I replaced my 30+ year old Mark Levinson 333 stereo amp with two new AHB2's about six months ago, and I was happy with the change. I believe that I heard an improvement. However, I recently tried out a new Bryston 4B3 stereo amp at home and decided to keep it, replacing the dual mono AHB2's (which I still have), and I am also happy with that change.
The 4B3 seems to sound less thin than the AHB2's.
The dollar cost of two AHB2's and one 4B3 are comparable.
The only explanation that I can think of for my preference is that the 4B3 is actually rated to drive a 4ohm load and actually delivers, whereas the mono AHB2 is not so rated and possibly does not deliver as well. My initial AHB2 investigation consisted of correspondence with Benchmark on this issue, and they stated that in spite of not officially rating the AHB2 in mono mode for a 4ohm load, it would nevertheless drive a 4ohm load effectively.
I lieu of having the means to actually measure these two amps for comparison, I must depend on analysis of the published specifications for a possible explanation. And, the specifications for the two components differ in my application.
My speakers have an impedance curve that sometimes dips below 4ohms.
The above curves show the THD+N vs Output level for the AHB2. The blue curve is 8-Ohms stereo. The Orange curve is 4-Ohms stereo. The green curve is 8-Ohms mono.
The horizontal and vertical units are expressed in dB SPL at the listening position. This assumes 90 dB
[email protected], 1m. It also assumes a 2m listening distance in a "typical" listening room. You can easily adjust the values of the X and Y axis by a few dB if your speakers have a higher or lower sensitivity relative to 90 dB.
From X axis above, we can see that with this configuration, we achieve 110 dB peak SPL in stereo and 116 dB mono. From the Y axis we can see that the noise floor is -117 dB SPL in stereo and -115 in mono. This 3dB difference is expected because the two channels are connected in series in bridged mono and the amplifier noise is uncorrelated white noise. Notice that the output is 6 dB higher in bridged mono, so the SNR is actually 3 dB better in mono than it is in stereo, even though the noise is 3 dB higher.
More importantly, if you look at the Y axis, you will see that all three curves are entirely below the 0 dB SPL threshold of normal hearing. This means that the noise and THD never exceed the threshold of hearing, even when the amplifier is delivering full output (100W, 190W, or 380W). The green curve appears to reach about +4 dBSPL, but this rise is largely caused by the limitations of APx555b analyzer, but that is another topic.
What this graph shows is that the AHB2 does not show an increase in THD when driving 4-Ohms instead of 8-Ohms. This is very unusual. Most amplifiers produce more THD into lower impedances. The graph also shows that there is absolutely no audible distortion. The distortion is below 0 dB SPL which means that it would be inaudible if the distortion could be played while the music was muted. With the AHB2, we are not using the music to mask the THD produced by the amplifier.
Just to provide a bit of context, here is the AHB2 response with that of 6 other top-rated amplifiers. Notice the significant difference in the noise floors and notice the significant rise in THD that occurs with most amplifiers as the output level increases. All of these curves are only showing the 8-Ohms response.
And, to provide a bit more context, here is what happens with many amplifiers when driving 4-Ohms vs. 8-Ohms:
Notice how the THD is 5 to 10 dB louder when this amplifier is driving a 4-Ohm load. Also notice that the THD is 40 to 50 dB above the 0 dB SPL threshold of hearing. With this amplifier, you would need to hope that the music masks the THD, because it is louder than normal conversation.
I have just written an application note on the topic of plotting THD+N in terms of dB SPL. This plotting method makes it easy to see the noise floor, the SPL of the noise, the SPL of the THD, the peak SPL of the system, and the differences that are caused by changes in impedance loading. None of these can easily be extracted from a traditional THD+N % vs Power graph, but all of the data is there and it can be extracted and replotted in THD+N SPL vs Output SPL.
Here is the link to the paper:
How Loud is the Distortion from Your Power Amplifier? - John Siau, Benchmark Media Systems, Inc.
