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Last Call for Autoformers

Posted by Paul Speltz (M) on April 3, 2001 at 08:39:09

Back in the beginning of March, I posted a message asking, "Who wants autoformers?". I actually got a response back from a lot more people than I expected. So with their support, I proceeded to develop a custom made autoformer designed to increase a speaker's impedance so that it becomes OTL friendly.

I think that the reason that there is interest in a high performance autoformer is because even though we fall in love with A-S OTLs, we may also love our OTL un-friendly speakers, or we may not be able to fund OTL friendly speakers for a year or two. It may be that even with OTL un-friendly speakers, the virtues of the OTL amps are so musical that we are not willing to use any other amplifiers. In any of these cases the autoformers can be used as an temporary or permanent solution.

I now have the first prototype pair of autoformers. They are wound on a toroid core and will handle up to 150 watts of power, so they are ideal for ST-30, M-60 and MA-1 amplifiers. Their frequency response goes from 1hz up to a few MHz!!!! I have structured them to multiply the impedance of the speaker by 1.5 or 2 or 3 or 4 or 8 or 16 times (depending on the tap selected). The 2x and 4x taps are completely balanced. This way any speaker can be made to be put into the maximum power band of the OTL (ie. 14-16 ohms for a M-60). Yes, your 4 ohm speakers can look like 16 ohms to the M-60!

I have named these autoformers the "ZEROs" because you will get them in their raw form, and their donut shape looks like a big zero. I will post a picture of them tomorrow.

There is now 11 people committed to the "Zs". All the free time that I have had since last Wednesday, I have dedicated to evaluating the sonic performance of the pair of prototype "ZEROs". I needed to make sure that first, they achieve the sonic benefits of multiplying the impedance of the speaker load, and second, they do this without showing any sonic signatures of their own. Wednesday through Friday I simply ran them on my system, played a lot of music and listened for any sonic clues that told me that I added "iron" to the system.

Harry Straub is on vacation, but Joe Stocco was kind enough to invite me over. Saturday I brought them over to Joe's house and ran them on his excellent system which include an MP-3 preamp, MA-1 mkII amps, and a pair of MG20 (top of the line Maggies). Joe was hoping for better bass from his Maggies. Using my "Zs" as an impedance doubler (2x) to make the 4 ohm Maggies feel like 8 ohms to the amps, it took about 3 seconds to hear that the bass came out of the background and became a balanced part of the rest of the music. Joe was thrilled, and started saying things like, "I'm sold.....count me in".... I reminded him not to worry, that he already had committed to a pair. What we started noticing next was an overall improvement top to bottom. A nice drop in distortion and an increase in resolution made it easier to hear a lot more of everything that was happening in the recording. This was not subtle either, we both quite quickly noticed and described to each other the same things, and this was listening to music a mild volume level.

Next we tried using my "Zs" as an impedance quadrupler (4x) to make the 4 ohm Maggies feel like 16 ohms to the amps. This time the improvement was a little more of the same. If the 2x gave us 5 steps forward, then the 4x gave us just 1 step more beyond that. Yet it was enough for Joe to say that he plans on using them that way.

In the 3 hours of listening with them on Joe's system, we were unable to hear any problems with the "ZEROs". Thanks to Joe for his time on Saturday. I know he was sold when he said that I should just have 100 of them made, because when the word gets out on how good these are, a lot of guys are going to want them.

In an attempt to find out if the "ZEROs" are truly transparent, I listened to music on my system back and forth with the "Zs" in and out, but compensating for the impedance change of the "Zs" by pulling output tubes. In other words 8ea 6336 tubes per amp biased up to 1 Amp (essentially a MA-1) without the "Zs" in place, compared to, 2ea 6336 tubes per amp biased at 250mA with the "ZEROs" used as a 4x speaker impedance multiplier. The thought here is that 1/4th the tubes at 1/4th the bias current driving what feels like 1/4th the load should sound exactly the same if the autoformers are truly transparent. Well not only were the "ZEROs" utterly transparent, I actually have to say that the music with just 2 tubes running through my "Zs" sounded a little cleaner in the bass, and the highs were better focused. This may be because fewer tubes are easier to drive and/or the power supply is taxed less.

I apologize if you happen to find fault with your "ZEROs", I have tried really hard and have not been able to find any problems. I have received down payments from 11 of you (you know who you are) and now is the time for anyone else to jump in. The total cost per pair including shipping is $383. I will find out later this week how much time it takes to produce them and will let you know. Anybody interested in buying a pair should contact me at:

PaulSpeltz@hotmail.com

Mail Checks to:
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Paul Speltz
2325 Wallingford Lane
Woodbury MN 55125
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