Steven Stone reviews the Ref 600m in the January issue of the Absolute Sound. In his conclusion, he states, “To my ears, it is good enough to qualify as the best all-around power amplifier I’ve heard to date, regardless of technology or circuit topology.” Chalk up another win for Bel Canto!
While Mr. Stone does not dwell on it, he does note, “ With the Ref600M powered up I could hear only the faintest hiss if I put my ears within a few inches of the Spatial’s coaxial driver [his review speaker]”. He later compares his recently refurbished Pass Labs X150.3. “After its return from the factory, the Pass was almost as quiet in terms of noise (without a signal) as the Ref600M, with only a slightly higher hiss level and zero hum.”
I find this very interesting. How common is this hiss/hum issue? Is it caused by class D topology? Seems not, given the reviewer experiences this even with his class A/B Pass amp. And is this issue caused by every product of a certain design or does it depend on what other components are in your system? All three of the class A/B amps I’ve previously owned were completely silent when powered on but playing no music. Putting my ear to the speakers revealed zero sound. Then I got my current Audio Research DSi200 integrated amp (class D) and it does produce a faint hiss from the tweeters and low level hum from the woofers of my Revel F208s. Close up, it is easily heard, but it’s faint enough that it can’t be heard from any further than a couple feet away, even in a silent room.
I don’t feel this hiss/hum thing is a deal breaker. My current system plays music very well. But ideally, I would never experience hiss at all.