub provides to its driver should be proportional to (Lvolts + Rvolts), which is a mono signal obtained by summing together the left and right channels.

That is what will happen with the black wire from the sub connected to amp chassis. However, if it is connected to, say, the left channel negative terminal of the amp instead, what will result is ((2Lvolts) + (Rvolts-(-Lvolts))) = (3Lvolts + Rvolts). So instead of the summation to mono giving equal weight to the two channels, the left channel would receive very disproportionate emphasis (the factor of 3 corresponding to about 9.5db).

Drew -- Upon re-reading my earlier post, I want to make sure it's clear that no disrespect at all was intended when I used the word "ouch." We've all made comparable mistakes at one time or another, including me.

Regards,
-- Al

Drubin -- Yes, you would want to connect both hots (yellow and red) to the plus output of the corresponding channel of the amp, and both subs' grounds to amp chassis.

The subs' electronics sums the difference between its red and black together with the difference between its yellow and black inputs. If you connect both red and yellow to the + output of a fully balanced amp, and black to ground of the amp, then since two identical signals are being summed together, the mono signal the sub provides to its driver will be proportional to the output of that channel of the amp.

If you were to connect sub black to amp negative for the corresponding channel, then as we discussed earlier in this thread you create, under some scenarios, the risk of damaging the amp. However, fwiw, the other downside of doing that which I had mentioned (creating an imbalance favoring one channel in the summation to mono) would not occur in this situation.

Regards,
-- Al

Hmmm (pun intended :)).

I feel quite certain that what I said is correct in terms of basic functionality, but I can't say with certainty that hum won't arise (just as it can arise in any system with unbalanced interconnections).

By connecting the black wires from both subs to amp ground, you are connecting the signal ground of both subs together. If the two subs do not have significant voltage offset between their signal grounds (such as might result from plugging their 3-prong power cords into separate dedicated ac lines), and if their power transformers do not have significant leakage between ac hot or ac neutral and signal ground, I wouldn't think that there would be a problem. Even more so because the signal levels involved are speaker-level, rather than being line-level (at lower voltages).

If a hum problem did arise, conceivably it could be fixed by connecting a heavy wire directly between ground points on the two subs. Or perhaps by using cheater plugs, to isolate each sub from ac safety ground.

That said, both the causes and the solutions to hum problems arising from single-ended interconnections can, as I'm sure you realize, be elusive. If REL answers you similarly to the rep you quoted, I would try as much as possible to pin them down as to whether they are basing that on hands-on real world experience, using their subs with balanced amps, or if they are perhaps just being overly idealistic.

Best,
-- Al