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Old 18th November 2008, 07:09 AM   #31
lineup is offline lineup  Sweden
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Quote:
Originally posted by jam
Have you guys tried to use complementary jfets for the input of the buffer, Helps offset drift as you don't have to deal with base currents and could be direct coupled.

Jam
As you very well know, jam
symmetrical JFET buffers I have used many times

The attached circuit is one of the basic configuration
in my
No Global Feedback symmetrical JFET module
It is based on 3 parts, blocks, where the last part
is the very high input impedance Complementary JFET/BJT 'sziklai' pair buffer.
Quote:
More to read and explore here:
Audio Virtues of JFET Transconductance Amplifiers
.
To adjust DC-offset balance in Output Buffer, you trim R5 and/or R15.
My original test setup uses 2SK170BL, BD140 and 2SJ74BL, BD139.
No problems run at 50 mA Class A, for Heavy LOADING.

My basic testings, as can be seen, used 100 Ohm load ...........
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Old 18th November 2008, 09:41 AM   #32
TimS is offline TimS  New Zealand
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Lineup, I really like different and unusual circuits and I think yours are the most unusual I have seen for a while.
The common gate source input for your transconductance amplifier is definitely counts as different, but I thought it would give a low input impedance and that one stage looks like a circuit I had designed as a low input impedance for a moving coil cartage pre-amplifier.

I tried the bootstrapping on my circuit instead of the constant current and the output could then swing within 1 volt of the rail but the distortion increased 2-4 times, i am not sure if the change gave me an overal win in performance, but it may be worth an extra look.

Jam, I like the idea of using complementary JFET buffers, though I thought using JFETs would increase the possibility of output offset since JFETs can have a larger variation of Vgs and Gm between devices, the difference would be harder to cancel.

I have attached my first draft of my PCB, I hope you like it.

Cheers

Tim
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Old 18th November 2008, 10:25 AM   #33
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Lineup this circuit is very clever indeed. The output buffer Ive used since 1992, it was published by JLH in his articles on preamp design. Those days I was still a student and JLH was my reference in audio design. I use them as buffers between cdplayer and monoblocks because of long interconnects leads so that I can place amps behind the speakers. I could drive headphones with them but there was some degration with low impedance ones. With the japanese parts one can ring some extra performance from this buffer. I still use my original JLH buffer but it uses junk 2n devices but even so driving amps is no problem. Together with that front end it is a very interesting circuit indeed.

Quite brilliant

Wild man with very good ideas. That one more than merits a build.

Two questions, is there any reason you choose to adjust offset with R5, R15 ?? I would it do it via R8 R13.

Whats the input impedance of the front end stages??

There is only one negative with these Jfet designs, it very difficult finding the P channel parts.

This is the JLH buffer
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Old 18th November 2008, 11:39 AM   #34
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CFP with JFET input and bipolar as "slave" is a favourite of mine. I can't see why it's not in wider use. I've tried it in a couple of different amps including a simple complementary amp (with a gain stage and EF run at unity gain, but otherwise similar to the picture above). I've tried it in a single end input amp and in a differential input amp, and all has improved a good deal.

Now that AKSA uses CFP input in his top of the line amp, I hope more of you give it a try.
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Old 18th November 2008, 11:55 AM   #35
jam is offline jam  United States
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nelson,

Do you like the bipolar or fet version? I have found theat the fet version requires a higher bias for the fets to sound best, and yes there is a difference betwwen the fet version and bipolar.

Regards,

Jam
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Old 18th November 2008, 12:04 PM   #36
jam is offline jam  United States
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lineup,

Yes I have built a version of your buffer, only two stages but I used the gates as the input and grounded the sources, (higher input z) I ran into some stability problems with the output stage but corrected it with layout and local bypassing.

Would some degeneration on the emitters of U10 and U16 help?

Regards,

Jam
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Old 18th November 2008, 12:09 PM   #37
jam is offline jam  United States
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Tim,

The offset broblem is easily solved by making one of the current sources adjustable, which rules out current diodes.
You layout looks good, I have always liked symetrical layouts and a ground plane might help.

Regards,

Jam
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Old 18th November 2008, 12:15 PM   #38
jam is offline jam  United States
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homemodder,

Yes lineup is a wildman......

I was looking at your circuit and noticed the split feedback path. Is there a advantage to this as opposed to a traditional feedback path as in lineup' design.

Regards,
Jam
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Old 18th November 2008, 04:55 PM   #39
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Jam, no its the exactly the same circuit, JLH did it this way to adjust the offset easily. He actually used a pot for adjustment but I prefer to stay away from them. With the values shown its very close to 0 although in actual building the values may differ. With 1k loading and by decreasing R3 to say around 150 ohm to increase the current one can get very good results. Below - 140db distortion figures with second harmonic dominating and nothing after 3rd. This is simmed results but it has very good performance real time too, superior to Jfets alone and it can handle heavy loading even 100 ohms with good distortion figures. In contrast to BJT cfp theres no stability issues.
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Old 18th November 2008, 05:25 PM   #40
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Quote:
Originally posted by jam
nelson,

Do you like the bipolar or fet version? I have found theat the fet version requires a higher bias for the fets to sound best, and yes there is a difference betwwen the fet version and bipolar.

Regards,

Jam
I've grown fond of the FET sound. It's warmer and gives some kind of texture and fulness to the sound. In the long run more pleasing to the ear. The downside is a slightly grainy sound compared to bipolar.
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