Autoformer vs Speaker impedance Curve

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

Thanks Al.  Guess that explains the bloated bass when I run 802D3s on my tube amp. 

Guess that explains the bloated bass when I run 802D3s on my tube amp.

No- it doesn't. If the amp is able to act as a voltage source, then it can make enough energy to be flat on the 4 ohm load of your woofer array. Since you say the bass is bloated, its doing that, so the bass bloat is something else. One possibility is that like at amps, yours is making more distortion into 4 ohms. The extra distortion could easily be perceived as bass bloat, since the 2nd harmonic is likely predominant. 

It could also be the power cord, as a cord that limited the amplifier's ability to replenish its power supplies might come off with more distortion as well. This effect is quite measurable- I've seen power cords rob a tube amp of nearly 30% of its total power!

Thanks atmasphere. Maybe bloated isn’t the correct word. The bass is markedly more pronounced from my tube amp (60 Watt push pull, KT88, “some” local negative feedback) than my SS CA-2300. It’s also seems “looser” or less defined.

Nonetheless, it seems reasonable to my novice brain that if an increase in speaker impedance increases the sound level “heard”,  that the bass should sound louder where the impedance spikes. No?

73max OP
bloated bass when I run 802D3s on my tube amp
60 Watt push pull, KT88, “some” local negative feedback

Without global feedback to keep the output impedance low, you probably have quite high output impedance giving you very mediocre damping factor, and no control over the bass hence your term "bloated" compared to the control the Classe has over the bass.

You could always bi-amp with the tubes on the mids/highs and the Classe on the bass, but you’ll need a passive volume control on the input of louder of the two amps to reduce and set it so it’s the same gain as the other amp.

Then use your master volume control on the main pre or dac to vary the whole (both amps) volume.

Cheers George