amp power & driver sensitivity in multiamp setup

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

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