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AUDIO FORUMS >> General Discussion and Support >> Speaker sensitivity measurement
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Message started by Burgermeester on 06/02/22 at 02:47:04

Title: Speaker sensitivity measurement
Post by Burgermeester on 06/02/22 at 02:47:04

Thought-provoking. I didn't know that tube power output was invariant with respect to impedance.

https://www.youtube.com/watch?v=PEcFkSQMc8g

FF to 10:45 for the short version if you are sensitive to, or (like me) susceptible to being driven mad by, ah... Well you'll see.

Title: Re: Speaker sensitivity measurement
Post by Lin on 06/02/22 at 18:49:30

Burgermeester wrote on 06/02/22 at 02:47:04:

Thought-provoking. I didn't know that tube power output was invariant with respect to impedance.

He was generalizing to make his point.
Tube amps have a maximum power at a certain impedance (based on the OPTs) and typically have less power at higher and lower impedance. Some times the difference is fairly significant and other times it is not much.
Speakers "rated" at 8 ohms (or 6, or 4, etc) can be 5 ohms at certain frequencies and they can be 30 ohms or more at other frequencies.

Check out Stereophile measurements of speaker impedance and tube amplifier power into different impedances.

An example of a single ended EL84 tube amp that I used to own that was switchable between pentode and triode, but only had one winding tap for the speakers.
Pentode 6 watts - 8 ohms; 3.5 watts - 4 ohms
Triode 1.8 watts - 8 ohms; 1.7 watts - 4 ohms

Amplifiers that have multiple output winding taps will come closer to the same wattage at the tap impedance, but remember 99% of speakers do not present a flat impedance to the amplifier, so the actual wattage will still vary.
That is why Decware and many tube enthusiasts will tell people to try all of the speaker taps regardless of what the speaker impedance "rating" is, the "wrong" one may actually work better and therefore sound better.

Title: Re: Speaker sensitivity measurement
Post by Lin on 06/02/22 at 19:58:16

JA makes 2 posts about the video and his measurements in the comments section here:

https://www.stereophile.com/content/gramophone-dreams-46-decware-zen-triode-amplifier

Title: Re: Speaker sensitivity measurement
Post by armstdav on 06/02/22 at 22:47:25

Here's the stated power output of my Atma-Sphere M-60s:

Output power
60 watts per channel into 8 Ω load
45 watts per channel into 4 Ω load
80 watts per channel into 16 Ω load

So at least for that amp the power does vary based on speaker impedance.

Title: Re: Speaker sensitivity measurement
Post by CAJames on 06/02/22 at 23:39:28

The Atma-Sphere M-60s are OTL, so they don't have a transformer to match the speaker impedance. They are "special."

Devore did actually say the power is the same only if the speaker impedance matches the OT tap impedance. It is less if they don't match.

To the larger point, matching impedance maximizes power transfer, but does it maximize sound quality? Everywhere else in your system you want the source impedance to be much less than the load.

Title: Re: Speaker sensitivity measurement
Post by Lin on 06/03/22 at 01:02:26

CAJames wrote on 06/02/22 at 23:39:28:

To the larger point, matching impedance maximizes power transfer, but does it maximize sound quality?

Everywhere else in your system you want the source impedance to be much less than the load.

2 unrelated and totally different things.

Having equal impedances from one component to another reduces transfer and is detrimental to sound quality.