2 plus 2, Acoustat transformers

Read more on the Acoustat 2 plus 2 in our home audio section

Dan Todd2008-12-13 03:13

Andy, I had owned a pair of 2+2s that I bought from Read Brothers back in 86\'. I know (the hard way) that the problem with these speakers is the low frequency transformer (TT-121L) is prone to arcing in the secondary if driven hard. (All transformers can be overdriven.) The most common problem is where the secondary wires are \"dressed\" and secured. The yellow wire tends to arc to opposite polarity (blue) outer windings due to the high voltages and close proximity. Anyway I do notice that the TTA-246B transformers utilized in the Spectra 44/4400 greatly reduce this by using separate cores for each polarity, but I also noticed a 10M ohm resistor in line between the two transformers common return and ground. I think this also helps to prevent arcing to the core, since the old TT-121L single core has a center tap that is connected with the ground of the bias power supply. I know that the 44/4400 and the 2+2 are different and have read all the postings over the years. It is possible to convert the 2+2 to spectra with the mod (cutting of the stators at mid point of each panel), purchasing the TTA-246B transformers which I have and then incorporating the necessary resistors of the spectra interface. All in all, not easy but for me not to bad. The only thing I dread is the frame change to bring the panels back to a straight plane. My questions are concerning the 10M ohm resistor and also the input 10uf caps on the primary of the TTA-246B. Are these caps used to prevent some sort of ringing (resonant frequency)? I think the 1ohm resistor is as described with the TT-121L, am I right?


Andy Szabo2009-01-01 17:06

The MK-121 low frequency transformer does have a tendancy to arc when driven very hard. The Medallion version of the transformer addressed this problem somewhat, but did not cure it. The basic problem still exists: both plus and minus voltage swings are on the same transformer. The full range Spectra models avoided this problem by using two transformers, one for each phase, thereby reducing the maximum voltage difference inside the transformer by a factor of two. I don\'t recall ever seeing an arced-over full-range Spectra transformer.

The 1-ohm resistor in series with the transformer primary has the same purpose in both MK-121 and Spectra: to make sure that the impedance at extreme low frequencies is above 1-ohm. The difference in the Spectra is that the resistor is paralled with capacitance, essentially bypassing the resistor at higher frequencies.

The 10-Mohm resistor in the Spectra transformer\'s secondary ground connection is there for shock protection. If you were to accidently connect your body across circuit ground and a high voltage audio point, the fault current would be limited by the 10-Mohm in series. This component is recommended for safety reasons, but is not required for audio reasons or for arc-protection.

It seems as though you have correctly identified the various challenges to convert a 2+2 into a Spectra 44. Not a project for the timid - good luck if you decide to embark on this project.

Dan Todd2009-01-01 19:34

Thanks Andy. You have been a GREAT asset to the enduring fans of Acoustat! I remember talking to you when Acoustat was still in Florida. I have went back to my old 2+2 speakers after trying the low watt, single-ended, direct heated triode realm of audio. If only it were possible to drive electrostatics with a 45 tube triode amp...oh well.

Yes, I know about the arc problems with the TT-121L transformers, and have talked to Ken at Galaxy transformers about getting a larger core with more mylar insulation. He\'s a great guy to deal with. The problems are still existing today even with their improved models, if you drive the transformers hard. Well it\'s going to be a mute point when I change over to the TTA-246 Spectra interface. I was fortunate to acquire a set of four to change over to Spectra 44. The 680K resistors and 75K resistors should ideally be high voltage (5KV) types but it doesn\'t appear that way when acoustat produced them. The .022uf poly cap will be substituted with a .02uf, 15KV high quality polypropylene from and ebay source. I\'m sure even the original acoustat type wasn\'t that close in tolerance. As I said before, I don\'t look forward to modifying the frame and may build a new one.

I\'ll post the results after the change. I too have a set of HSU/Dayton subs to crossover on the very low end. Thanks again...Andy. God bless Jim Strickland.

Andy Szabo2009-01-02 01:49

The 680K and 75K-ohm sector resistors are indeed high-voltage types, rated at 5-kV. The .022-uF was likely 5% tolerance, but .02-uF would probably be fine. I believe the bare high voltage board is still available as surplus on eBay, which would sure make construction easier. I have even seen fully stuffed boards for sale (with component values for the 44/66) but they may be gone by now. Contact sellers \"dealsbyjason\" and \"soundvalves\" and let them know what you are looking for.

The Acoustat CD available from our webmaster has frame drawings, and I believe the Spectra 44 is amoung them. I\'m not sure that modifying a 2+2 frame will work, as there may not be enough space within the frame rails for two panels laid flat. I think it would be easier to build new fgrames from scratch.

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