Jfet Line preamp, to cascode or not. - diyAudio
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Old 19th August 2012, 07:25 AM   #1
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Default Jfet Line preamp, to cascode or not.

Hi

I am considering building this line amp, but unsure weather there would be any improvements to be had by cascodeing the input Jfets with either BJT, or higher VP fets.

Any comments welcome.

Thanks.
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Old 21st August 2012, 04:18 AM   #2
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What I was wondering was why you propose to use it at all let alone replete with cascodes.
rcw
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Old 21st August 2012, 05:28 AM   #3
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Hi,

Well i know a similar topology has been used open loop as a phono amp. It occoured to me it may make a reasonable line amp either with the gain lowered by current feedback as shown or even open loop with say a diamond buffer in place of the output emitter followers.

Do you have a specific reason for disliking the circuit?

Last edited by velvetsunrise; 21st August 2012 at 05:40 AM.
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Old 21st August 2012, 05:33 AM   #4
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Have you build it ?
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Old 21st August 2012, 05:39 AM   #5
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Hello Joachim

No, not yet.

But i probably will
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Old 21st August 2012, 05:47 AM   #6
RNMarsh is offline RNMarsh  United States
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cascode will improve distortion and power supply rejection. best done with bipolar for highest cmr. -RNM
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Old 21st August 2012, 05:50 AM   #7
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It should work. I have ideas to improve it but you should build a prototype first.
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Old 21st August 2012, 05:58 AM   #8
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Thanks, i will add some BJT cascodes to the input fets, biased with LED strings, and give it a try.
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Old 21st August 2012, 08:18 AM   #9
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It is just that I have never been convinced about the merits of symmetrical designs for audio, or the use of cascodes in them, it seems to me to be a great deal of needles complexity for very dubious benefits.

Symmetry certainly has esthetic appeal but that is all the merit I can see in it.
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Old 21st August 2012, 12:21 PM   #10
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I'd think about the F5 amp, this is somewhat akin...

I'd want to lose the cap coupled output myself.
perhaps a servo or other means to stabilize the DC point.
Also, not sure I want diodes for the biasing string... maybe, but maybe not... people do seem to use them a lot...
Mosfets for the output?
What is the purpose of the current mirror (just asking)?
Also the input pot is drawn backwards...

_-_-bear

PS. did you simulate it yet and see what it specs at?
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