amp power & driver sensitivity in multiamp setup


I'm building an active 4-way system and would like to use SET amps for midrange and tweeters. I'm getting confused about needed power for a given midrange. The driver is 94 dB/W sensitive and has a rather flat impedance of 6 ohm throughout the range.

I usually listen at around 80 dB SPL as measured with a Radio Shack meter C-weighted & set to slow, sometimes go up to 85dB, and very occasionally 90dB. But of course this is for the summation of bass, midrange and tweeter.

From this a 2A3 SET delivering 2W should be more than enough to drive the midrange only. Right?
lewinskih01
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?
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
Al,

I did mean 101 dB, but that doesn't mean I had it right :-)
My rationale: if the given amp driving the 8 ohm driver, which is 94 dB/W, delivers 98 dB SPL, then when driving a 16 ohm version the amp will need to deliver half the current and hence twice as much voltage, adding 3 dB to the 98 dB.

As far as the amp goes, you are right I'm making lots of assumptions. I thought amps generally were challenged doubling Wattage as load impedance halved, but not the other way around. Not sure either if SET are the same in this regard.

Thanks for helping me think through this!

Best regards,
Horacio