On Tue, 14 Jul 2009 18:26:33 -0700 (PDT), Rick Karlquist wrote:
>Also, the waveform on the output of the controller is only a few
>hundred Hz. The capacitive reactance is much larger than the resistance
>at these frequencies. Microphones have to go to 20 kHz and the impedance
>is at least 600 ohms, not a few dozen, so that may not be a great analogy.
You may be right. I haven't looked at the waveform, but a 300 Hz square
wave will have harmonics well above 3 kHz. As to the capacitance -- 40pF/ft
is typical for the generic sorts of cable that SteppIR is using.
On the audio side of things, the relatively low reactance of the large
capacitance of a lot of cable can current-starve the live driver, causing
distortion peaks above about 10 kHz. Also, a relatively high value of
capacitance as a load can interact with the feedback network around the
output stage make it unstable. Deane Jensen published a classic paper on
that about 40 years ago, and ever since, audio output stages have 50 ohm
resistors in series with each leg to prevent the instability.
BTW -- pro audio has not used 600 ohm interfaces for about 50 years. Line
drivers (mics and output stages) have output impedances on the order of 100
ohms. Line level outputs are designed to drive input circuits on the order
of 10K ohms, and mics are designed to work into at least 5X their nominal
output impedance. We describe these as "voltage-matched" systems.
Yes, these are low frequency circuits, but things are not always as simple
as they look.
73,
Jim K9YC
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