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DIY Output Trannies

I have noticed a few threads previously regarding winding output transformers. I would like to point out an alternative suitable for DIYers. On page 888 of the Radio Designer's Handbook (Langford-Smith) there is mentioned a technique where the usual speaker crossover network is moved forward so as to be between the output tubes and transformers. This uses two trannies, one is used for low frequencies (say to 400 Hz) and the other covers the high freq. range. The low freq. trans. can be just a cheap 60 Hz power line xfmr. with the right turns ratio. (ie. need a center tapped high voltage xfmr) This is something that can often be found surplus. The high freq. trans. does not need nearly as many turns on it as the low freq. one, but needs the same turns ratio. This would be easier as the DIYer part. Use a big ferrite toroid core to wind it on yourself. Use small teflon wire or Kynar wire wrap for a low distributed capacitance primary. The toroid shape itself minimizes leakage inductance nicely if the windings cover 360 degrees of core. An additional advantage of this dual xfmr. scheme is that the bandwidth will extend up into the near megahertz range with a ferrite core, so one can use global feedback to much greater effect if desired. One would have to provide two feedback paths, one from each xfmr. secondary to get the full freq. range. The Handbook article references two articles in Electronics magazine, both by P. W. Klipsch: Vol. 21.11 (Nov. 1948) and Vol. 18.11 (Nov. 1945). I will try to get a copy of these and any subsequent references this winter at the library.
The Handbook (pg. 888) gives a diagram and parts values for a 12dB crossover version which requires two inductors. If one only wants a 6dB crossover then L2 and the two C1s can be eliminated as per fig. 4.53A (parallel) on page 185. It might be possible to use the high freq. xfmr. as the inductor itself (Lo) if it has an air gap and is connected per fig. 4.53A (series).
 
dual trannies simplified

I have done a little more research on this idea and I'm confidant that a fairly simple design can be made to work well. To actually perform the speaker crossover function one would use the design on page 888 of Langford-Smith to get the usual preferred 12dB rate.
But, if one just wants to make a cheap equivalent to a conventional output xfmr. (but with greatly increased bandwidth) all one has to do is use a center tapped HV 60 Hz xfmr. for the low freq. half with a small capacitor across its total primary (maybe put half of its impedance transformed value on the secondary side and split the primary side cap into two caps, connected to the center tap B+ point). Then connect the high frequency xfmr. primaries in series with the LF xfmr. primary winding ends. So one needs two separate (instead of one center tapped) primary windings on the HF xfmr. The HF xfmr. needs an internal primary inductance such that its reactance at the crossover freq. is equal to the primary side equivalent of the speaker load. The small cap. on the LF xfmr. needs likewise to have a reactance at the crossover freq. equal to the primary side equivalent of the speaker load. (This is essentially just the equivalent of fig. 4.53A in Langford-Smith, page 185, and the associated design formulas for a 6dB crossover - adapted for a center tapped output xfmr.) The two xfmrs then have their 8 ohm or whatever secondaries connected in series. (The two xfmrs. should have the same turns ratio from input to output.) The resultant output impedance is still 8 ohms since only different frequency pathes are split. Normally, one would use an air gapped core for the HF core to get a constant inductance for a stable speaker crossover freq. But in this case, who cares if the crossover freq. changes a little since the two outputs are summed again anyway. Especially using a ferrite toroid for the HF xfmr. since ferrite has a better constancy of permeability with signal level than steel laminate. A further advantage is that the number of turns on the HF core will be rather low, hence an EASY winding job. Should be able to do a primary winding in a single layer on the core. Combining the secondaries also allows one to still use the conventional feedback arrangement from the single speaker output back to the input gain stage.
 

Brett

Member
2002-01-07 6:02 pm
Before you try to wind one, ask the torroid manufacturers what the high frequency performance of the transformers are, or buy some and test them. One manufacturer here, when I questioned them told me (though he didn't want to admit it) they use basically the same transformer for 70/100V audio distribution as for mains with the same turns ratio. These are rated for ~60Hz - 15000Hz

Mains transformers are specced at 50/60 Hz, so you would need a higher rated core to get low audio frequencies, all else being equal.

<a href="http://www.plitron.com/">Plitron</a> sell a reange of torroid power and audio transformers, including electrostatic stepup. A bit pricey, but apparently very good. BAT tube amps use a custom o/p from Plitron. Menno Vanderveen has designed all their audio trannies, and has written a book on the subject that has been well received called "Modern High-end Valve Amplifiers based on toroidal output transformers", which apparently details all the design difficulties. Audio transformer design is a non-trivial task for high performance work. If you want to design audio torroids I would suggest you buy this book from Plitron.

It is an interesting idea, and a number of people on AudioAsylum have tried it with varying degrees of success with 1 o/p tx per channel. I have picked up some small torroids surplus (cheap) and am going to try them in the o/p of the small PP tube amp I am building for my biamped horn system. I only need 5W/ch above 200Hz due to 114dB sensitivity. I have a couple of 30VA types. Below 200Hz will be SS.

Let us know the results or your experiments.
Cheers
 
Hammond Organs did this with their Tone Cabinets at least back into the 40s. I have seen an amp with power supply, 2 large chokes, high voltage oil caps intead of electrolitics and 2 output transformers each running off of 4 6V6s. The cabinet was a H-40 if you want to find a schematic. Pretty interesting. One thing I have thought about was building Heathkits active xover into a Bi-amp Mono using PP for the bass and SE for the mid/highs. I think the schematic for that xover is in the Virtual Heathkit Museum site. Sorry, I don't have the URL.
 
Thatch_Ear said:
One thing I have thought about was building Heathkits active xover into a Bi-amp Mono using PP for the bass and SE for the mid/highs.

Couldn'y find the XO-1 schematic at the V-Museum, but i have it on my site at http://www.t-linespeakers.org/tubes/xos.html -- a few others there as well.

dave