Pass Discrete Opamp in DIP-8 Package

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In the past, I have published the odd schematics of projects that I have built, plus the odd photos. So this is going to be the first publication from me for a project from start to finish.

A few disclaimers first :

a) All the information I have to offer is as published.
b) You can copy, modify, DIY, commercialised, ... as you want.
I have no illusion of any intellectual property rights by publishing on the Internet.
c) I cannot offer any components (PCBs, matched FETs, ...) or finished products.
You have to do it yourself.
d) This is not a project for beginners.
You need to be able to etch double sided PCBs to finer than 0.5mm.
You should be able to drill to 0.5mm diameter.
And you need to be able to match SMD FETs in SOT-23.


Now the story.

Like many, I started audio DIY using opamps (ICs) but have gone discrete over the years. A few years back, I read the article by Nelson Pass on discrete opamps, and thought how nice it would be if one can make something similar in a compatible size. Back then, there were few choices of SMD JFETs or bipolar transistors for that matter, not to speak of matched duals. So nice idea, but I did not need it bad enough to follow on.

There are literally hundreds of websites on the internet for discrete opamps, including finished products, schematics, photos, ...., etc. I have attached a few links for well known circuits here (FETs). There are equally many circuits based on bipolar transistors, e.g. from Jensen & D. Self. You just need to google :

http://www.borbelyaudio.com/pics/305borbely-new.pdf
http://www.passdiy.com/pdf/diyopamp.pdf
http://www.forsselltech.com/download...et%20Opamp.PDF
Improved discrete op-amp

Although they are claimed to be sonically better than their IC counter parts, and with finished modules selling around US$40 per opamp for DIY upgrades, I have yet to see a discrete opamp design which is size compatible to a standard DIP-8 package, like e.g. a NE5534.

Well, so what !! Except that you try to use them on a PCB that will fit nothing but a standard DIP-8.

Around Christmas 2007, I was looking through the website of Linear Systems and realised that they could offer LSK389s in SOT-23-6 packages. It re-ignited my thoughts on a discrete opamp in DIP-8 size, and a quick layout followed which would have fitted a 10x10mm footprint.Well, would have. Of course the LSK389 is only available in SOT-23-6 in theory.

The attached document describes my approach to doing just that - a discrete all-FET opamp in DIP-8 package.

Enjoy.



Patrick

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Top View.

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Top isometric

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Bottom View.

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Bottom isometric

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With heatsink in a glueing fixture.

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Pretty nifty


Magura

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Supply voltage +/- 12V
Gain =11 (10k / 1k, non-inverting)
Load = 1k
Ccomp = 100p (not optimised, just what I had at hand)
Input signal = 0.1V

Heatsink temp = 55 degC.

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100kHz square wave.

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100kHz sine wave.

(Green is output, cyan is signal generator)