Autoformer vs Speaker impedance Curve

One thing George is ignoring here is that in a Mac, the autoformers are inside the feedback loop

One thing Ralph is ignoring here, is if the solid state Mac was deigned and built correctly, it wouldn’t need autoformers, inside or outside the feedback loop!
Ask him directly if he honestly believes the sound will improve with autoformer on better solid state amps, like Classe’s, Krell’s, Halo’s, Gryphon’s ect ect ect, driving speakers like 73max has, in the B&W 802D3’s! then do the comparison yourself.

As I said before, put them on your Classe CA-2300 and see what they do to it’s sound, (I’ve done it on a few good ss amps and it’s not pretty) and one of the amps was a Classe DR25!, it’s the only sensible advice given so far from the pro autoformer side.

Cheers George

I design and manufacture audio transformers for all Music Reference amplifiers. I also make OTL amplifiers with and without autotransformers which actually extend the frequency response in the high end. 

Here are a few things to note. An autotranformer is not like a standard output transformer in that there is no need for HV insulation and the winding scheme is totally different. Where we interleave sections in an output transformer we can mulitfilar wind in a autoformer which reduces the leakage reactance markedly.. 

Also, in a 2 to 1 (4 to 1 impedance ratio) autoformer half the current is direct and half is transformed. So they are smaller and lower loss. 

As to McIntosh, their use is very clever and wise. If you note they drive the autoformer typically at the 2 ohm tap. This allows them to use lower rail voltages and stay out of the second breakdown region that all bipolar transistors suffer from. 

This is just a glance, if you want more just ask. As many of you know I have been doing this since 1980 and I find the art of transformers facsinating. Every design brings new challenges.

How fortunate are we in having two of the very best hi-fi amplifier designers/engineers working today (Ralph Karsten of Atma-Sphere and Roger Modjeski of Music Reference) sharing their knowledge with us here? That products of the quality they offer are made by not only such talented engineers, but also such nice guys, I am very grateful for. 

I note that this discussion has many questions concerning the use of an autotransformer with high damping solid state (voltage paradigm) amplifiers. I would say there is little to gain and a lot to lose using one. Here's why. This does not apply to the McIntosh amps and I previously stated their reasons for using autotransformers in their SS amps. 

First they place an almost zero DC resistance to the SS amplifier. That means a small offset voltage will create a large offset current which may be large enough to bring a torroid close to saturation. It takes very little DC to bother a non-gapped core. This is why tube amps should never have a torroid output transformer. 

Second, modern SS amps already have good current capability and low output impedance. They don't need any help

Third, good autoformers will do nothing to modify the wild impedance curve and reactive load that some speakers present. Some discussion of load line limiting in SS amps needs to enter here.

Heres what they will do. 

High output impedance OTL amps like the Atmasphere will benefit as their ideal load is typicaly higher than 8 ohms. My single pair OTL likes 64 ohms so I autoform it down to 16, 8 and 4 ohms and wrap 6db of feedback around the autoformer. 

All OTL amplifiers suffer from lack of current though there is plenty of voltage. Hey they are tubes... However the Futterman family has inherently low output impedance so the autoformer does not help with impedance but does exchange this excess voltage for improved current. A 2/1 autoformer will double the current available to the speaker. 

+1 Eric (bdp24).

We are privileged to have highly experienced and accomplished designers of outstanding audio electronics, such as Ralph and Roger, sharing their knowledge with us here.

Best regards,
-- Al