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Old 19th November 2013, 05:57 PM   #501
1audio is offline 1audio  United States
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I think this will illustrate what I'm asking:
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File Type: png Test setup.PNG (7.2 KB, 293 views)
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Old 19th November 2013, 07:31 PM   #502
RNMarsh is offline RNMarsh  United States
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Quote:
Originally Posted by 1audio View Post
I think this will illustrate what I'm asking:
The gen used was a 600 Ohm output Z (HP 339A)....
?

-RM
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Old 19th November 2013, 08:57 PM   #503
1audio is offline 1audio  United States
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The point being that the QA400 etc. will introduce distortion at its input. You can measure the distortion introduced by the input of the QA400 (or whatever) with this test.
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Old 19th November 2013, 09:10 PM   #504
RNMarsh is offline RNMarsh  United States
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I will do that test and let everyone know if any change due to QA400 input loading and put up data here.

Thx-RNMarsh
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Old 20th November 2013, 05:39 AM   #505
RNMarsh is offline RNMarsh  United States
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Default comparison A-P vs QA400

Using the ShibaSoku AG16 source (600 ohms/.7v rms), the QA400 added .0001% THD+N when applied in parallel with Audio-Precision 2722.

That is when the QA400 is in data acquisition mode... when halted and you have only the screen capture display, the THD+N is 2-3%.

The AG16 shows on the A-P as only having 2H and 3H at about -120dB and nothing above the noise floor of -135dB above those two harmonics.

The QA400 however, shows a lot of harmonics and extrainious freqs beyond the first two harmonics and shows the 2H and 3h at a higher level than the A-P.

RNM


DSC01190.jpg

A-P with Shibasoku AG16.jpg

QA400 with ShibaSoku Ag16.jpg

Last edited by RNMarsh; 20th November 2013 at 05:57 AM.
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Old 20th November 2013, 11:47 AM   #506
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If the "stock" '400 input and output stages may be the "weak link" in an otherwise excellent product, how about adapting/modifying the outboard I/O stage that Demian has already developed to "tap into and replace" the stock I/O amps?

Charles
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Old 20th November 2013, 12:37 PM   #507
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It may be enough to make sure the QA400 input is driven from a very low impedance, i.e. a buffer.
Many opamps show somewhat nonlinear input impedance and that may become a factor at the very low distortion levels we are looking at here.
Possible Demian's interface is better in this respect?

Jan
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Old 20th November 2013, 01:15 PM   #508
davada is offline davada  Canada
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I suspect the ESI chip and possibly the input blocking capacitor. it would be interesting to see
what we have with these parts removed or replaced.
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Old 20th November 2013, 03:36 PM   #509
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David, what is this ESI chip - is it the input opamp/buffer or what?

jan
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Old 20th November 2013, 03:55 PM   #510
1audio is offline 1audio  United States
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I saw the input induced distortion as well. The interface reduces it but has a similar problem of its own. However the interface does reduce the extraneous spurs.

Adding a buffer/driver to replace the one on the board could work but its still more stuff to deal with. That only makes sense when the interface is inside the box. Connecting flying leads to holes vacated by the BNC connectors is easy. Flying wired to pads on surface mount PCB's demands an additional level of skill. Also, ideal would be an integrated diff in to diff out solution like the one I'm using. Otherwise it will need a number of .1% resistors to have decent common mode rejection.
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