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.
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
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.
I think he is and I'm not sure what he means by "your poor design"?
realdaeal, I don't understand this either
in OTL design manufacturer don't mention about the specs about the tubes they used which is the most important in OTL designin,.transconductance of the tube is the sweetness of an OTL amplifier.they only built since the parts are cheap.
The dynamics of transformers are very good. Just think how many transformers are in the recording chain from the one in the MIC to the final output.
There is a current thread here on AudiogoN from a guy who took two pair of LFT-8b's and "stacked" them. Each 8b speaker is comprised of a 1' w x 4' h panel containing two LFT "midrange" drivers (180Hz-10kHz) and one ribbon tweeter (10kHz up), and an 8" dynamic woofer (180Hz down) in a sealed enclosure. He made new metal frames for stacking, into which he installed two LFT drivers side-by-side, with two more above them, and one ribbon tweeter beside each pair of LFT drivers---two total. The two woofer enclosures sit below the frame, the stacked LFT-8b measuring about 5' tall by 2' wide. A pair of LFT-8b's retail for $2495, so for less than the price of a pair of Magneplanar MG3.7i you can have stacked LFT-8b's. Think about it!
A single pair of LFT-8b's are one of the best deals in all of hi-fi. Harry Weisfeld of VPI recently stated he considers the midrange reproduction afforded by the LFT-8b the finest he has ever heard. I disagree only to the extent that 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!
in OTL design manufacturer don't mention about the specs about the tubes they used which is the most important in OTL designin,.transconductance of the tube is the sweetness of an OTL amplifier.they only built since the parts are cheap.
Paul Speltz came out with the ZEROs. I think our device was better in some ways- it was EI core rather than toroid, and it had 1 ohm taps! But the ZERO proved easier to explain and since that wasn't our main business we just went with it.
I am curious in reading you white paper and posts here that while you
admit that an ESL can have a 10 to 1 impedance range you feel that it is
appropriate to run such a speaker with high impedance drive
Like you, I didn't think it would work. It totally did. The reason is as I mentioned- the impedance curve of an ESL is unlike that of a driver in a box, where impedance peaks represent a resonance. ESLs OTOH don't have that behavior. Their curve is based on a capacitor (despite a matching transformer usually in use). So they don't respond all that well to the voltage rules. As a result, an amplifier that makes constant power rather than constant voltage can do quite well on them.
While some may consider a transformer a band-aid, I consider it a device that makes the problem go away.
I totally agree with Roger’s summation for autoformers, they are an "answer looking for a problem."
I've examined these two statements for a while and they seem to be at odds. George, I think you missed Roger's point.
Note to people who love the sound of both magnetic-planar loudspeakers
and tube amps: the Eminent Technology LFT-8b is a much better candidate
for use with tube amps (especially OTL’s) than are Magneplanars. I love
and have owned three pair of Maggies (currently one---the legendary
Tympani-IVa), but they present a 3-4 ohm load to the power amp, while
the LFT-8b an 8 ohm. And, if you bi-amp the LFT-8b (easy to do---it has
separate binding posts for the m-p drivers and the dynamic woofer), the
Linear Field Transducer itself is a mostly-resistive 11 ohm load---ideal
for tubes! The Atma-Sphere M60 is known to be a great amp for the ET
LFT-8b, as is the Music Reference RM-200.
The Maggies were also not push-pull drivers until more recently. ET was often quite gracious in the old days and sold individual panels to hobbiests. I have a friend who built up a system using multiple panels so that the overall impedance was 16 ohms. The speaker was not only wide bandwidth and revealing, it was also quite easy to drive full range- 100 watts was plenty!
