Hissing sound coming from tweeters

Lots of good thoughts above. Given that the 89db s/n number is referenced to 1 watt (as opposed to full power), I would expect that with 101 db speakers it would result in a hiss level that can be heard at a typical listening distance in a quiet room, but not to a degree that is "very annoying." Even if the number is based on A-weighting, which would de-emphasize high frequency hiss somewhat.

However the very high 32db gain, in combination with the 101 db speakers, means that the noise level that is heard will be extremely sensitive to any noise that is picked up or introduced at the input circuits of the amp, including rfi/emi (radio frequency interference/electromagnetic interference) that may be picked up through the air. Are the amps located away from other components? Are the interconnect cables routed so that they do not run close to and in parallel with other cables or power cords? Are there any fluorescent lights, dimmer switches, computers, or other known rfi generators in the vicinity? If so, try turning them off.

Another contributing factor, btw, which can't be helped, is that your ears are only 25 years old, which means greater sensitivity to high frequency hiss than most of us would have :-)

Regards,
-- Al

A further thought. It would probably be worthwhile to invest a few dollars in a pair of shorting plugs such as these, and to put them on the unused rca inputs of the amps.

I see that the amp has a switch to select between the rca and xlr inputs. It is possible that noise being picked up at the rca input and its associated wiring or circuitry is coupling past the switch into the signal path, where it is then multiplied by the amp's high gain factor. Shorting plugs would probably help that.

Also, using the shorting plugs and selecting the rca inputs would allow you to make a more definitive assessment of the intrinsic noise levels of the amp, as others have said.

Before using the rca shorting plugs while using the xlr inputs, however, it would be a good idea to use a multimeter to verify that no continuity exists between the center pin of the rca connector and pins 2 and 3 of the xlr connector. Although I suspect it is not the case, I'm envisioning the possibility that the rca center pin may be connected directly to one of those two xlr pins, and the switch simply grounds the other pin when the rca input is used. In that situation using a shorting plug would ground one of the two input signals in the balanced signal pair.

BTW, be sure that the amp is turned off when and if you change the position of the input select switch. Also, for the benefit of others who may read this, shorting plugs should not be used where a component provides both rca and xlr inputs but does not have a switch to select between them. And of course they should not be used on the outputs of any component.

Regards,
-- Al

When I have the XLR cables hooked up to the pre-amp in balanced mode the hiss is amplified.

Does engaging the preamp's mute function reduce the hiss level?

Regards,
-- Al

I'm beginning to think that the Ayre just doesn't pair well with my amps.

Given the findings you've described, I suspect that you're right. When you connect the cables to the preamp, you are introducing the possibility of ground loop-related noise, but I would expect the xlr connections to be much less susceptible to that than the rca connections, and you are saying that the results are fairly similar either way.

So even though the noise performance of the Ayre preamp is probably very good (I note that John Atkinson describes the noise performance of the presumably similar K5-xeMP as "superb"), the little bit of noise it generates in the circuitry that follows the volume control is being boosted to bothersome levels by the combination of the high gain of the amp and the high sensitivity of the speakers.

The only solution that occurs to me other than replacing the amps or the speakers would be to put in-line attenuators at the inputs to the amps. The Rothwell's, which are available in both balanced and unbalanced form, are an example, although the 10db of attenuation they provide might not be as much as you would want. I believe some others are available that provide 20db or thereabouts, but I'm not sure if they are available in balanced as well as unbalanced form.

Regards,
-- Al

12-28-11: Magfan
Pre / Power amp impedance mismatch?

No. Preamp output impedance is 31 ohms/62 ohms unbalanced/balanced (per JA); power amp input impedance is 23.5K/33K (per mfr). No problem.

Best regards,
-- Al

Are there any ill effects from using the attenuators?

A lot of people are happy with them, but there have been some reports of adverse effects on dynamics, etc. I suspect that they would do fine when driven from a component having very low output impedance, such as your Ayre preamp, and assuming that the input impedance of the amp does not vary significantly as a function of frequency (which I would expect to be the case).

I wonder if any Mcintosh power amps would work well with the Ayre?

The key thing is that you want an amp having significantly lower gain than your present amp. More often than not that will correlate with having a lower maximum power rating than your present amp. The MC501 is an exception, though, having greater power capability but significantly lower gain. See JA's measurements.

The hum you are now hearing with rca's is most likely due to a ground loop problem, assuming that the interconnects are not running close to and in parallel with any power cords. I suspect that if you are hearing just hiss and not hum with the xlr's that the hiss is not being caused by ground loop issues, but you can verify that by using cheater plugs as DTC suggested. As I had indicated, xlr interfaces are inherently less susceptible to ground loop issues than rca interfaces, at least if they are well implemented.

Regards,
-- Al

No, the hiss won't cause any damage.

The MC252 will reduce the 32db gain of your present amp to roughly around 23db, according to my calculations. Not sure that will be enough of a reduction to be satisfactory. Also, using a 250 watt amp with 101db speakers most likely means that you will be paying for a lot of watts that you'll never use, unless you are envisioning going to less efficient speakers in the future.

Regards,
-- Al

Al, how do you calculate the reduction in gain?

As a rough approximation, I neglected dynamic headroom and calculated the gain for the MC252 based on the continuous maximum power rating of 250 watts. Based on P = (Esquared)/R, 250 watts into 8 ohms corresponds to 44.7 volts. The specified sensitivity for the balanced inputs is 3.2 volts. Therefore the voltage gain, neglecting dynamic headroom, is 44.7/3.2 = 13.97. Based on 20log(Vout/Vin), that corresponds to 22.9db.

I now see in one of the spec sheets that dynamic headroom is specified as 1.8db. So depending on what power level the sensitivity spec is defined with respect to, the gain may be as much as 22.9 + 1.8 = 24.7db, which is only 7.3db less than the gain spec of your present amplifier.

how much reduction do you think is needed to obtain satisfactory levels?

A commonly cited rule of thumb is that a 10db reduction in volume is subjectively perceived as being about "half as loud." Based on that, and on your initial description of the present hiss level as "very annoying," my rough guess is that you would want to reduce the amplifier gain by significantly more than 10db with respect to what it is now, perhaps reducing the present 32db gain to the area of 16 to 20db or thereabouts.

Regards,
-- Al