Experimental Design - Comments, Flames Welcome

[Tom2]

DIGORA said:
It can work in class AB ?

Yes.

The patent(4229706) refers to the diodes-resistors
(D1 R6 D2 R7, my schematic) connections as to help operate the circuit in class B.
I assume this can apply to class AB operation also.

Tom

[EUVL]

To continue the discussion of a F5 like current feedback circlotron :

http://www.diyaudio.com/forums/showt...34#post1549534

This would have been the circuit I would have proposed. I don't think it matters whether we use 2SK170 or 2SK369. I do think the J111 cascode is necessarily.

But I am also puzzled by Klaus's simulation results. Need to think first.


Patrick

[EUVL]

Klaus,

I presume you were using balanced input signals.

http://www.diyaudio.com/forums/attac...amp=1214589765

http://www.diyaudio.com/forums/attac...amp=1214590037

Could you show plots of V across R6 and Vgs across J2 alongside the input signal ?


Thanks,
Patrick

[KSTR]

Patrick,

Yes, I used pefectly balanced input (which is also quite important, of course).

Here Vgs at J2 (green) and Voltage accross R6 (blue). Not very nice at all....

Hhm, you might want to install LTSpice yourself, this makes evaluation much easier (models used for the transistors are found here on this forum, the JFETs were posted by syn08 and the MOSFETs by andy_c).

- Klaus

[Michael Rothacher]

I have built and tested such a circuit and I'm working on the article now. I will find an appropriate place to publish once finished.

[EUVL]

Did you experience the problem Klaus described, and if yes did you find a cure ?

Patrick

[Michael Rothacher]

I haven't seen that problem, but our circuits aren't exactly the same.

[juma]

Originally posted by EUVL
...
This would have been the circuit I would have proposed. I don't think it matters whether we use 2SK170 or 2SK369. I do think the J111 cascode is necessarily.
...
Patrick

Patrick's schematic as .gif file:

[EUVL]

Thanks for converting to gif.
I just discover a mistake. The batteries should be the other way round.

Sorry guys.

Patrick


PS I believe I might have an explanation for the wild common mode bias swing, which is (mainly) at twice the frequency of the signal, but I do not have a cure yet. Need more thinking.

[KSTR]

Seems that I found a remedy, maybe not the most elegant but it's working:
Buffering the gate drive with the right type of buffer (I missed this in my first try). This buffering also would help thermal compensation and is compatible to the current limiting (mabye a base reverse bias protection diode is needed). Freq. compensation seems necessary as the buffer unloads the resistors, I tried here this time with slightly inductive source resistors and an output snubber. It appears to be stable with capacitive loads in the nanofarads range and no resistive load (the situation when a cable is attached but not the speaker itself).

Now, at 20kHz, that common mode modulation is almost gone (reduced by the buffer's current gain) and the circuit behaves well and has good perfomance. Still it is not very intuitive what the root cause was, originally. At any rate, both capacitive gate currents got routed somehow through both resistors because the return points of these currents were on the "wrong side" of the load.

Matching of R7 to R12 is very critical... or a point where any mismatch introduces even order distortion if that is wanted. Other parameters seem less critical wrt matching.

Last not least, I'm very curious to see mrothacher's circuit....

- Klaus