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Old 6th September 2003, 05:52 PM   #1
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Default Simple amp design JFET and follower

What are the advantages and disadvantages of:

using 1 JFET input running from a 40v supply... gain of 10 or so.

DC coupled to a darlington pair using say a MPSA18 and a suitable output transistor...

Run the output at 3A use a 6 ohm emitter resistor and cap couple to load.

Battery supply (3x12v SLA)..

Just wondering....
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Old 7th September 2003, 02:57 AM   #2
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Default Re: Simple amp design JFET and follower

Quote:
Originally posted by RichJones
What are the advantages and disadvantages of:

using 1 JFET input running from a 40v supply... gain of 10 or so.

DC coupled to a darlington pair using say a MPSA18 and a suitable output transistor...

Run the output at 3A use a 6 ohm emitter resistor and cap couple to load.

Battery supply (3x12v SLA)..

Just wondering....
Advantages: It's really simple.

Disadvantages: It's really simple.



Sorry, but without knowing just what your goals are, it's difficult to speak to any particular advantages or disadvantages. What you propose is ultimately doable. Whether you'd be satisfied with it in the end is something only you can answer after having built it and given it a listen.

se
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Old 7th September 2003, 02:53 PM   #3
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Exclamation Jfet Amp

"Sorry, but without knowing just what your goals are, it's difficult to speak"

That never stopped you before.

Almost all jfets with enough bias current through them for good linearity will need to be cascoded for that high of a supply voltage and enough voltage swing for a loud speaker. You will need a higher voltage supply for the voltage gain stage for any decent voltage swing. Most importantly you will need to ignore vague advice from people who have never designed an amplifier and make post with no actual useful content. It would be advantageous to avoid overly simple advice.
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Old 7th September 2003, 04:16 PM   #4
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Welcome back, Fred. How was rehab?

se
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Old 7th September 2003, 05:09 PM   #5
jam is offline jam  United States
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Default Warning Food Fight Zone!

Guys,

Now that Grey is back, let the games begin.............................


I am standing on the sidelines for now!

Jam
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Old 7th September 2003, 05:10 PM   #6
jam is offline jam  United States
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or.....
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Old 7th September 2003, 05:46 PM   #7
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Default Re: Warning Food Fight Zone!

Quote:
Originally posted by jam
I am standing on the sidelines for now!
Better stay low. You might get hit by a stray sesame seed. And for all you bun-grabbers out there, remember, buns don't kill people. People kill people.

se
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Old 7th September 2003, 09:12 PM   #8
djk is offline djk
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Maybe you should look at the ZenV4, it may fill the bill.

One follower + one gain stage.

You could substitute your resistor for the current source (or use a choke, lightbulb, whatever).

http://www.passdiy.com/pdf/zen-v4.pdf

ZenV5 looks interesting too. With its low input impedance it would be nice to have a simple J-FET follower on the input.

http://www.passdiy.com/pdf/zen-v5-hires.pdf

Nelson Pass remarked (somewhere) that he liked the sound of J-FET and BJT together, but not J-FET and MOSFET.

Adhering to this, it may be better to use the follower portion of Zen4 grafted onto the front of Zen5.
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Old 8th September 2003, 01:59 AM   #9
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Default Re: Jfet Amp

Quote:
Originally posted by Fred Dieckmann
"Sorry, but without knowing just what your goals are, it's difficult to speak"

That never stopped you before.

Almost all jfets with enough bias current through them for good linearity will need to be cascoded for that high of a supply voltage and enough voltage swing for a loud speaker. You will need a higher voltage supply for the voltage gain stage for any decent voltage swing. Most importantly you will need to ignore vague advice from people who have never designed an amplifier and make post with no actual useful content. It would be advantageous to avoid overly simple advice.

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Old 8th September 2003, 02:24 PM   #10
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Default Fet Fight At The......

Jam,
Your pics always crack me up - just where do you keep getting them from ?.

Keep it up.

Eric.
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I believe not to believe in any fixed belief system.
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