OTL Asylum

(New?) OTL candidate scheme

77.43.10.3

cellai@space.it

Based on my recent (positive) experience with the construction of an OTL (mono) guitar amplifier I have decided to try the construction of a stereo OTL too.
After a careful evaluation of the published schemes available over Internet (and in the links of this valuable forum)I have synthesized the attached one, a sort of pout-pourri of ideas of others that suits fine to me.
My design driver has been a personal desire for the simplest(and less harmful) power supply (+140V/-140V and negative bias only)and for low parts count.
At the same time I have also tried to minimize the number of coupling capacitors (2 + 2 non electrolytic caps only).

I will greatly appreciate any hints, comments and suggestions on the attached scheme (before construction).

Follow Ups:

RE: (New?) OTL candidate scheme - tubetvr 00:23:51 04/13/08 (1)
As far as I can see this is a classic SEPP output stage where the upper tube works as a cathode follower and the lower tube as an ordinary cathode grounded stage. The main problem with this stage is that it is very unbalanced as the upper cathode follower has much lower output impedance than the lower tube, this give a lot of 2nd order harmonic distortion. Futterman solved this problem by feedback from the output to the phase splitter cathode, (he originally used a split load phase splitter) thereby both tubes give the same output impedance and the unbalance problem is solved. An inverted variant of the Futterman give the lowest possible output impedance with any given tubes and the tubes then both works as cathode followers with very good balance and lower output impedance than the Circlotron. Please see this link for more theory about OTL circuits http://members.aol.com/aria3/otlpaper/otlhist.htm
- RE: (New?) OTL candidate scheme - cellai@space.it 01:33:10 04/14/08 (0)
  Dear Tubetvr,
  
  Thank you for your suggestions.
  Based on your input I am now condidering the attached modification of the input stage with a split load phase splitter.
  Unfortunately the modification implies one more cap too.
  
  Best Regards
  Luca Cellai
RE: (New?) OTL candidate scheme - Lew 10:07:04 04/10/08 (1)
Without consulting any source material, that looks like a Futterman-type totem pole output stage. Because the output is attached to the plate of one phase and to the cathode of the other phase, there is likely to be some DC offset. This is why the classic Futterman design used coupling capacitors betw amp and spkr. Your amp might have a problem w/direct coupling. The input and driver stages look like cascaded dual-differential amplifiers. Will the output impedance of the driver stage be low enough (and the voltage swing great enough) to drive the output stage properly? Also, you've got coupling caps between each stage; it'd be nice to get rid of at least one pair.
- RE: (New?) OTL candidate scheme - cellai@space.it 23:57:51 04/10/08 (0)
  Dear Lew,
  
  Thank you for your valuable advice.
  According to my calculations (and my previous experience) the driver voltage swing (6SN7 output) should be of the order of 150V (+75/-75), that is, enough to get 0V on the 6080 tube grids.
  The calculated small signal open loop gain (RL=8Ohm) is 50*14*6.5mA/V*4tubes*8Ohm=145.6.
  Provided that DC output offset is sufficiently stable and low (less than 50 mV and I hope future tests will confirm this assumption) I hope to get the theoretical closed loop gain of about 21.
  As far as the coupling caps are concerned, in order to get rid of a couple of them, I suppose I should add one more HV tension (350 V or the like) and I would rather to stick to the original idea of using a simplified power supply. I will keep you informed about the progress of this project as I have just finished the PS section and, as soon as I get the ordered tubes, I am going to start final integration and test.
  
  Best Regards
  Luca Cellai
RE: (New?) OTL candidate scheme - cellai@space.it 08:59:06 04/08/08 (0)
I hope the picture can now be seen.

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