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[DPRG] Audio filtering

Subject: [DPRG] Audio filtering
From: John Swindle swindle at compuserve.com
Date: Wed Nov 4 18:44:06 CST 2009

Scott and Ed:

I agree that most audio problems are solved by eliminating ground 
loops and isolating power. It seems that some consumer devices 
have internal leakage pathways that are sufficient to create 
loops within the signal pathways, even if the grounds are 
isolated, maybe because of an unbalanced signal going over what 
looks like a balanced line. For low-power devices, the signal 
lines can source or sink power unintentionally.

A few times, I've transfomer-coupled the systems, with pleasing 
results. Fortunately, audio transformers are small. I personally 
have never had satisfactory success with filters if I want to 
actually listen to the audio. Filtering seems OK to reduce the 
noise when the audio is being measured by a program, such as for 
speech recognition. But if I want to listen to the audio, a 
filter is unsatisfactory.

Beware that the loops can be anywhere. For example, the USB spec 
allows the signal ground to be connected to the chassis ground at 
both ends of the cable. (The original USB spec prohibited that 
behavior, but later specs relented to a "best-practices" 
statement instead of a specification.)

Older stereo systems connected the signal lines through 
high-value resistors to the safety ground (which is one side of 
the AC mains). I don't know if that practice is still done, but 
those megohm resistors sometimes created problems with low-level 
audio signals.

Audio amplifiers can be so sensitive that even a capacitive loop 
can create problems. It doesn't have to be a DC pathway. If 
you're lucky, you can measure the AC current between the isolated 
systems. Kinda spooky. Sometimes, you can feel the current. Even 
more spooky.

Good luck.
John Swindle

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