Will this isolation transformer limit power draw?

El: That's a good point about the "down time" on a sine as it is crossing over. Forgot about that : )

I can also see how a square wave would be FAR more efficient due to the longer duty cycle and more consistent peak to average ratio.

While it is true that many designs use an outboard power supply, the separate chassis typically just houses the transformer and / or rectifiers. Some designs do place the main filter caps in the outboard chassis, which is a poorer design in my experience. That's because the "umbilical cord" between the active circuitry and the output power supply acts as an antenna with minimal filtering at the point of entry into the circuitry.

Drlazybones: Work your way back through your system piece by piece to see where the hum is coming from. That is, hook up the amp to the speakers with nothing hooked up to the amp and see if you hear any hum. If nothing, turn the amp off, hook up interconnects to the amp and then turn it back on. If no hum, turn off the amp and connect the interconnects to your preamp with nothing else connected to the preamp. Do this until you find the source of the hum. It may be a bad set of cables and / or a ground loop that occurs only when specific components are tied together into the system. This can be VERY annoying and difficult to find, hence the need for step by step analysis.

As a side note, if you are running any gear that has a non-polarized two prong power cord, you might want to try flipping it around in the AC outlet. Some gear is VERY sensitive to proper AC polarity. Sean
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I did this test last night

With the Bryston powered up and connected to the speakers, without any inputs, there is a low-level hum/hiss from the tweeter and midrange of my speakers!

Also, now mind you this is a 19 year old amp, the transformers emit a fairly loud humming noise (audible out to 10 feet away) when plugged in, even with nothing connected!

I figured, from reading some posts, it's either a ground loop hum, or it's 60Hz hum from the AC line. My understanding is that a balanced or isolated power line may solve this problem. If a balanced transformer doesn't address line noise, then what the heck is it, good for nothing?

I've also decided to send the amp back to Bryston for service, but I'm not sure they'll agree with me that something's wrong with it! I'll have to see how that goes. The amp will be shipped out tomorrow or the next day...

Thanks for the input, everyone!

The hiss is normal with a lot of amps. If you can't hear it from the listening position I wouldn't worry about it. The hum from the speakers doesn't suprise me especially with an amp that old. The filter caps should be replaced in a 19 year old amp.

If you can hear the transformer humming from 10 feet away then you have a problem. If it is a toroid then it may be due to crap on the AC line. Light dimmers are notorious for causing them to hum. Even though on opposite ends of my house, the light dimmer in my stove hood would cause a toriodal isolation transformer I once had to hum. If it is a convential transformer then the core laminations may have separated. In that case you are looking at a replacement. That won't be cheap.

If the amp is 19 years old, it still has at least a few months left of Bryston's 20 year factory warranty. In plain English, your amp has a problem. Call up Bryston, give them your serial number and see if it is still covered. If it is, make arrangements with them to send it in for a check-up ASAP before the warranty expires. Once there, Dennis at their US repair facility will check the unit out. If it needs help, he'll do what is necessary to bring the amp back up to spec. All of the parts and labor will be covered but you'll have to cover the shipping. This is a phenomenal deal to say the least. Even if you don't like / keep the amp long-term, you'll at least have verifiable proof that the amp was just inspected by the manufacturer and meets factory spec. In this regards, Bryston is unrivalled in terms of customer support.

As a side note, all the older original Bryston series of amps used "iron core" transformers. They switched over to toroidal designs somewhere along the line in their newer series of amps. Sean
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I'd be happy to solder in some new filter caps. I'll pop open the case and take a wild guess on where to find them... :) I'm getting bolder with my soldering iron in any case...

Thanks again!