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Balanced Soundcard Interface for Power Amplifier Distortion Measurement
Balanced Soundcard Interface for Power Amplifier Distortion Measurement
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Old 11th December 2013, 02:30 PM   #31
lhquam is offline lhquam  United States
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Quote:
Originally Posted by Speedskater View Post
I'm not sure how the circuit works with a balanced circuit, as a balanced circuit has no direct reference to ground. The XLR connector pin #1 is chassis not circuit ground.
I have found that most consumer balanced audio has pin 1 connected to signal ground rather than chassis ground. Whether pin 1 is connected to signal ground or chassis ground, the attenuators shown will work correctly. Any issues with the ground will be common mode.

However, balanced attenuators are normally built as shown in the following schematic. (Note that the voltages to the clamp diodes are reversed from the previous drawing which was incorrect). When I compared the noise levels using these balanced attenuators vs. the grounded attenuators shown in the previous post, I found slightly lower noise levels in the grounded attenuators. This is with the signal source being an M-Audio Audiophile-192 soundcard.
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File Type: jpg soundcard-interface-2-asc.jpg (271.6 KB, 433 views)
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Old 11th December 2013, 02:36 PM   #32
fpitas is offline fpitas  United States
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Wherever pin 1 is connected, the circuit common has to be close to the chassis ground potential, since otherwise the output common mode voltage would be undefined, and might exceed the input common mode voltage tolerance of the next piece of equipment.
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Old 11th December 2013, 05:22 PM   #33
lhquam is offline lhquam  United States
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I presented a soundcard interface that can be used for either balanced or unbalanced sources and sinks. The attenuators were primarily designed to match the voltage swing from an amplifier output to the permissible voltage swing of the soundcard input. There is no intent for these attenuators to be used for more general balanced audio interconnection.

Here is a simple analysis of the grounded attenuator in post #28 Balanced Soundcard Interface for Power Amplifier Distortion Measurement, " and the ungrounded attenuator in post #31 Balanced Soundcard Interface for Power Amplifier Distortion Measurement. The major difference is in the common mode output. The grounded attenuator reduces the common mode by the same factor as the difference signal whereas the ungrounded attenuator retains the full common mode level.
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File Type: jpg ungrounded-atten.jpg (87.3 KB, 400 views)
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Old 11th December 2013, 05:27 PM   #34
lhquam is offline lhquam  United States
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Quote:
Originally Posted by ArthurDK View Post
Will the clamping diodes only work with batteries ?

Best regards
Arthur.
Post #31 shows an alternative without batteries. I chose batteries so that additional power supplies were not required. It might work fine with just the Zeners and switching diodes and no extra bias current, but I have not measured the distortion created by leaving the Zeners unbiased. What I would expect is that the Zeners will charge to the levels defined by the min and max peaks of the signals.
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Old 11th December 2013, 05:55 PM   #35
ArthurDK is offline ArthurDK
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Thank you :-)
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