amp power & driver sensitivity in multiamp setup

Hi Lewinski,

First, I wouldn't rely on an SPL reading with the meter set to "slow." The necessary power will typically depend on the amplitude of very brief musical peaks, which might even be too brief for the meter to fully capture when set to "fast."

Also, I believe that a rough rule of thumb for the frequency above and below which music often requires roughly equal amounts of power is 350 Hz. And while I recall from your past posts that you will be using an electronic crossover ahead of the power amps, so that most bass content will be kept out of the 2A3 amp, depending on the crossover point you choose the mid-range driver may still therefore often have to provide SPL's not a great deal lower than the total contribution from all of the drivers that is necessary on those brief peaks. Perhaps just 3 db or so lower, which in power terms is a factor of 2.

See my post here for a description of how to approximately calculate maximum SPL at a given listening distance, as a function of amplifier power and speaker efficiency. Keep in mind, though, that this methodology neglects room effects, and also neglects thermal or other forms of compression that may occur in the driver at high volumes. It also assumes, of course, that the efficiency or sensitivity specification of the driver is accurate.

Based on that methodology and on those assumptions, I calculate that the 2 watt amplifier and two (left and right) 94 db/W drivers will be able to produce a maximum volume of around 90 db at a 10 foot listening distance, roughly corresponding to a total SPL produced by all drivers in the area of perhaps 93 db or so. That probably figures to be sufficient for a considerable majority of recordings, but I would not feel comfortable that it would be sufficient for some recordings having particularly wide dynamic range (i.e., large differences in volume between the loudest notes and the softest notes). And of course the lower the crossover point you are intending to use between the midrange drivers and the low frequency drivers, the greater that concern would be.

Finally, keep in mind a point Ralph/Atmasphere has stated many times, that SET amps do not sound their best when asked to provide more than just a small fraction (perhaps 20 or 25%) of their rated power capability.

Good luck. Best regards,
-- Al

Thanks for the nice words, Lewinski. I agree with all of your comments, aside from a minor typo in no. 2 (you meant to say "2 watts" instead of "3 watts").

Re "is this 95 dB peak or RMS," that would be "peak" in the sense of "maximum," if that makes sense.

The 3 dB added to reflect two speakers is indeed conservative, given your centered listening position, and room effects will help also, with 3 dB perhaps being a conservative assumption as well.

So as I indicated in my previous post I suspect you would do well with most recordings, but not necessarily with all recordings. For example, I have in my collection a goodly number of classical symphonic recordings on audiophile-oriented labels such as Telarc, Sheffield, Reference Recordings, etc., that were subjected to minimal or no dynamic compression when they were engineered and mastered. When those recordings are played at average levels of perhaps 75 db at the listening position, some of them will reach brief peaks in the area of 100 to 105 db (measured at the listening position, with a Radio Shack digital SPL meter set to "fast" and C-weighting).

On the other hand, though, as you've probably seen in past threads here some members report surprisingly good results using low power SETs with speakers that are considerably less efficient than 94 dB. But FWIW my own bias is that I don't want to be marginal when it comes to power.

So the bottom line would seem to be that it comes down to an individual judgment call, with the most significant variable probably being the kinds of recordings that are listened to. Re-doing your SPL measurements with the meter set to "fast," and using recordings you may have which have particularly wide dynamic range, would probably be helpful in making that call.

Best regards,
-- Al

Hi Horacio,

There are a number of variables and unknowns (to me, at least) that enter into your question. And while I therefore don't know what the answer would be I suspect that chances are it would be significantly different than 101 dB (or 100 dB, which you may have meant, that being 94 + 3 + 3).

First, without a specific indication from the manufacturer I wouldn't assume that the per watt efficiency of the doubled impedance driver goes up by 3 dB. Also, I wouldn't assume that the amp has the same power rating into 16 ohms as into 8 ohms, especially if it does not provide a 16 ohm output tap. And if it does not provide a 16 ohm tap, but only provides say an 8 ohm tap, I would not necessarily assume that it can perform at its sonic best when working into 16 ohms.

Best regards,
-- Al

Hi Horacio,

In this case it was me who wasn't quite right, about the 98 and 101 dB. When I wrote my last response I didn't re-read your post which preceded the one I was responding to, so I had forgotten where the 98 dB (at the listening position) had come from.

In any event, as you realize, decreasing load impedance will increasingly challenge an amplifier with respect to the correspondingly increased demand for current (everything else being equal). And also thermally in the case of non-class A designs (SETs are class A), due to the temperature rise caused by the increased current passing through their output circuits.

However, if the amp does not have an output transformer, or for a given output tap if it does have an output transformer (as I assume any 2A3 amp does), if the load impedance rises to high values output power capability will be limited by the amp's voltage swing capability (i.e., the maximum amount of voltage it can put out).

With many solid state amps, that will probably cause the maximum amount of power that can be delivered into 16 ohms to be not a great deal more than 1/2 of what can be delivered into 8 ohms, corresponding to there not being a great deal of voltage headroom relative to the voltage capability required for an 8 ohm load. Some amps that are rated to have several dB or more of dynamic headroom, though, would probably do better than that. Although many times large amounts of dynamic headroom may simply reflect that the amp is not robust enough to sustain high output currents continuously.

With a tube amp operated from its 8 ohm tap, as the load impedance increases significantly above 8 ohms max power capability may initially increase, but will eventually also reach a point where voltage swing capability, and perhaps also increased distortion, will limit its output capability. Where that point occurs will depend on the design of the specific amplifier.

If the amp provides a 16 ohm tap, though, I would generally expect power capability from that tap into 16 ohms to be in the same ballpark as power capability into 8 ohms from the 8 ohm tap.

Best regards,
-- Al

Thanks, Ralph. I don't think, though, that Lewinski was asking about adding more drivers or putting drivers in series. As I read it he was asking about the possibility of choosing a single 16 ohm driver instead of an ostensibly similar 8 ohm driver, and whether doing so would be beneficial with respect to maximum volume capability.

Best regards,
-- Al