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Old 17th August 2002, 10:37 PM   #1
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Question Does Pre-Matching 9610's Really Work?

Hi Dale, Guys,

I realized this question was probably in the wrong forum so I re-ask it here.

I went ahead and ordered some extra 9610's (from Dale)
1.)The hope is to build a second Aleph at a later date.
2.)It sticks in the back of my head that in an old thread (un-findable with "Search") Mr. Pass said (not a quote, just from memory) that it does little good to match the differiential FETs. That the Pass method is to substitute 9610's until the DC offset disapears.

Does any one have experience with matched 9610's providing good results with a low DC offset voltage, or will it still be necessary to try several devices?

Slowly, but surely, I'm getting there.
Rodd Yamas***a
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Old 17th August 2002, 10:41 PM   #2
fcel is offline fcel  United States
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My understanding is that to obtain low DC offset, the output transistors would have to be matched as closely as possible.
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Old 17th August 2002, 11:34 PM   #3
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If you match them for Vgs, then you usually don't
have to go to any extra effort to lower the DC offset
at the output.

Actually substituting devices into a working amplifier is
really the best way to do it, as you can test for a bunch
of performance parameters. It is tedious, but worth it.

Did I mention that we turn the amp off while performing
the substitution?
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Old 18th August 2002, 12:54 AM   #4
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Hi Rodd,

I will probably have plenty of input 9610's so I will try to sort in batches of three of four and send out more devices. Maybe you can report back on your success in doing this. I would expect that you would still want devices that match pretty close on Vgs.

doing my part to Keep Austin Wierd.
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Old 18th August 2002, 01:02 AM   #5
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Hi Dale,

Thanks, much appreciated.

Rodd Yamas***a
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Old 19th October 2003, 04:32 PM   #6
Asen is offline Asen  Bulgaria
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Mr. Pass

Once you said you substitute the input devices in order to obtain lower distortion, not lower offset. From your experience, do they go together? I mean the lower offset and the lower distortion, or there are no rules of that kind?

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Old 19th October 2003, 05:06 PM   #7
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I'm sure there's some kind of relationship, but I don't
necessarily count on it. It depends on how much time I
want to devote to an amplifier. If I have lots of time, I'll
sub devices in and out, picking the best measuring combo
for offset, noise, and distortion, or maybe any one of these
three depending on where I'm trying to go. It's a very
tedious process.
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Old 19th October 2003, 06:21 PM   #8
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I feel that replacing parts may because of lack of proper Engineering of the circuit but it is the cheapest route in the orginal approach....there are so many variances when designing and building that we have to decide what is best when making a circuit work...hence subbing transistors is the best approach rather than a new board design

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Old 19th October 2003, 08:31 PM   #9
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It might be a design issue, but more commonly it
is the result of imperfect parts, specifically gain devices
which distort or vary with temperature, voltage, current
and so on.
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Old 19th October 2003, 09:09 PM   #10
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The impact of differing Vgs values will vary depending on what circuit you intend to put them into. Something like an Aleph can tolerate more mis-match than an Aleph-X because the 'back' MOSFET (i.e. the one that takes the NFB and negative phase of a balanced input) is free to swing; its output doesn't go anywhere. If you try a mis-matched input pair in an Aleph-X (or any X, for that matter) the 'other' side will try to swing in order to bring the circuit into balance. Unfortunately, that will cause the outputs that it's hooked to to go along for the ride and the differential voltage offset at the outputs will get lopsided pretty quickly.
That said, it's still a good idea to match inputs, even in a 'normal' Aleph. The front end won't have to work so hard to achieve zero offset (or at least close to it) at the output.

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