It is not often we reference the use of autoformers with amplifiers, as the more popular use is as an attenuator in a preamplifier (passive or active). It is known that people make them for use with amplifiers, one being Paul Speltz (https://anticables.com/autoformers), and McIntosh has a long history of manufacturing solid state amplifiers that use autoformers. In fact, there is quite the debate on the use of autoformers in McIntosh circuits. However, their autoformer output coupled amplifiers have enjoyed quite an avid following over the years. With autoformers in the circuit these amplifiers will output constant power into 2, 4, and 8 ohm loads.
McIntosh aside, the use of autoformers is more common with tube amplifiers that have high output impedance. Ralph Karsten of Atma-Sphere was making the Z-Music autoformers for use with his output transformer less (OTL) amplifiers prior to Paul Speltz making the Zero Boxes. Atma-Sphere and Magnepan are in Minnesota and Ralph had a lot of local customers that wanted the transparency of the Magnepans combined with the transparency of OTLs. Magnepans have a 4 ohm (dipping to 3-ohm) resistive load, and the higher output impedance of the Atma-Sphere amplifiers meant they put out less power and current into the load. Not to mention the damping factor was 1, not exactly ideal for a Magnepan. As such, the autoformer was the answer allowing the amplifier to “see” a higher load and lessen the stress on it.
Now some speakers do not mind an amplifier with high output impedance, traditional horns and speakers with a nominal 16-ohm impedance immediately come to mind, but there are others. However, from my experience with Atma-Sphere M-60 amplifiers and their 4-ohm output impedance matching them with a speaker is a bit challenging. For example, I have heard quite a few audiophiles comment that OTLs and electrostatic (ESL) speakers are a match made in heaven. However, with ESLs being a capacitive load that comment is a bit off base. For example, when my QUAD ESLs start nosediving from its 30-ohm resonance peak at 90 Hz towards 2 ohms in the 15 KHz to 18 KHz range things can get a little hairy for any amplifier, let alone an Atma-Sphere OTL, and by that point the amplifier is working entirely too hard.
It should be noted that Atma-Sphere OTL amplifiers are based on the Circlotron circuit (designed by Alpha Wiggins of Electro-Voice) and use little, 1 dB in my M-60s, to zero feedback. On the other hand, the Futterman influenced circuits use larger amounts of feedback thereby greatly reducing the output impedance and making the 2-ohm load seen by the amplifier in the 15 KHz to 18 KHz range of a Quad ESL much more manageable. In my system using the Paul Speltz Zero Boxes between the M-60s and the QUAD ESLs presented a minimum 8 ohm reflected load to the amplifier based on connecting the amplifier to the 4x tap. I can say that overall, this resulted in a happier amplifier and listener.
On the other hand, Roger A. Modjeski designed his OTL amplifier with the autoformer built into the circuit. The amplifier was stable to 2 ohms, perfect for my Quad ESLs, and regardless of whether you connected the speaker to the 2, 4, or 8 ohm taps the amplifier “saw” a reflected load of 32 ohms. In addition, Roger added a direct output tap that by-passed the autoformer so if you had very efficient 16 ohm or 32 ohm speakers that tap could be utilized. As feedback was used in the design the amplifier had low output impedance and adequate, but not overly excessive, damping. If you have not already done so, give an autoformer a test run. Regardless of whether you own a solid state or tube amplifier, or the type of speakers the amplifier is connected to, the results may surprise you. I might be willing to bet you will hear the difference immediately as opposed to what you supposedly hear “listening” to your cables, fuses, etc. for hours on end.
Autotransformers: The Benefits in Matching an Amplifier to Speaker
By Roger A. Modjeski
There has been a great deal of conversation about autotransformers (autoformers) in this forum. Many think they are like the output transformers we use in tube amplifiers. They are not for some very important reasons. First, they are not wound the same way as they have no high voltage insulation. Autoformers are wound with heavy low resistance wire and all the winding is used all the time. In addition, part of the signal current is direct, and part is transformed. Now let us consider a few more points: