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Old 22nd August 2012, 05:26 PM   #26831
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Quote:
Originally Posted by RNMarsh View Post
The 990 claim to fame was in the patent -- using small inductance in series with the emitter resistor, Re. The example circuit used to demo the affect on noise was pretty funky even for the times back then. Of course, the principle could be used in anyone's differential input circuit.
Dick Burwen did this in 1966?, I think Walt's history has a schematic.

EDIT - http://www.analog.com/library/analog..._ChH_final.pdf AD Model 121

IIRC - When the patent was obviously unenforcable Deane placed it into the public domain (moot now).
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Last edited by scott wurcer; 22nd August 2012 at 05:36 PM.
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Old 22nd August 2012, 05:28 PM   #26832
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Quote:
Originally Posted by john curl View Post
Scott, would you like to 'critique' the Jensen 990. Did you know Dean Jensen? I did. Have you ever used a version of the 990? I have.
In fact, the Taiwanese, for an early Parasound preamp, more than 20 years old, adapted the general Jensen topology, adding an 'improvement' of using dual jfets on the input, instead of bipolar transistors. It was the first preamp that I worked on for Parasound. I have one in a closet, somewhere.
I have no problem but this technique is violently opposed to the mantra here in that it increases dramatically the low frequency Aol as opposed to the gain at high frequencies.
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Old 22nd August 2012, 05:28 PM   #26833
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Right you are, Scott, however a jfet input does NOT need the inductor to get the same noise (or better), because the gm is lower in the jfet compared to the bipolar.
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Old 22nd August 2012, 05:32 PM   #26834
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Of course, Scott! That is part of the 'critique' '-) As a drop in module, it is pretty good, certainly better than any IC I have used. Constellation does not use them, and they are about as expensive and exotic as Boulder.
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Old 22nd August 2012, 05:34 PM   #26835
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Quote:
Originally Posted by scott wurcer View Post
I have no problem but this technique is violently opposed to the mantra here in that it increases dramatically the low frequency Aol as opposed to the gain at high frequencies.
If I design an audio opamp using several IC opamps with local feedbacks, will it be considered as discrete design?
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Old 22nd August 2012, 05:45 PM   #26836
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Before I go further let's see if we can get agreement on a really simple circuit.

Pavel, Wavey, Scott, SY and anyone else interested, would you each please post an answer to what is the power dissipated in the resistor in the attached circuit?
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Old 22nd August 2012, 05:49 PM   #26837
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Quote:
Originally Posted by EUVL View Post
>A group design of a discrete op-amp is a great idea.
> First, choose the basic topology.
> This thread or new? -RNM

New thread please, so that it does not have to be mixed with all other on-going discussions here.

Topology :
JFET input, 2 stages, possibly SMD parts ?


Patrick
OK, new thread. I will dig out my simple suggestion, you guys will have to help on the discrete NPN/PNP choices. I was going to use LSK170/74's on the input.
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Old 22nd August 2012, 05:51 PM   #26838
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One must ask what are the performance advantages of ICs? Among them, intrinsic matching of adjacent devices; reduction of some parasitics. Deane used what is in essence an IC in the input stage, the LM394 pair, which is an array of interconnected devices. Good parts, although the same or better performance can be achieved in other ways.

Jensen's white paper on the design was instructive, and included the way that the compensation was done to extend bandwidth where it was thought useful. And as remarked, he thought he was the first to conceive of the use of inductors in the emitters to limit gain at high frequencies, but it was, like so many things, prior art, which (like they do more times than not) the USPTO missed.
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Old 22nd August 2012, 05:54 PM   #26839
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Here is My challenge for a descrete opamp --> using any topology and transistor types (except expensive and/rare/obsolete/exotic types): no more than 8 transistors; drive 30 ohms or lower with no more than -105db harmonics, 20-20Khz; S/N ref 1v -unweighted to be at least -110db (input shorted). ; no use of dc servo, allowed - dc offset at output less than 2mV over time and normal use temp range.

How much better than these requirments/specs can be made??

RNM
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Old 22nd August 2012, 05:57 PM   #26840
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Quote:
Originally Posted by bcarso View Post
One must ask what are the performance advantages of ICs? Among them, intrinsic matching of adjacent devices; reduction of some parasitics. Deane used what is in essence an IC in the input stage, the LM394 pair, which is an array of interconnected devices. Good parts, although the same or better performance can be achieved in other ways.

Jensen's white paper on the design was instructive, and included the way that the compensation was done to extend bandwidth where it was thought useful. And as remarked, he thought he was the first to conceive of the use of inductors in the emitters to limit gain at high frequencies, but it was, like so many things, prior art, which (like they do more times than not) the USPTO missed.
Today it is not needed. I would take one low noise dual opamp and combine it with one more single fast opamp that has better output stage, adding few resistors. The result will be one opamp that you can't beat by discrete designed opamp.
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