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Old 18th November 2008, 08:02 PM   #1
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Default M-Audio Transit DC offset


I was doing a new round of measurements last night and found that my M-Audio Transit USB sound card has high DC offsets.

The Right Out has 1.3V. The Right In has 2V. The soundcard was only connected to a desktop computer.

Upon the discovery, I connected a pair of cheap, active desktop computer loudspeakers and played a CD - the sound is normal and beautiful.

So I added an input capacitor to the preamp then did some measurements and they looked OK.

Is there something wrong with the soundcard? Have you measured yours?

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Old 22nd November 2008, 01:03 PM   #2
y8s is offline y8s  United States
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what is the DC offset with a load connected? does it drop to tenths of a volt?
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Old 23rd November 2008, 08:18 PM   #3
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I did not measure it directly with a load, but I suspect you are right that the voltage dropped when connected to a load.

I initially discovered the problem when I measured 300mV from my power amplifier output to the speakers. I then switched off the power amp and preamp and then measured the soundcard outputs while the the R In was connected to the R Out. The measurement showed higher voltages (1.3V and 2V).

Since I have neither DC blocking capacitors in the preamp nor power amp, if the voltage did not get dropped with a load, I would have measured very high voltage, instead of 300mV, at the power amp output.

So what does that say?
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Old 24th November 2008, 06:22 PM   #4
kevinkr is offline kevinkr  United States
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Originally posted by HiFiNutNut

So what does that say?
Probably that you need coupling capacitors.. I have one of these as well that I use for audio measurements, but I use an outboard box which is ac coupled to connect it to the DUT. I posted a schematic somewhere here on diyaudio in the past that outlines the details of my interface box.

I haven't measured mine recently, but I have some vague recollection that there was some dc present on the output when connected to high impedance loads. It is likely that the coupling capacitors are a bit leaky and this in conjunction with dc coupling and a high input impedance in the following device could lead to offset issues.
"To argue with a person who has renounced the use of reason is like administering medicine to the dead." - Thomas Paine
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Old 25th November 2008, 07:15 PM   #5
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Thanks for your inputs.

I have now the DC blocking cap in my measurement preamp and since the measurements look fine I will live with it.

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