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Old 28th April 2014, 02:04 PM   #1
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Default Improved Sound Card Interface - Ideas and Suggestions?

There has been a lot of discussion about tweaks and improvements that people have made to my original sound card interface PCB. The thread about the original is here:

Test & Measurement interface for Soundcard

This got me wondering of it would make sense to do an update/improvement to the original design?

I thought it might be best if I started a new thread for the purpose of gathering suggestions for improvements.

I'm not committing to do this yet, but if there is enough interest, it might make sense.

So please post your suggestions. Especially valuable are those that you have implemented on the original board, and found an improvement in performance.

Thanks!

Pete
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Old 28th April 2014, 02:14 PM   #2
SY is offline SY  United States
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Balanced inputs (XLR maybe?).

1M input impedance (so it can be used with 10x probes).
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Old 28th April 2014, 02:23 PM   #3
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Old 28th April 2014, 02:56 PM   #4
AKN is offline AKN  Sweden
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My suggestions.

  • True balanced input with XLR
  • Power supply improvements. I experienced common mode switching noise originating from the Murata DC-DC converter (as mentioned in the original thread).
  • Shielding.
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Old 28th April 2014, 04:00 PM   #5
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Input protection. See the ideas populating the Quantasylum 400 thread.

I second the notion of balanced input. You can configure OPA4140's in parallel for very low noise.

Much of the switching noise can be eliminated with a high quality 3-legged regulator, albeit at the expense of some overhead voltage.
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Old 28th April 2014, 04:25 PM   #6
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Old 28th April 2014, 04:39 PM   #7
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Balanced inputs: with a restricted common mode voltage range (like +/-15V) balanced is not difficult. But if something larger (like 200V) is needed, it gets difficult, and expensive. If I recall, Audio Precision actually used transformers and some fancy circuitry to linearize them. What is needed (and what are we willing to pay for it)?

Protection: The existing unit is protected pretty well, at least the input is. I've applied 200V to the 200mV range without any problem. What more is needed?

Pete
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Old 28th April 2014, 04:53 PM   #8
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Quote:
Originally Posted by pmillett View Post
Balanced inputs: with a restricted common mode voltage range (like +/-15V) balanced is not difficult. But if something larger (like 200V) is needed, it gets difficult, and expensive.
That's why God invented capacitors.
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Old 28th April 2014, 04:54 PM   #9
kevinkr is offline kevinkr  United States
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* Balanced input
* Higher input impedance option
* 20mV range (Lowest on my wish list)
* Flexible reference connection option using other channel input from sound card.
* 1:1 transformer isolation option on sound card IO connections (compact Lundahl audio transformers) to deal with conducted noise from the PC. (No ground connection between PC and interface, interface floats and is grounded to the DUT)
* common and diff mode filtering on input/output sides of DC to DC converter
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Old 28th April 2014, 06:02 PM   #10
_Wim_ is offline _Wim_  Belgium
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* balanced input
* 20 mV input range
* Sound card output switchable attenuation: useful when measuring for crossover distortion (very low high resolution input signal for amp required)


* USB control to facilitate automated measurements (for example to measure distortion vs power)

For this USB control part, I would be willing to write the software (using http://www.waitingforfriday.com/index.php/Open_Source_Framework_for_USB_Generic_HID_devices_ based_on_the_PIC18F_and_Windows)

Maybe USB control can also lower the cost as no front panel with knobs and switches is necessary. PIC controller can be powered by USB (including the small relays for switching), analogue section of course needs a separate power supply to avoid damage to the pc.

* and maybe a complex one: the bob Cordell distortion magnifier (I have a “separate” one in the making)
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