The Amazing Fet Circlotron by Mike Rothacher

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nagini262

nagini262

Member
Joined 2008
This is mine, hope all of you love it!:)
The input transformer is modify for RCA input:p

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cnpope

Member
Joined 2009
I recently got around to measuring the distortion of my FET circlotron (which I built to Michael Rothacher's design). I get THD figures that are very poor compared to those reported in the original article. Into 8 ohms I get about 1.3% at 1 Watt, rising to about 4% at 5 Watts. These are hugely greater than what the article reports (e.g. about 0.005% at 1 Watt).

Does anyone with experience of this design have any suggestions as to what might be amiss in my amplifier? Both channels, by the way, give about the same results. FWIW, I tried simming it in LTSpice, and it gives THD figures in line with those reported in the original article; way better than I'm getting.

Chris
 
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cnpope

Member
Joined 2009
Michael Rothacher said:
Chris, describe your balanced measurement technique for us. There could be an issue there.
Click to expand...

Hello Michael. Glad to find you here!

I think my measuring setup is OK. I'm using a Keithley 2015 THD multimeter. The + and - inputs are both floating with respect to ground. The lower limit on the distortion I can measure is effectively set by my Tenma audio signal generator, which is nominally about 0.05% distortion.

I'm driving your circlotron amplifier by means of an unbalanced to balanced preamp, based on the Balanced Zen Line Stage of Nelson Pass. If I measure the distortion across the balanced output of the line stage, I get a very low figure (essentially unmeasurable given my signal source). This reinforces my confidence that all is well with my measuring setup.

One other thing I have just tried is measuring the distortion on the balanced signal between the drains of the two 2SJ74 jfets. It is again very high, and almost identical to the figures I reported in my previous post.

I suppose this suggests that the 2SJ74 jfets are the likely culprit. It was several years ago now when I built the amplifier, and I can't now recall where I got them (I do remember it was not trivial to find them). So I don't know how to answer Gyuri's question about their provenance. There is nothing on them that specifically indicates either Toshiba or Linear Systems.

One other thing occurs to me. I find the Toshiba data sheet for the 2SJ74 completely ambiguous as to which leadout is the drain and which is the source. There is a side view with the three leads clearly numbered, but in it one cannot tell if the 2SJ74 is lying with its semicircular curved face upwards or downwards. There is also a end-on view, which would in principle be unambiguously clear, except that the leads are not explicitly numbered in the end-on view. Maybe, or maybe not, the numbering is assumed to be in the same order as in the adjacent side-view picture. Considering that they could so easily have made it unambiguous with a little more effort, it is rather frustrating that some uncertainty remains!

I understand that with some, but not all, jfets, the drain and the source are essentially interchangeable. I don't know whether this is the case with the 2SJ74.

So, in summary, I am pretty confident that my measurement setup is correctly reporting the distortion that is occurring. The finger of suspicion would seem to be pointing, I suppose, at the 2SJ74 driver stage. Maybe because I am using the "wrong make" of 2SJ74? Or, just possibly, might it be because of a wiring mistake with drain and source interchanged in the driver stage? The latter would easily be settled, if someone could tell me unambiguously which lead is drain and which is source on the 2SJ74.

Thanks for your help!

Chris
Edit: I've now found a Linear Systems datasheet for the 2SJ74, and they at least have an unambiguous indication of which lead is which. And I did in fact wire them up correctly, according to their identification of the leads.
 
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cnpope

Member
Joined 2009
Gyuri said:
I suppose you need BL grade JFETs with 8-10mA IDSS.

Transistor matching
Click to expand...

Hi Gyuri,

Yes, I remember I did go through that matching process when I was building the amplifier. I had bought ten 2SJ74s, as far as I recall, and I did select pairs that matched nicely. And they are BL grade.

That diagram in you previous post is precisely the one, from the Toshiba datasheet, that I was grumbling about. I am indeed familiar with the fact that the end-on view is a view from the bottom; that was not the issue. The problem was that the bottom-view picture does not have an explicit 1,2,3 numbering indication for the three pins. One is evidently supposed to assume that the numbering is in the same order as in the upper side-view picture in the same diagram. I hate it when things are left imprecise, when they could so easily have been made precise with a little more effort!

Anyway, the main question at this point concerns the suitability of the 2SJ74 jfets that I am using; might they be responsible for the poor distortion performance I am getting? Since you asked me whether mine were Toshiba or Linear Systems, does that mean you have reasons to think the Linear Systems ones would not perform well here? Again, let me emphasise that the ones I got did claim to be BL grade, and I did match them for Idss.

Thanks,
Chris
 
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