[Grit]

I was reading the other day a post about benchmarks/specifications on amplifiers vs sound quality. The overwhelming conclusion is (of course) to listen to the amp yourself. But there was one spec and potential results that caught my eye: power doubling as load decreases. One post talked about the amplifier being able to provide enough power for the speakers with a lower resistance. The other mentioned that in order to double power, many amps use greater current feedback, which the poster believed decreases the quality of the sound.

I've often wondered why some amplifiers can double power output and others do not. Bryson doesn't do it, and I believe they use a feedback design? Ayre does double power, but they use a no-feedback design. So, what component is it that allows power to double as resistance decreases (300 watts @ 8 ohms, 600 watts at 4 ohms, 1200 watts @ 2 ohms, etc.), and what impact, if any, does it have on sound quality?

And, what is the difference between current feedback and voltage feedback, with respect to impact on sound quality?

[95Dyna]

Marketing people are well aware of this "conventional wisdom" regarding amplifier performance.  The reality is that not many amplifiers perform exactly in this manner.  Typically what a company will do is start with their measured power into 4 ohms then will understate the 8 ohm measured reading such that in the brochure it appears as if their amp is doubling down.

Bryston markets their amps truthfully by telling you what the actual measured power rating is into 4 or 8 ohms.  My 7B SST2's for example measured @ 643 WPC in 8 ohms and just shy of 1KW into (4) ohms according to docs I received from the factory with the amps.  Bryston publishes the 7B ratings at 600 into 8 and 900 into 4 ohms.  They could easily claim that the 7B provides 450 WPC into 8 and 900 into 4 ohms and still be telling the truth.

It's always best I think to tell the unvarnished truth then explain why it is what it is.  That explanation I'll leave up to the far more qualified Chris or James 

[Phil A]

Marketing people are well aware of this "conventional wisdom" regarding amplifier performance.  The reality is that not many amplifiers perform exactly in this manner.  Typically what a company will do is start with their measured power into 4 ohms then will understate the 8 ohm measured reading such that in the brochure it appears as if their amp is doubling down.

Yep - here's a review of Levinson 33Hs (which I help deliver and set-up a couple of times and they are about as good as it gets as far as increasing power with a a decrease in impedance):


http://www.stereophile.com/content/mark-levinson-no33h-monoblock-power-amplifier-specifications-and-associated-equipment

“Specifications
Description:Solid-state monoblock power amplifier. Rated continuous-power outputs: 150W into 8 ohms (21.8dBW), 300W into 4 ohms (21.8dBW), 600W into 2 ohms (21.8dBW), 1200W into 1 ohm (21.8dBW).”

http://www.stereophile.com/content/mark-levinson-no33h-monoblock-power-amplifier-measurements-part-2

“Finally, the big Levinson is a powerhouse of an amplifier, comfortably exceeding its rated power. Specified at 150W into 8 ohms, it actually didn't clip (defined as 1% THD+N) into that load until 265W (24.2dBW)! And the wall AC supply, at 114.5V, was starting to droop—this means that, with its own dedicated 30A line, this amplifier will probably put out 300W into 8 ohms.
Into 4 ohms, the maximum output power almost doubled, to 500W (24dBW); into 2 ohms, 900W was available (23.4dBW). (The wall AC voltages for these power figures were 113.3V and 112.5V, respectively.) “

[R. Daneel]

The B135 is specified as producing 135 Wpc into 8 Ohm and 180 Wpc into 4 Ohm loads. Does anyone know how much does it actually deliver?

I received no reply from ATC on their SCM19 monitor speakers and I wonder whether B135 is a good partner for these considering they have a flat impedance at 8 Ohm and a sensitivity of 85 dB/W/m at 1 kHz. These speakers need a lot of voltage to be properly driven so I am trying to see whether a Bryston integrated amplifier would be suitable for the task.

I have a strong feeling a 4B SST2 would be a better choice but it is only a power amplifier and I am not keen on using the BHA-1 headphone amplifier as a preamplifier despite the claims it performs admirably well in that area.

As far as specifications go, one important area is inter-modulation distortion (IMD). A few years ago I was at Faculty of electrical engineering here and one of the professors was demonstrating his own "Status" amplifier. He explained different principles and the possible advantages and disadvantages of each principle of amplifier design. Anyway, IMD was something that was more than measurable and with careful listening, it could be heard and eventually, discriminated as being the cause of a less transparent, less engaging sound. As he explained, this is because IMD occurs at upper and lower frequencies, not just at the base frequency. From a musical perspective, it is the equivalent of having one string slightly out of tune in an otherwise perfect arrangement. It sounds good and yet it doesn't sound right.

My post is a bit off-topic, sorry about that!
Cheers!
Antun