Volume difference with 4 ohm AR's vs. 8 ohm AR's

[Guest postjob62]

In comparing vintage AR speakers against each other through a 3-way speaker selector box (Adcom GFS-3; doesn't seem to affect sound noticeably) I have noticed that the 4 ohm 3a's and TSW-610's are significantly louder at the same amp volume settings than the 8 ohm 2ax's, 2x's and 4x's. I actually first noticed this when trying to A-B a single pair of 4x's (8 ohm) against stacked 4x's (4 ohms). I would have expected the "stacked" 4x's to have a bigger sound, which they do, but I was surprised to find that they were also significantly louder than a single pair through the same amp a the same volume dial setting.

Knowing that an amp has to "work harder" with a 4 ohm load than an 8 ohm load, I would have expected just the opposite to be true. Can anyone explain this phenomenon in fairly layman-ish terms? I don't think I'm imagining it. FWIW, this amp has no trouble at all with 4 ohm loads.

Obviously one can increase the volume dial setting on the amp to compensate, at least to a point. But this begs the question as to whether the clipping level of the amp is more related to the relative loudness level coming from the speakers, or the volume dial setting on the amp?

I'm sure this is very elementary stuff to many of you but it has me a little flummoxed. Any help is appreciated.

Ed

[tysontom]

>In comparing vintage AR speakers against each other through a

>3-way speaker selector box (Adcom GFS-3; doesn't seem to

>affect sound noticeably) I have noticed that the 4 ohm 3a's

>and TSW-610's are significantly louder at the same amp volume

>settings than the 8 ohm 2ax's, 2x's and 4x's. I actually first

>noticed this when trying to A-B a single pair of 4x's (8 ohm)

>against stacked 4x's (4 ohms). I would have expected the

>"stacked" 4x's to have a bigger sound, which they

>do, but I was surprised to find that they were also

>significantly louder than a single pair through the same amp a

>the same volume dial setting.

>

It is all in Ohm's Law (Wattage = voltage squared divided by resistance): the 4-ohm AR-3a is approximately the same efficiency as the 8-ohm AR-2ax, but for a given volume setting (re: "voltage setting"), the lower-impedance (almost the same as resistance) speaker will draw almost twice the power from an amplifier and thus sound louder for a given volume setting. More wattage -- for a given volume-control setting -- is actually flowing to give that louder sound. The greater power comes about because of increased amperage flowing at that given volume setting due to the lower impedance, thus the amplifier is indeed working "harder."

--Tom Tyson

[dynaco_dan]

Excellent explanation, Tom.

Pre AR-1 speakers, I believe, most hifi expensive speakers were 16 ohm impedance for the tube equipment of that day.

When you buy any amplifier you need to read the 4, 8 and 16 ohm ratings.

Some lower cost amplifiers may not even list 4 ohms, you can assume there will be little output, if any.

There may also be a manufacturers caution if the speakers are less than 8 or 6 ohms.

If we use the AR-3A speaker system as just one example, the speaker is rated @ 4 ohms impedance.

The actual impedance swings from a high of about 35 ohms in the lower bass, 4 ohms @ 1khz and 2.75 ohms in the highs.

A good match would be an amplifier with a high 4 ohms rating and increasing output in the 16 ohms would be better than the reverse.

An example of an amplifier with 50 watts @ 4 ohms, 25 watts @ 8 ohms and 12 watts @ 16 ohms would be bass shy at the 35 ohms area.

[Guest postjob62]

Thanks Tom, that of course does indeed make sense in a way even I can understand.

Going forward from there, would it be safe to say that in very general terms the onset of amplifier clipping would occur at similar overall volume levels produced by the same amp pushing first 3a's then separately 2ax's? Obviously the volume dial setting would have to be advanced more with the 2ax's to produce the same volume level as with the 3a's, so this would seem to make sense. I guess I'm asking if clipping is more directly related to volume level produced in this example more so than it is related to the amp's "o'clock" volume dial position.

I ask this in part because I have always had difficulty accurately recognizing the onset of clipping, and I feel compelled to try to protect these old treasures. Although I normally power my 3a's with lots of clean power, I am constantly surprised at how good they sound with an old 40 wpc receiver when the mood strikes. This of course assumes the receiver in question puts out as much or more wattage at 4 ohms as it does at 8.

This also helps me to understand that more is involved than just the speaker's efficiency; the 3a's are of course pretty notoriously inefficient.

Ed