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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.
ramtubes
bdp24's avatar
bdp24
10,229 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
912 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?
ramtubes's avatar
ramtubes OP
460 posts
 

09-01-2018 3:38pm

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?
I would hope your 2A3 amplifier has a measured tap that is equal to the impedance of the driver you select. Low power ampifiers need this more than ever at 4 and 8 ohms. 

The popular Zero is just too much iron for a 2A3 to deal with. You would be hooking up a 200 watt transformer to a 2.5 watt amp. I do have a nice small autoformer that I made for my two tube OTL that would do the job nicely. Very low loss, easy to drive, 4 taps to match any driver.



ramtubes's avatar
ramtubes OP
460 posts
 
To Lewinski and others thinking about multi amping please consider this.

The crossover in a typical two or three way speaker is entirely responsible for widely varying impedance, distortion and is just a bad idea in a high end system. Its fine if you need a speaker you can carry around and hook up anywhere, but we aren't doing that in our stationary home systems. We have the option of doing something much better.

Dynamic drivers have very flat impedance over their useful range. I know and hope I will get some disagreement on this, so go ahead.

For all the money people invest in expensive speakers and amplifiers there is a much better solution. If you don't care to make your own speakers then ask the maker whose speaker you like to "hold the crossover and give me direct terminals to the drivers".

Then get a good active crossover or have one made. Many of us offer them. Choose some amps approiate for those drivers like Lewinski is doing. You will have a great system, no impedance problems, no crossover saturation or the EQ tricks that are going on in most of them to obtain good frequency response at the expense of flat impedance.

I would hope that many of the readers here are tired of searching for the right amp for their speaker. I think that's a fools errand because the information is hard to find, some amplifier makers don't tell the truth, and most speaker makers assume you have a big SS amp with infinite damping, 60 amps of current just like they do. 

Here are a few examples of what you can do.

Drive your woofer with some inexpensive big SS amp, don't sweat the midrange, there wont be any here.

Drive the high efficiency midrange with a small tube of class A SS amp. Small amplifiers always sound better than big amps used at low power. 

Drive the tweeter similarly or use the midrange amp with a simple 6dB per octave passive crossover consisting of a choke and cap. Properly done these will not affect the drivers of the load. At these frequencies both the choke and cap are small, easy to obtain and have vanishingly small losses. Such a crossover will sum perfectly.

If you are into ESLs, DIrect drive is the way to go. 5,000 volts at 1/4 amp is 1250 VA and equivalent to a 1250  watt SS amp which is what many ESLs need.
weebeesdad's avatar
weebeesdad
142 posts
 
Roger - 
Would your "light loading" recommendation apply to all transformer based tube amplifiers, or just to your designs? I have read many comments stating that the higher output impedance taps allow for better performance assuming compatibility with a given speaker. I have a pair of DeHavilland 50A monoblocks which have only one set of binding posts which are connected to the 8 ohm transformer winding. One must open the amp and rewire to access the 4 ohm winding. I assumed that was because the designer felt that the 8 ohm tap was preferable sonically (or perhaps it was because it allowed for the greatest power output as you pointed out).
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