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Old 22nd August 2012, 05:00 PM   #1021
lhquam is offline lhquam  United States
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Quote:
Originally Posted by flg View Post
1 MOSFET does not output a semetrical sinewave at the input!
2 MOSFETs of the same type "may" have a curve sufficient to find an operating point where, the non lineararities(unsemetrical performance)(distorion) of one, is mirrored by the other, in which case the nonlinearities cancel. But, have you won the lottery lately?
Otherwise, I agree, the Circlotron has the best oportunity to be such a balanced, symetrical, linear design.
I'm not building an F6 after I'm done with my F4 & 5, I'm building a SiClotron
The problem with parasitic capacitances is that their effects are frequency dependent. It is very unlikely that picking a point on the MOSFET I-V curve could overcome these asymmetries.
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Old 22nd August 2012, 05:45 PM   #1022
lhquam is offline lhquam  United States
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Originally Posted by lhquam View Post
I hate capacitors.

I have been experimenting with "tweaks" in my bias circuit in an attempt to lower THD at low frequencies and discovered that it is possible to provoke oscillation at .2-.3Hz, depending on the RC time constants related to the capacitors. Looking at the LTSpice AC analysis plots, there is a peak in the response at these frequencies and very bad phase margin.

I need to crack open the books and learn how to solve this problem.
Here is the AC Analysis plot
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File Type: jpg F6-R100-1x-v3-a2-ACanal.jpg (118.3 KB, 390 views)
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Old 22nd August 2012, 06:30 PM   #1023
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We have to add some studies on the behavior of the transformers. I had some telex converter coils laying around, and thanks to ZM is tried them.

Two pictures:

Audio 2012 238 49.jpg
Without proper loading
Here we see much ringing.


Audio 2012 239 48.jpg
With proper loading.
With 680 ohm loaded it becomes well behaved. Maybe a little lower would work in my case; my coil has 200 mH inductance.
Loaded this way, the Xfrm (see below) has a bandwidth of 70 Hz- >50 kHz, with a tilt at 200 KHz of about 10 dB.
With feedback it will change it's behavior of course . . .

My work on an "F6":

F6 ak.jpg
  • input with 2SK170/2SJ74
  • a bias with the jFET 2SK30AGR/bias resistor of 1k2.
  • this works well in principle though the large swing of the output WILL influence the drain current; that is why I have a low pass filter added. The 2SK30 can accept up to 80V loaded this way.

and
  1. with IRF130 or
  2. with 2SK1020 (as far as I can see, in specs similar to the R100)

Now testing . . .

This is my prototype board:
Audio 2012 242 46.jpg
Left the bias circuits; center the buffer and feedback stages; right the transformers (1:1), so I use two per channel.

The transformers came from this unit (thanks to ZM!!)
Audio 2012 234 42.jpg
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File Type: pdf F6 AK.pdf (47.8 KB, 59 views)
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Old 22nd August 2012, 06:56 PM   #1024
Zen Mod is offline Zen Mod  Serbia
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what's source impedance , when you measure F3 of 70Hz ?
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Old 22nd August 2012, 07:19 PM   #1025
Zen Mod is offline Zen Mod  Serbia
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Quote:
Originally Posted by lhquam View Post
Here is the AC Analysis plot
I'm really puzzled- are we going to have that in real life .........

in fact , I'm not puzzled

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Old 22nd August 2012, 07:31 PM   #1026
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Quote:
Originally Posted by Zen Mod View Post
what's source impedance , when you measure F3 of 70Hz ?
I had the signal generator directly on the coil, <10 ohm I think. No input resistor.
albert
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Old 22nd August 2012, 07:39 PM   #1027
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I'm afraid that , in that case , those repeaters are not good enough ones ;I fed mines with 50R siggene , and they were flat from 15Hz

well , I'm cheating - I measured that strictly with sine
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Old 22nd August 2012, 08:46 PM   #1028
lhquam is offline lhquam  United States
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Originally Posted by Zen Mod View Post
I'm really puzzled- are we going to have that in real life .........

in fact , I'm not puzzled

I suspect this is the same as what is called "moter-boating" in tube amplifiers (with transformers). I need to study the solutions used there.
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Old 22nd August 2012, 09:31 PM   #1029
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solution for motor-boating is always easy - brute force - use zillion of uF

it wasn't easy in time when uF was expensive
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Old 22nd August 2012, 09:48 PM   #1030
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triode_ al: I like [and hopefully others will appreciate] your schematic in post #1023 because:
  • The bias to the upper MOSFET is referenced to the output and not to ground. Thus; this MOSFET will always operate Class A. By contrast, lhquam has the corresponding bias in his prototype referenced instead to ground.
  • The bias to the upper MOSFET is [hopefully] stable due to the CCS you used to establish it. I would have used [on paper] an independent power supply of positive voltage higher than the rail voltage shown. Bias in this newer instance will be granite solid or better. By comparison, kasey197, uses a resistor voltage divider to establish bias in his prototype. It may suffice. But; it may also appear to be less robust [rock stable] than the one you are using.
  • The bias to the bottom MOSFET can be anything else that is stable; which you do show, and others do.
  • The blocking capacitors in series with the gates of the MOSFETs eliminate the dreaded direct current through the secondaries.

Last edited by Antoinel; 22nd August 2012 at 10:03 PM.
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