Note to people who love the sound of both magnetic-planar loudspeakers and tube amps: the Eminent Technology LFT-8b is a much better candidate for use with tube amps (especially OTL’s) than are Magneplanars. I love and have owned three pair of Maggies (currently one---the legendary Tympani-IVa), but they present a 3-4 ohm load to the power amp, while the LFT-8b an 8 ohm. And, if you bi-amp the LFT-8b (easy to do---it has separate binding posts for the m-p drivers and the dynamic woofer), the Linear Field Transducer itself is a mostly-resistive 11 ohm load---ideal for tubes! The Atma-Sphere M60 is known to be a great amp for the ET LFT-8b, as is the Music Reference RM-200.
While some may consider a transformer a band-aid, I consider it a device that makes the problem go away.
I totally agree with Roger’s summation for autoformers, they are an "answer looking for a problem." A bit like having rubber roads and looking for concrete tires.
It’s better not to have the problem to start with yes?
And have the right amp/combo yes? And if you have this correct amp/speaker combo and introduce an autoformer, the sound will take a hit for the worse.
Damn, this is great! For those in the San Francisco Bay Area, Roger Modjeski (RAM Tubes, Music Reference) teaches a course in amplifier design at The Berkeley Hi Fi School. You can build your own amp in the class, any kind you want. My home town is San Jose, and if I still lived there I’d do it myself. If you Google the school name, a website will be at the top of the list, and it contains all kinds of great amplifier design information.
If you truly want to "Walk the walk", this is the way to do it, not sticking pieces of wood and rubber in a 70’s Japanese receiver. Beware of false Prophets!
I agree your amplifier is fine for the Maggies as the impedance is constant however the lowness of it is hard on the tubes. I look at low impedances are like driving a short. I did put a one ohm tap in the RM-200 and it will indeed output 100 watts into a 1 ohm load with good damping. So why did you stop making the Autoformers?
I am curious in reading you white paper and posts here that while you admit that an ESL can have a 10 to 1 impedance range you feel that it is appropriate to run such a speaker with high impedance drive. I was working for Beveridge when we did the model 2 and 3 speaker and was involved in the specification of the transformer for both(I was not winding yet). While we came up with something that kinda worked for the system 3 we found that system 2 was impossible due to the bass extension down to 30 Hz while the system 3 only required 200 Hz.
The original Beveridge direct drive amplifier produced 1500 VA (similar to watts). We could not make a transformer or hardly find a high enough current ampifier at the time to produce 1500 VA. This was in 1978. I did find a Mitsubishi amp that produced 60 amps but getting all those VA through the transformer was another challenge. When I questioned Bev as to why one needs 1500 VA he said. "go play some trumpet music and measure the current". I measured 1500 VA! This is why I make 5,000 volt direct drive amplifiers for ESLs.
Much thanks for the information! Well I am certainly not literate in EE so a bit fuzzy on all of this but I did recently purchase some MC601 monos but have not heard them yet. Is your definition of the issue the same one as Mac has in their design?
I would be happy to comment on particular Mac designs using Autoformers. If anyone has a question and can provide a link to a schematic that would be a big help. Im not having much luck finding the SS amp schematics.
I did find an early Mac SS amp that they admitted ran in class B, that was not such a good idea and I doubt they do that now.
I consider it a device that makes the problem go away.
When we introduced our Z-Music autoformer back about 1990, we always regarded it as a problem solver. The same is true of the ZEROs, which is a similar product. Since we make OTLs, the problem to be solved is lower impedance speakers. We are in Magnaplanar's back yard and have a lot of local customers that wanted the the transparency of the Maggies combined with the transparency of OTLs. The autoformer was the answer, as its introduction didn't introduce the same loss of transparency that seems to be part of the conventional transformer route. Paul Speltz, who makes the ZEROs, has a letter from Steve McCormick, a well-known manufacturer of solid state amps. In the letter, Steve says that his amps can drive 4 ohm loads easily, but he finds that when he uses the ZEROs to interface between the amp and 4 ohm load, it sounds better. Paul was pretty pleased to get that letter :)
Much thanks for the information! Well I am certainly not literate in EE so a bit fuzzy on all of this but I did recently purchase some MC601 monos but have not heard them yet. Is your definition of the issue the same one as Mac has in their design?
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