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Autoformer vs Speaker impedance Curve

Autoformers vs speakers with wild impedance curve swings (for instance; MC601 amp paired with B&W 802D3 speakers).

There’s a wealth of information about tube amp audio transformers interaction with speaker impedance, but I can’t find anything regarding Autoformer and speaker impedance/phase curve relationships. 

Can any techies enlighten me? 

Thanks!

(I tacked a similar post onto the end of a 10 year old thread but thought I might get a few more hits with a new thread.  Sorry for the redundancy)



73max
unsound
7,365 posts
 

I previously posted this on another similar thread, but it might be more appropriate here:

https://www.stereophile.com/content/mcintosh-mc501-monoblock-power-amplifier-measurements-0

georgehifi's avatar
georgehifi
10,312 posts
 
ramtubes
Second, modern SS amps already have good current capability and low output impedance. They "don’t need any help"

And I’ll add to that, they will also make that amp sound worse if used, as I’ve proved to myself with 3 or 4 amps that were very capable without the autoformers, they made the amps sound like they were cushioned, like looking through opaque glass instead of clear glass.

They are for amps "that need help", but I say save your money and get the right amp instead, or change the speaker to one that the amp in question is happy with.

Cheers George
ramtubes's avatar
ramtubes
460 posts
 
I have started a new topic to explain how conventional output transformers differ from Autoformers. 

https://forum.audiogon.com/discussions/autoformers-the-benefits-in-matching-amp-to-speaker
73max OP
46 posts
 
A twist on the original question.  Numbers and graphs aside, what we hear is the ultimate arbiter. 

With a tube audio transformer:

What happens to what we hear when the speaker impedance dips hard at a specific frequency? Will that “frequency” be heard louder or softer?

...and the opposite. What happens to what we hear when the impedance spikes?  Will that frequency be heard louder or softer. 

Same question, but with an Autoformer?
almarg's avatar
almarg
9,670 posts
 
For both transformers and autoformers, softer on dips and louder on peaks, ***relative to the response that the particular speaker would provide if the amp were an ideal voltage source, meaning a voltage source having an output impedance of zero.*** However in the case of an autoformer (which would be used in a solid state design) the differences will be extremely small, and essentially negligible with most speakers. Not because of the difference between an autoformer and a transformer, but because of the differences in output impedance between tube amps and nearly all solid state amps. As I said in my post dated 8-16-2018:

A major factor contributing to this, and what is probably the most major factor in many cases, is not the output transformer itself, but the interaction of the output impedance of a tube amp with the speaker impedance variations that you are referring to. In contrast to nearly all solid state amps, most tube amps have output impedances that are a significant fraction of speaker impedance, usually somewhere between a large fraction of an ohm and several ohms. That in turn causes the voltage divider effect to have significant effects on tonality, to the extent that the speaker’s impedance varies as a function of frequency.

In the case of McIntosh solid state amps which use autoformers that particular effect is essentially negligible with most speakers, because as a consequence of being solid state their output impedance is much smaller than the output impedance of most tube amps. (Although that certainly does not mean that an amplifier having low output impedance is necessarily the best match for a given speaker, in terms of tonality). For example the MC302 has a specified damping factor of "greater than 40," which for the 8 ohm tap theoretically corresponds to an output impedance of less than 8/40 = 0.2 ohms.
Regards,
-- Al
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