Autoformers, The Benefits in matching amp to speaker

There has been a great deal of conversation about Autotransformers in this forum. Many think they are similar to the output transformers we use on Tube Amplifiers. They are not for some very important reasons. They are not wound the same way, they have no High Voltage insulation, they are wound with heavy low resistance wire and all the winding is used all the time. In addidtion part of the signal current is direct and part is transformed. 

• THE WINDING.  When we make a traditional output transformer we have to insulate the primary from the secondary for over 1000 volts. This insulation takes up space and winding space is most dear to the designer as we want as much copper in there as possible. We then have to section the windings and interleave them. An interleave of 5 is good and some think 7 or 9 or even 11 is better but that raises the capacitance of the transformer and is hard on the tubes at high frequencies. An autotransformer has no DC voltage in the windings and thus can be bifilar wound (taking 2 or 3 or more wires at once). This increases the coupling and extends the high frequency response by a factor of 2 or more. My ouput transformers are good to 65 KHz and the Autofomer is good to 140 KHz. 
  

• THE CORE: As to the core, an EI core is preferable over a torroid as the torroid will have saturation problems if connected to an amplifier that has a DC offset. An offest as low as 20 mV can swing the core in one direction toward saturation. An EI core has a very small air gap that will allow it to ignore rather large offesets. 
  

• IN THE AMPLIFIER: Here's where the difference is between a conventional output transformer and a Autoformer occurrs. This is why Wiggins at Electro Voice created the CIrclotron circuit. In a conventional tube amplifier. for most of the signal, only one half of the output transformer is active. It is very difficult to make the two halves of a push pull transformer identical above 20 KHz where the feedback really cares about phase shift. Even the taps on an Ultralinear transformer can go out of phase at high frequencies. This causes the amplifier to ring on one half of the square wave. Though not widely talked about, we who design amplifiers are very familair with this problem. Wiggins realized that if he put the transformer in a bridge circuit that the primary would act as a whole and this problem would go away. That is the essence of the WIggins Circlotron. Because he wanted to keep the ampifier efficient he did use a high ratio transformer with conventional taps. BTW, we do not put taps on an amplifier to "match" the impedance of the speaker as we know it varies. We put them on there to deliver the proper ratio of voltage and current to make the amplifier happy. You can always use a lower tap and enjoy lower distortion, better damping, lower noise and extended tube life. You also extend the class A region. The only reason to use a higher or matched tap is to get the most power out of the amplifier if you play it loud. In the RM-4 manual I suggest this strongly and have termed it "Light Loading"

Now, what is an Autoformer going to do for you? If you have an OTL amplifier you should know that the power is greatly reduced into low impedance loads. Even worse is that low impedance loads will overheat the tubes at high power levels as most of the power supply voltage is being dropped across the tube not the load. So low impedance loads are hard on the tubes and cause higher distortortion All of these ills can be solved by the use of a proper Autoformer.
  
For OTL amplifiers that have high output impedance and produce their best performance into 16-32 ohms one needs a 6 or 8 to one step down ratio. This will make the speaker and amplifier very happy and still preserve the qualities of the OTL. A 4 to 1 is not enough. This is no problem to make and I have been using mine for many years.

An Autoformer can also be used in reverse if one has a low voltage, high current amplifier like an ML-2 which is 25 watts into 8 ohms but 100 into 2. Again a 4 to one will get you 100 watts and and an 8 to one even more. Remember the impedance ratio is the turns squared. So an even a 9 to 1 impedance is only 3 to 1 turns and 1/3 of the signal is direct through the primary.

I hope this clears up the differences in these two very different types of transformers and we can stop considering them as the same. While some may consider a transformer a band-aid, I consider it a device that makes the problem go away.

Please feel free to ask your questions.