The ML2 runs in Class A only.
Since the FR-30 Aspen speakers are rated at 4.0 ohms nominal impedance to 3.6 minimum, it is not as helpful to rate the PMG amps Class A output at 8.0 ohms.
If the M400 for example, has 30 watts Class A at 8 ohms, even if it is 15 or 35 watts at 4 ohms, it is still meaningful information for Aspen owners, as most other PSA speakers are also 3.3 to 4.0 ohms as well.
It is not a limitation of class A amplifiers, they can be designed to almost double the class A power when impedance is halved. If the standing bias current is set high enough it is definitely achievable, it just not practical at higher power levels as the amplifier would be as big as a refrigerator without forced cooling.
Folks:
Ohm’s Law is as follows:
P1/P2 = R1/R2 where P is the Power in Watts and R is the impedance. So do the math. But current is more important than Power for driving speakers. Just my 2 cents.
Yes, but just because the amplifier has the current to nearly double when impedance is halved in class A/B does not mean that the class A bias current is high enough to almost double when impedance is halved, in fact it gets cut in half on class A/b amplifiers.
Thanks for the input and I could be wrong but the above equation that I listed comes from P = I^2R and under conditions 1 and 2 if you assume constant current becomes P1/P2 = Ri/R2. I am assuming that class A yields constant current. So, Class A current stays the same but Class A Power halves. Just my 2 cents.
I believe what Paul is saying here is that the power will double even while in class A mode.
P = I squared / R, so if the current is constant the power will double if you have the resistance. That’s what the math says, anyway. Of course, the power supply has to be able to actually supply the constant current.
You are correct Joe, some are getting class A/B output current mixed up with class A standing bias current which is 1.37 amps of standing bias current for 30 watts of class A, which only gives 15 watts of class A wattage into 4ohms.
This may be read as a pedantic response and for that I apologize in advance.
The voice coil in a speaker would be more accurately modeled as an RCL circuit having an impedance (Z) than as a purely resistive element (R).
Impedance (Z) is the geometric sum of the speaker’s fixed DC resistance (R) and frequency dependent reactance (X)
Z = SQRT[R^2 + X^2]
The overall reactance element (X) includes both an inductive and capacitive reactance component that are functions of frequency.
Inductive Reactance = 2(pi)fL
Capacitive Reactance = 1/[2(pi)fC]
X (overall reactance) = Inductive reactance - Capacitive reactance
All of this is to say that things are a bit more complicated once the speaker is acknowledged as not being a fixed resistance as viewed by the amplifier. A speaker’s impedance is not fixed either and is typically quoted at a specific frequency, i.e. 400Hz. Lastly, when multiple drivers are involved, impedance gets even more crazy depending on how the drivers are connected (series, parallel or a combination of both) and the characteristics of crossover circuits. Fun!
Class A amps run full bore, and are current limited, constrained by their bias as determined by their ability to throw off the heat. They do not double as a traditional Class AB amp would. Those so called class A amplifiers are either using a sliding bias, dynamic bias, or are shifting into class AB. Pass Labs push pull Class A amps would be an example of the latter. Biased class A, the output devices do not switch off as class AB would.
But real world they are power limited, heat sink sizing, power supply margin and output device bias current come into play. Dynamic biasing is actually not true class A, nor is Pass Labs push-pull class A. Both being hybrids, when power increases above their class A power limit distortion rises exponentially.
A class A amplifier can come close to doubling it’s class A power if it’s bias current is high enough to do it. The Mark Levinson ml2 was purposely designed to do this down to 2ohm loads, but the voltage rails are only 27v+ and 27v- and the bias current is set very high, it draws a constant 400 watts. I say comes close because no amplifier can actually double output power when impedance is halved.
Can you help with this question? It clearly has provided multiple interpretations. Can you provide a Class A wattage measurement into 4 ohms for the M400 and explain the logic?
Even if it drops from 30 watts at 8 ohms to15 watts at 4 ohms, it would be good to know. Or maybe it is somewhere in-between.
For Aspen FR30’s the ohms are 3.6 to 4.0, so it seems appropriate to indicate the 4-ohm number and if it even has any bearing on sound output in Class A, before the amp switches to alternative watt headroom.
Thanks
I am thinking of using iso-acoustics Orea Indigo feet under the M400. Any thoughts? Four at 16 pound each are a good match for the 56-pound M400.
Will there be any noticeable sound difference?
I used the Oreas under BHK components, DAC, pre and the 600 amps. They did clean up the mids and treble but at the expense of clarity; the sound became too dull so I removed them and found the stock feet much better.
The PMG transport, DAC and preamp were a large enough improvement I just used the stock feet for a while. I then tried the VooDoo isopods which gave a nice improvement over the stock feet. Each isopod supports 250 lbs. max so no worries there. I will eventually try them with the M800s and S400 which should arrive next week (here’s hoping).
Thanks. I sure do not want these to change the amp to “dull”! I am only interested in the PMG amps and if vibration in it matters on a concrete floor. I certainly want clean sound, but love detail and clarity. My other Arcici stands take care of the other source components.
I wonder if Chris has tried any iso’s on these, as he mentioned to me once they were pretty effective on some other things in the past.
I’ve used Oreas under equipment in the past, but I moved to the VooDoo Iso-Pods and had them everywhere. . . until, darn it, I discovered the Stack Audio feet. These have become my favorite and I now have them under every component in my main system, except under my speakers. For now at least there are Iso-Pods under those.
isn’t it lousy when you were satisfied until you find the next thing? Luckily, I haven’t tried the Stack Audio feet 
Yes! Ignorance is bliss!