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Old 18th January 2007, 08:18 PM   #1
lineup is offline lineup  Sweden
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Lightbulb New JFET Hifi Op-Amp release!

High Fidelity Performance Op-Amp.

No need to make too complicated
when such good results with fairly simple 3 stage circuit

Set up this amplifier design model
in your simulator
and you will see!
And why not layout your own Hifi Op-Amp PCB
and make a few small killer OPamp devices of your own.


Version 1 of mine used 17mA current total
at +/-15 Volt DC
and tested for practically 0% THD distortion, in various loads and output voltages.
Fourier Analyse shows very nice even spectrum of harmonics.


Regards
Lineup Audio Lab Productions
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Old 18th January 2007, 08:55 PM   #2
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Did you made this circuit to test? Or you test in one circuit design program?

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Old 18th January 2007, 09:20 PM   #3
lineup is offline lineup  Sweden
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Quote:
Originally posted by mod_evil
Did you made this circuit to test?
Or you test in one circuit design program?

Best Regards
I tested simulation only with using 2SK170GR input JFET:s
For the Current sources, J3 / J4, I used BF245B.
Bipolar transistors, Q1-Q5, are my good spice models of BC560C / BC550C.

The output are some good TO39 transistors I have models for.
These 2 Output stage TO39 NPN, I have here at home
because they came from some old junk circuits PCB
that I have found at our electronics recycling station.
I got them for a few cents.

I say, what transistors you use, is not so critical.
As long as they are normal good audio small JFET / Bipolar.
This is why I did not specify any special transistors in my circuit.

But I recommend you try to use like those currents I have in circuit.
Because these currents will work!
I did add one little compensation cap. Somewhere ...


I use Electronics WorkBench, EWB, MultiSim9
with good spice models for transistors I downloaded from internet
and also found here in forum.

http://www.electronicsworkbench.com/

Regards
lineup
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Old 19th January 2007, 04:44 AM   #4
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resistor values?
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Old 19th January 2007, 11:05 AM   #5
lineup is offline lineup  Sweden
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I will as usual be back with a Version 2.
After further testings in extreme loads.

But as the Version 1 shows so very good behavior
I can not imagine there is very much final trimming needed.

Input JFETs, 2SK170GR should be as good MATCHED as possbile.
Best solution for this is of course use a DUAL JFET.
With 2 matched jfet in same case.
Like 2SK389 and several other.
There is another dual matched pair, with very low noise and very good matched devices.
But I do not remember the name. (maybe J404 or something like that???)

There are some extremely fine such N-JFET Pairs
excellent for Audio Amp input.
JFET also have almost no input bias current which can cause offset
when you want to use different resistors in input/feedback
------------------------------

For fine tuning of open loop output DC offset
I used R5, which can be replaced with 500 Ohm trimpot.

Use for example my recommended power supply +/-15 Volt.
1. You attach both IN+ and IN- to GND.
2. You use 100/1.000 Ohm for load. From OUT to GND.
3. Use no feedback from output = Open Loop setup.
4. You use this trimpot (R5) until output is close to 0.00 Volt
5. Measure this resistance of trimpot.
6. Replace the trimpot with a resistor with same value = R5



Until I come back with eventual updates
Regards
lineup
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Old 19th January 2007, 04:44 PM   #6
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Seems like a workable idea. I would degenerate the current mirror for less noise. With 2SK170's in there it's not so much of a problem but with lower gm JFETs you will get a lot of noise gain from the mirror to the input. The ouput stage has some limitations too. I wonder how this would work with an auto biasing complimentary N/P FET follower?

You also might want to cascode J1 and J2, 2SK170's have lots of excess gate current at 15V

BTW I tested some BF862 Philips' JFETs and was pleasantly surprised, <1nV/rt-Hz @ 1 mA and a corner well below 100Hz. The data sheet almost reads right on the 2SK170. These cost me 15 cents each. SMT only though and same excess gate current (impact ionization from the short channel).
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Old 19th January 2007, 09:06 PM   #7
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Quote:
Originally posted by scott wurcer
... You also might want to cascode J1 and J2, 2SK170's have lots of excess gate current at 15V

...BTW I tested some BF862 Philips' JFETs and was pleasantly surprised, <1nV/rt-Hz @ 1 mA and a corner well below 100Hz. The data sheet almost reads right on the 2SK170. These cost me 15 cents each. SMT only though and same excess gate current (impact ionization from the short channel).
M. Wurcer, is the excess gate current really a problem? (I mean audio wise) It's still very low compared to BJT's. Maybe J.C. or C.H. could also enlighten us on this point?
Do you know a P counterpart for the BF862?
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Old 19th January 2007, 09:50 PM   #8
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Hi Lineup

I like the common base VAS. I like it better (IMHO) than a common emitter VAS. Also (IMHO) I like differential VAS as well. Refreshing it is too see some new ideas. I have one too. Since I don't have a really good simulator, I usually just build a test circuit...it's more fun anyway Haven't made this yet, what do you think? Bias of first stage is dependent on 2nd stage CCS. Just another idea to try....



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Old 19th January 2007, 11:13 PM   #9
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Quote:
Originally posted by lineup
[BLike 2SK389 and several other. [/B]

these are as hard to find as hen's teeth...that being said, I would arrange a group buy of LSK389's (Linearsystems) if there is interest --
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Old 19th January 2007, 11:27 PM   #10
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Quote:
Originally posted by jackinnj



these are as hard to find as hen's teeth...that being said, I would arrange a group buy of LSK389's (Linearsystems) if there is interest --




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