The under appreciated impedance curve

The quick answer: The weaker the amp, the more its output in the frequency domain will look like the impedance curve.

Thanks Eric.  I guess that's a good rule of thumb.  Though I always hear "it's totally different between SS and tube amps how they react to speaker loads" so I think I'm still a bit in the dark.

I have heard various things about matching a tube amps output tap to the speaker impedance.

On one hand some say if you have an 8 ohm speaker, you should be using an 8ohm tap output for your tube amps or you aren't getting all the power from your amp.

On the other, some have said that outputting from the 4 ohm tap will generally entail better sound/less distortion at the expense of less full output power, so *if* you will still have enough power for your speakers from the 4 ohm tap, better to use it even with 8 ohm speakers.

Confusing things more to me are that stereophile measured the CJ premier 12 amps I own as actually dropping power output into higher impedances (139W into 4 ohm speaker load, only 98W into 8 ohm speaker load).  Though the review amps came set for 4 ohm tap output...I don't know if that explains it and if switching the amps to an 8 ohm tap would retain their full power output to an 8 ohm speaker load?

Prof,

It really only takes a little math to understand how it works.


An ideal amp:


[ input voltage x gain ] = Output Voltage


Gain is often around 20x (26 dB)


The reality :


[Input voltage x gain ] = Voltage lost in output stage + Voltage at Speaker terminals


Tube amps will loose more in the output stage. You can analyze it more or less like series resistors.

Vr1 = Vamp out x (   R1 / ( R1 + R2))


Where R1 = Amp output impedance  and R2 = Speaker impedance


Of course, R2 varies based on frequency.

For Solid State, R1 may be < 0.001 Ohms. For tube it is often in the range of 1-2

The quick answer: The weaker the amp, the more its output in the frequency domain will look like the impedance curve.

And for those who want to know, here is the proof of that, with a well known push pull tube into the Stereophile simulated Kantor speaker load (which is a moderate easy load)

Simulated Kantor load (solid black wavy line)
https://www.stereophile.com/images/archivesart/scan58.jpg

Push pull tube 4ohm tap frequency response (+ - 5dB!!)  into that same Kantor simulated load (solid black wavy line)
https://www.audiofast.pl/assets/Uploads/pict2-prima-rec.jpg

See the amps frequency response take a dive at 30hz, 200hz and 4.5khz (more of a tone control than a ruler flat response)
Now look at the Kantor loading at 30hz, 200hz and 4.5khz, see how the load dictates what the response of the amp is doing at the same frequencies??? No wonder they sound coloured!!!

Cheers George