What is the best premp for Atma-Sphere MA-1 3.3 amp?

...  I felt like I was back in my Electrical Engineering class at MIT 40 years ago.

@gdnrbob, thank you kindly for the nice words.

I of course agree that the major factor in determining how much power an amp needs to be able to supply is the ability of the amp/speaker combination to be able to cleanly handle the peaks of brief high volume transients in the music. With recordings having particularly wide dynamic range (dynamic range referring to the **difference** in volume between the loudest and the softest notes) being the most challenging in that respect. Many classical symphonic recordings, for example, being far more challenging in that respect than most pop and rock recordings. As I’m sure you know.

In choosing between two amplifiers which differ greatly in terms of their maximum power capability, though, such as the MA-1 and the X350.5, what I would do initially is to determine if the lower powered amp is powerful enough to support the listener’s desired peak volume capability, that may occur on brief transients. And if that criterion is satisfied I would then look at other variables in choosing between them. And if one amp is a solid state model and the other is tube-based, given that the impedance of the speaker varies widely over the frequency range the other variable I would look at first is which amp would result in the best tonal balance, taking into account the interaction of the amp’s output impedance and the speaker’s variations of impedance as a function of frequency. In short, what Ralph has discussed in his “Paradigms” paper.

I see that the Sound Lab 545 models have sensitivity specs of 86 db/2.83 volts/1 meter, with a specified nominal impedance of 8 ohms. Presumably the 8 ohm nominal impedance occurs somewhere in the mid-range, e.g., at or near 1 kHz. While the X350.5 is presumably capable of providing only 90 watts or thereabouts into the impedance of the 545 at low frequencies, let’s make an optimistic assumption in its favor that it can provide 350 watts into the speaker at all frequencies. (And in any event the difference in maximum volume capability between 90 watts and 350 watts is only about 6 db, for a given speaker). And given the OP’s indication that his listening room is 400 square feet let’s assume the listening distance is around 4 meters.

Let’s then use the following calculator to determine the maximum SPL 350 watts can produce with 86 db/2.83 volts/1 meter/8 ohm speakers at a listening distance of 4 meters, noting that 2.83 volts into 8 ohms is 1 watt. But let’s use a listening distance of 2 meters in the calculator to reflect the fact that SPL falls off much less rapidly as a function of distance in the case of planar (or line source) speakers than in the case of point source and most dynamic speakers (which are what the calculator is based on).

https://myhometheater.homestead.com/splcalculator.html

Converting meters into feet and neglecting room reflections the result is a maximum volume capability of 108.4 db at the listening position. And if we make the less optimistic assumption that the X350.5 can only provide 90 watts at all frequencies the result would be 102.5 db. Those levels will be adequate for many and probably most listeners, but not for some. For example I know that Ralph sometimes listens at considerably higher peak volume levels than those, in part because his system can handle higher volume levels without significant distortion. And I listen to many classical symphonic recordings having very wide dynamic range (e.g., on labels such as Telarc, Sheffield, Reference Recordings, etc.), and for my purposes an amp/speaker combination that can’t comfortably reproduce 105 db peaks at my 12 foot listening distance, plus a little margin, is a non-starter.

In any event, the specified power capability of the MA-1 is only about 2 db higher than 90 watts, and is only about 4 db less than the 350 watt specified capability of the X350.5. So given the OP’s indication that he doesn’t listen at rock concert levels let’s assume that a little more than 100 db at the listening position is adequate for his purposes. Which would be satisfied by both amps. So I would then focus on other variables, starting with the tonal consequences resulting from the interaction of amplifier output impedance and the speaker’s wide impedance variations over the frequency range. And that would clearly seem to favor the MA-1, given numerous anecdotal reports I have seen over the years here and elsewhere involving Sound Lab speakers, as well as what Ralph has explained in his “Paradigms” paper.

Best regards,

--Al