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Just beginning my experiments

0

Posted by Al Sekela on July 8, 2004 at 14:36:56

In Reply to: Al, what value for R? posted by Lew on July 7, 2004 at 11:09:52:

I used a 4 ohm resistor in conjunction with the 0.39 mFd cap to achieve a corner frequency
of 102 KHz. I chose the 4 ohms to match the nominal impedance of the Magnepan speaker,
and 100 KHz as the corner frequency target because it is 2 octaves above the theoretical
band of useful information from the Redbook CD (and 3 above the practical limit of my
hearing...).

I'd like to use a resistance value to match the characteristic impedance of the speaker cable,
if I can find the cable value or some way to measure it.

My operating theory of what is going on here is that the speaker cable is an RF resonator,
and high frequency noise sets up oscillations in it because the speaker impedance is not
matched to the characteristic impedance of the cable. We know that transmission lines
terminated in impedances different from their characteristic impedance exhibit reflections at
the terminations. If the lines are low-loss (as most good speaker cables are, to achieve
good bass response), electric waves can endure multiple reflections. Think of standing
between two facing mirrors, and seeing multiple reflections of yourself.

RFI can enter the cables directly from nearby power cords or through the amplifier itself if
there is RF contamination of either the input or output connections.

Most speakers are somewhat inductive, and become essentially open-circuits at very high
frequencies. Even electrostatics, with transformer coupling, have stray inductance as part
of the transformers and look to the cable like open-circuits. The RC network at the speaker
terminal offers an alternative place for high frequency noise to be dissipated, rather than
within the cable resistance itself. The Power Wraps are an alternative means to dissipate RF
energy. A combination of a single Power Wrap twelve inches long and two parallel networks
for 16 ohms and 100KHz and 200 KHz corners gives very smooth cello sound with my MA-
1s driving the mid/tweeter inputs of my Maggies through Speltz ZEROs set for 4:1
impedance shift. This is with short Silver Audio Silver Sympony 48 cables.

The 10 ohm, 0.01 mFd combination has a corner frequency of 1.6 MHz. I heard an
improvement when I added the 100 KHz networks to the 200 KHz networks on my setup, so
I'd recommend experimenting with several values of corner frequency. I don't know where
the optimum values are for my own setup, and cannot predict how other setups would
respond. It is cheap to experiment, however.

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