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Analog Line Level Preamplifiers , Passive Pre-amps, Crossovers, etc.

Thoughts on making J-FET Preamp of a 2N5546
Thoughts on making J-FET Preamp of a 2N5546
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Old 23rd June 2020, 12:14 PM   #1
clew is online now clew
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Default Thoughts on making J-FET Preamp of a 2N5546


Ive taken apart an old oscilloscope and found some interesting old gold pin BJT and JFETS (and some opamps i havent looked into yet.
I have built a JLH 1969-ish clone before and a normalt simple BJT preamp, and been tinkering with electronics a alot, but not in audio very much.

But I would like to try this as an input stage to JLH or maybe a separate preamp.
It beaing a dual jfet should mean they are pretty well matched up?

It is a 2N5546 that a find most appropriate of what I have.
Link to datasheet: 2n5545-7.pdf PDF datasheet. ALL TRANSISTORS DATASHEET. POWER MOSFET, IGBT, IC, TRIACS DATABASE. Electronic Supply. INNOVATION CATALOG. Service

I have done alot of searching and havent found anybody using this, I read up on what resistors etc would be appropriate i believe. But what I just seem to not find any good info on is what makes a particular JFET good for audio purposes?

Is it even probable that a osciloscope jfet would be of any use? It was a crt oscilloscope but i dont believe it was part of that circuit but somewhere near the inputs.

Sorry if any grammar is messed up, english is not my first language.
Would very much appreciate some tips in the right direction.

Best regards,
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Old 23rd June 2020, 07:46 PM   #2
duncan2 is online now duncan2  United Kingdom
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You haven't found many using this dual fet as fet technology has advanced considerably with fets designed for audio having vanishingly small amounts of noise etc like-


Even the one you posted has been superseded.

A JLH 1969 clone using j-fets ?

Do you know how many audio enthusiasts think John,s original design OR his slight modifications to it represent a high level of audio satisfaction ?
To such an extent many well known audio design engineers have built it and applauded it in tech magazines like EW inhabited by guys in the industry.

Sure you can experiment but don't expect the same quality of reproduction that circuit is a world classic.
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Old 25th June 2020, 02:21 AM   #3
Bigun is offline Bigun  Canada
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Thoughts on making J-FET Preamp of a 2N5546
maybe build a simple Nelson Pass buffer with that dual jfet, even if just for kicks it would be a handy circuit to have around for various experiments
"The test of the machine is the satisfaction it gives you. There isn't any other test. If the machine produces tranquility it's right. If it disturbs you it's wrong until either the machine or your mind is changed." Robert M Pirsig.
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Old 26th June 2020, 05:04 AM   #4
clew is online now clew
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Yeah this is obviously not the absolute best stuff, I'm aware. I just got into building audio electronics and I try to see what I can make with stuff I have around.
But youre right, I saw some expensive looking old parts with gold on them and saw prices on ebay without looking very close at the specs.

I do have some input fet opamps from amps and theres some K373 JFET in an old amp pcb. I will try them instead.
And I dont need a preamp really for signal level, I guess a buffer is more what Im after. More specifically I was looking into getting more and consistant input impedance while building a passive input selector and attenuator.

Does that sound like a reasonable idea_
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Old 26th June 2020, 10:52 AM   #5
Mooly is offline Mooly  United Kingdom
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Thoughts on making J-FET Preamp of a 2N5546
Scopes used these dual matched FET's in the input stages because being matched and in the same package meant the circuit was less prone to drift. Any imbalance would result in the trace on the screen continually drifting until everything was warmed up and stable.

The Siliconix E421 is one I remember:

E 421, Tube E421; Rohre E 421 ID45956, Transistor
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Old 27th June 2020, 05:25 AM   #6
johnhenryharris is offline johnhenryharris  United States
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for Audio the 2n5556 was used in the front end differential input of the Threshold SA/1 -3 can't be all bad.


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