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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.
128x128ramtubes
ramtubes's avatar
ramtubes OP
460 posts
 

bdp24     I consider the old Quad (which I own, in addition to LFT-8b's
and Magneplanar Tympani T-IVa's) still the gold standard in that regard.
But the ET's play a lot louder!  
I agree the 57s are the gold standard for midrange due to the virtually massless diapragm and excellent step up transformer. I used them to voice my ESL. 
Post removed 
ramtubes's avatar
ramtubes OP
460 posts
 
I tried to understand the OP, but I'm not getting it or something. How does a transformer only act on 180 degrees of a wave form? That doesn't make sense. And if the transformers is making the load easier to drive, how is it not taking away the control the amplifier has over the load? I get it with the shrinking class A envelope, but all that really does is drive a class A amp into class AB with a little lump of distortion at the envelope edge. And isn't a transformer really just feedback in the form of degeneration?

Although at first read is appears that kosst_amojan is indeed confused but as I am considering his points his answer does beg some clarification on my part. Along with some assumptions and further questions. So here we go. Thanks for your reply and I hope this clarifices this for others.

The transformer acts on the entire signal not just 180 degrees. What I was saying is that in a push pull amplifier there is one half of the primary used for the positive going signal and the other half used for the negative going signal. Making those equal is difficult above 45 KHz where feedback can cause ringing on one half and not the other half of a square wave. In an Autoformer the entire primary is used all the time.

In the application of making a speaker easier to drive we use the transformer in a step down configuration, the step up would be a rare but still valid usage. Stepping down the voltage by a factor of two doubles the current and current is what is lacking in all OTL amps. There is plenty of voltage. Although this discussion is based around impedance I would like readers to consider voltage and current. Impedance is just a simple way to combine the two into one term. 

By presenting a higher impedance to the amplifier the amplifier has MORE control over the speaker, control=damping. 

The  Aurtoformer actually extends the class A region when used to increase the impedance as in this discussion. Similarly going to a lower tap on a tube amp extends the class A region and can reduce the distortion by a factor of 5-10.

A transformer is not a form of feedback, Perhaps you are talking about what ARC and some others do when they use the secondary for cathode degeneration. I tried that in the early RM-200 and later found it was not so useful. 


bdp24's avatar
bdp24
9,868 posts
 

Roger, It’s great to have you contributing here on AudiogoN. I really miss your AudioCircle Forum! We met twice when you came to Brooks Berdan’s shop in Monrovia to give talks. The first time, I was the long-hair who asked you why you recommended a single RM-9 for stacked Quads rather than a pair of RM-10’s. You told me you considered the headroom afforded by the RM-9 to be of great significance and benefit. I asked if the load of paired quads on each channel of an RM-9---whether run in series or parallel---would not be of more concern than headroom? And if wouldn’t a single Quad on each of four RM-10 channels would not be preferable? The second time you told me you had been reconsidering my question, and had changed your mind. :-)

Side note: I know very few musicians who are audiophiles (most are far too poor, for one thing). The one I do is, as am I, a drummer (he’s currently working with guitar-great Albert Lee), and his speakers are---stacked Quads! Maggies are also great at reproducing drums, but they don’t have the liquid, see-through transparency of ESL’s. Roger, I’m dying to hear your direct-drive ESL’s; maybe on my next trip to the Bay Area.

ATENTION ALL TUBE AMP OWNERS! I just spent 90 minutes watching and listening to a video of Roger giving a lecture at the 2015 Burning Amp Festival. That 90 minutes is the best investment in time (something of increasing value to me as I approach the end of the line ;-) I have EVER made. Roger covers many aspects of tube amplifier design in deep detail, then takes questions from the attendees, some of them audiophiles, but many amateur and professional tube amp designers, one of whom appears (off screen) to be Mike Sanders of Quicksilver. If you own a tube amp, or are considering getting one, you NEED to watch and listen to the lecture too.

I’m one of those idiots who doesn’t know how to attach websites, so here’s how to see the video:

1- Go to "berekelyhifischool.com". Along the top of the website you will see "COURSES", "CLASS NOTES", etc. Go over to "RESOURCES" and a drop-down menu will appear. The first of three choices will be "VIDEOS". Click on it, and you will be taken to a new page. On the right hand side of the page will be "ARCHIVES". Find October 2015 and click on it. The video will start.

2- I suppose you could also find the video by doing a search on You Tube, but you may as well look at the rest of the Berkeley Hi Fi School site too!

lewinskih01
909 posts
 
I'm building an active system where the midrange will be driven by a 2A3 SET. SETs like constant, high impedance. And need high efficiency too. Many high end midrange drivers have relatively flat yet low impedance. Would an autoformer be a good solution?

If I place an autoformer between a 2A3 and a 4 ohm 90 dB/2.83V midrange such that the amp sees a 16 ohm load, would that be a good solution? How would efficiency behave? Any detrimental side effects to be expected?
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