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

Al,

As usual, great insightful answer!

I do keep in mind Ralph's comments about the 25% or so. That was an insightful comment as well. Keeping this in mind I will run the numbers assuming 50% of amp rating as max working power, thinking at max power I would allow distortion to be higher than ideal, but far from the amps rated max power. So a 2A3 amp rated at 4W+4W, I will use 2+2W.

Running the math from the post you linked to:

1)Compute how many db greater than 1 watt the amp's specifiedi power rating is, based on the relation db = 10log(P1/P2), where "log" is the base 10 logarithm.

In this case 10log(2 watts/1 watt) = 3 db.

2)The driver is rated at 94 db/1 watt/1 meter. Therefore 3 watts will produce 94+3= 97 db at 1 meter.

3)Add 3 db for the second speaker.
97 + 3 = 100 db at 1 meter.
The listening position is indeed centered between the speakers. Maybe 3 dB gain is conservative in this case?

4)Calculate the reduction in SPL at distances greater than 1 meter as 20log(1 meter/distance in meters). In my case 2.4m. 20log(1/2.4) = approximately -8 db. 100 - 8 = 92db.

The room is made of brick, 5m x 5m x 2.4m high. The speakers are placed 1m from the front wall and 1.4m from the side walls. Is it safe to assume 3dB room gain? That would put me to 95dB.

But then, what does this mean? Is this 95 dB peak or RMS?

BTW, the crossover to the bass will be in the 350 to 400Hz region.

Thanks a lot!

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

Thanks Al.

Last night I only had a chance for a brief listen and set the Radio Shack meter to fast and C-weighting. When measuring 80dB in "slow" I would get about 86dB peaks in "fast". This was on a 30-year old rock recording that I believe was pre pervasive-compression era. I need to do more listening and measuring, but my current guess is my albums are going to generally have dynamic peaks of maybe 10-15dB. Can't be proud of that, but I gues it is what it is :-)

If I re-run the numbers above using 4.5dB gain for the second driver and 4.5dB room gain, for 2W and 94 dB/W I would get 98dB SPL at the listening position. Borderline, it seems.
If the driver was instead 100 dB/W then I would get 104dB, which seems more than enough. Even a 45 amp, at 1W (2Wpc rating), would deliver 101dB. Of course, such midranges are not easy to come across.

I need to go back and pick my poison.

Thanks for the very helpful input!

Regards,
Horacio

Hello Al.

Follow-up question: how would the above change if I doubled the driver's impedance? Say the above calculations were for an 8-ohm-rated driver. If that manufacturer had the same driver in 16 ohm version, then current would be halved for a given SPL, right?

So if the 94dB/W, 8 ohm driver delivers 98dB SPL with 2W, would the 16 ohm version of the same driver deliver 101dB SPL with the same 2W?