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Old 2nd November 2011, 11:53 PM   #11
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Originally Posted by dggs View Post
I have my PA-7 for almost 20 years. Have always been wondering how easy/difficult/possible it is to convert it to balanced input. Thanks Apogee for posting the input stage schematics. Can I just disconnect C102 and feed negative input to R108(?) (2.21K) ? Any comment would be appreciated!
Any answer to this??
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Old 7th November 2011, 07:48 AM   #12
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Originally Posted by dude2010 View Post
Any answer to this??
No. You can not short C102. C102 is the feedback..

The best upgrade to these amplifiers is to short C102, it will then be dc coupled. I did that with my own, and the sound came to another level.
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Old 7th November 2011, 08:23 AM   #13
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Originally Posted by Lars Nielsen View Post
No. You can not short C102. C102 is the feedback..

The best upgrade to these amplifiers is to short C102, it will then be dc coupled. I did that with my own, and the sound came to another level.

You can't short it... But you did short it!?? Did I miss something here?
But great! Tell us more about your modds!
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Old 14th November 2011, 07:45 AM   #14
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Originally Posted by dude2010 View Post
You can't short it... But you did short it!?? Did I miss something here?
But great! Tell us more about your modds!
Sorry.. You can not disconnect C102..

If you short C102, the amp will be DC coupled.

My mods are described in another thread.. just search
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Old 15th November 2011, 03:02 AM   #15
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I see Apogee is exploring the whole Stasis thing, too, only in a far more intelligent and informed manner than I am. I would hope to learn as much as I can about these amps, since I, too, have recently developed a Stasis jones, only know zip about circuitry and hands on design, upgrades, etc., but since you chappy's are on it here, I'll just hang out and lurk, ask dumb questions, hopefully get to the point where I can even understand what you are talking about! Also interested in just how much of this is Pass 9000 and how much Nakamichi. I cannot afford Threshold to do any a/b listening tests. I've been pulsing Nelson San in emails and I must say he's been quite patient but I think this is the better path to Wisdom and Understanding, and it will spare the gent my crazy missives.

BP
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Old 6th February 2013, 05:12 AM   #16
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Having worked on a few of both units. Here are my thoughts: The small electolytics on the PCBs are important to change. Some control the protection circuit and can fail. The trimmers get replaced with multi-turn devices. The main caps are hard to find for reasonable cost, but new brackets could be fitted. The relay can fail. Some of the parts like 2SC3333 and 2SA1320 are getting hard to find. I was able to source some from an obsolete part warehouse. Once the bias is set higher like Nelson indicates the sound is quite nice and somewhat compares to an S500 The cap of interest dictates the roll off freq. It can range from the 10uf found here to 470uF found on a older stasis circuit. Other designs use 220uF or 330uF What sounds best? Sometimes even, the design is de-tuned to not sound as good as the bigger flagship units.
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Old 23rd May 2013, 10:43 AM   #17
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Default PA-7 to balanced?

Is there an answer as to how or if it is possible to convert the PA-7 to balanced input?
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Old 27th May 2013, 08:58 AM   #18
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Originally Posted by warren64c View Post
Is there an answer as to how or if it is possible to convert the PA-7 to balanced input?
I would say: impossible.. You need to change the input circuit to a new design.
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Old 27th May 2013, 01:16 PM   #19
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change R102 from 75K to 47Klift left side of C102 from GND , connect it via 220K to same point (gnd) and use that node (lifted left side of C102) as neg input
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Old 27th May 2013, 02:00 PM   #20
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Take a look at the Parasound HCA-2200ii for an example of creating a balanced input.

C102 is shorted (i.e. not used) and an IC DC servo is used to control output offset.

A J-FET source follower is used to buffer the negative input. Output of source follower then drives negative input through a resistor (which sets gain).
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