amp power & driver sensitivity in multiamp setup

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 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?

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

Ralph, sorry I missed your post. And thanks for chiming in!

Al is right: I was considering using a single 16 ohm driver.

Maybe using specific examples helps. The original 94dB/2.83V driver was a B&W FST. PHL Audio makes high sensitivity drivers, and I was considering some of their 6.5" and 8" midranges as well. For example 2520 is an 8" 4 ohm driver spec'd as 100dB/2.83V and the 2530 is the same but 16 ohm.

Also as example, I'm looking into Yamamoto A-08S (45 SET) and A-011 (2A3). These only have one set of speaker connectors, though. I'm looking into what kind of impedance they were designed for, but so far haven't discovered it.

BTW, what's the impact you would expect if I ran one channel on the 5 ohm tweeter and the other channel on a 16 ohm midrange? Would that tax the amp in any way?

Thank you!

When you say "if the amp has only one output and no taps", what are you referring to by taps? I initially I thought you meant speaker output connectors, but maybe not.

PHL Audio is a French manufacturer, well regarded in EU.

Thank you Ralph.

Let's assume for now the amp only has 8ohm taps (I've asked the manufacturer), and let's assume it's a stereo SET amp and one channel will be used to drive a 5 ohm tweeter (it measures 5 ohm flat across the usable bandwidth) and the other channel will be used to drive one of these midranges. Let's say the 16 ohm driver. Both drivers directly connected to the amp, with the crossover upstream from the amp.

Would you expect any downside in terms of sound performance or expected life of the amp by running different impedance loads on each channel?

Thanks again!