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

It could be your system is not grounded properly. Our cable company, comcast was doing work and and did not properly ground the system. I was having hissing through the tweeters also. To pinpoint I disconnected the cable into the preamp and the hissing stopped.

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

Your in a world of hurt.

Wow. You guys are extremely helpful and full of good ideas. Thanks for all the replies.

This morning I rearanged my equipment and made sure all cables, cords, etc. are nice and neat. Unfortunately when I powered everything back on the hiss is still there. I disconnected my XLR cables from the pre-amp and mono blocks again, and the hiss is much less noticeable. When disconnected you have to put your ear close to the tweeter to hear the hiss. When I have the XLR cables hooked up to the pre-amp in balanced mode the hiss is amplified.