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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
 
Should have just bought an automatic car to begin with. (The right amp to do the job)

But automatic transmissions still have gears. If there is a right amp for every speaker then does one have to get a new one with every speaker change?

I've designed all my amplifiers to be as universal as possible to sound and measure well with a wide variety of speakers. Music Reference owners tend to keep their amps for a long time, you don't see them often and there are many cases of Sellers Remorse.

I do this by reasonable choices of damping factor around 10, distortion below 1% at full power and typically 0.1% at listening levels. The RM-9 and RM-200 provide ample current, and are easy on the tubes. A resaonable amount of feedback is used to achieve these things. I don't care to use so much as to get the distortion down to 0.001% or the damping up to 100. Those will result in an unstable amplifier.

I would like to say one important thing about damping. Paul Klipsch said many times "Who cares if the amplifier is 0.1 ohm or 0.001 ohm output impedance. The woofer is a 6 ohm resistor, so how much do you think you are going to damp that". This thought of the amplifier having a grip on the woofer is another misconception so lets look at another way 

The following is intended to dispel some of the misconceptions on how dynamic speakers work. You may find it hard to accept.

Certainly a low damping amplifier is going to produce boomy bass in many speakers though not in all. The extent to which it does this is determined by the impedance peak at the bottom end. One peak for sealed boxes and two peaks plus a rather deep dip between the peaks in a ported speaker. What happens at these peaks. To the extent that the impedance rises to 20 or 30 or 50 ohms the voltage of a low damping amplifier will rise as much at 6-10 dB, a very noticable amount and a very narrow peak. It is this rise in voltage and rise in efficency at resonance that produces the peak.not that the woofer is out of control. It is perfectly in control and resonates because the air load on the cone now equals the mass load of the cone. If one designs a woofer without a impedance peak then damping doesn't matter in the bass. 

Here is the impedace curve of a well designed driver from Fostex. Note how flat the impedance curve is; only 8 to 10 ohms over the usable range.Of course the bass peak has to be handled by the proper enclosure. These curves are open baffle. Also note the 90 dB efficiency!

https://www.madisoundspeakerstore.com/approx-4-fullrange/fostex-fe108ez-4-full-range-sigma-series/

Now what happens in the rest of the range? The other drivers, if operated in their range of comfort, will present constant impedance in that range.Once you accept the fact that all dynamic drivers are mass loaded devices you will understand that. The impedance peaks and dips in a mulitway speaker are entirely due to driver overlap, driver underlap, and components added to voice the speaker. 

Here is a nice tweeter  https://www.madisoundspeakerstore.com/soft-dome-tweeters-peerless-vifa/peerless-dx20bf00-08-3/4-text...

One might now see that using a woofer down to its resonance is not such a good idea as that is beyond its usable range. We don't do that with  midranges or tweeters.




georgehifi's avatar
georgehifi
10,312 posts
 
But automatic transmissions still have gears.


Yes but at least with an automatic you’ll won't get: 
" When going up a steep hill (difficult load) the car sometimes shutters, engine knocks."

Cheers George
clio09
3,346 posts
 
Roger has done a drop in board modification for the Acoustat direct drive amp. The main upgrade is the elimination of the solid state input (opamp) in favor of a tube input using 3 x 6DJ8/6922 tubes. I use am using the Acoustat direct drives on my Model 2's right now. The only thing the interface boxes are doing is holding up the speaker. I hope to either get the drop in board mod or a set of Roger's direct drive amps at some point.
georgehifi's avatar
georgehifi
10,312 posts
 
Guess who getting off on music, to his Acoustat 3’s?

http://4.bp.blogspot.com/_NVQujPOPx2c/TSUhBAtkNeI/AAAAAAAATmg/RIIOAT-VuFc/s1600/steve-jobs-stereo.jp...

I went through nearly all the Acoustat’s, the best for me were the 2+2’s with Kef B1814 (fs18hz) bass drivers for 80hz down in 14cu ft boxes and my MP-02 Plasmas’s from 10kHz up all passive xovers driven by my 120w water cooled pure class-a S/S beast of an amp two man struggle to pick up. 24 Hirel EB or ED 203/4’s output Devices per channel.

Cheers George
georgehifi's avatar
georgehifi
10,312 posts
 
clio09
The main upgrade is the elimination of the solid state input (opamp)
I can’t remember as I never fiddled with mine as I soon upgraded to the two+two's, but knowing the vintage, it was probably tlo72/74 about the only thing around at the time, not a very good sounding opamp.

Cheers George
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