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From: sjc@mips.UUCP (Steve Correll)
Newsgroups: rec.audio,sci.electronics
Subject: Re: INTERCONNECTS
Message-ID: <1000@mips.UUCP>
Date: Fri, 24-Apr-87 13:20:26 EST
Article-I.D.: mips.1000
Posted: Fri Apr 24 13:20:26 1987
Date-Received: Sun, 26-Apr-87 00:33:01 EST
References: <18511@ucbvax.BERKELEY.EDU>
Lines: 48
Xref: mnetor rec.audio:1397 sci.electronics:604

In article <18511@ucbvax.BERKELEY.EDU>, bks@oz.berkeley.edu (Brian K. Shiratsuki) writes:
> ...i also replaced the cable which connects my record player to the
> preamp.  the difference which resulted from this change is quite obvious;
> it now seems that the original cable was a low-pass filter...
> 
> i would assume that standard measurements (resistance, impedance) between
> regular and fancier interconnects do not differ...

Some magnetic cartridges are fairly sensitive to load capacitance, and
cable capacitance does vary. (So does preamp input-stage capacitance.
A few years ago, the audio press noticed--apparently for the first
time--that you could make a cartridge's response vary by several dB at
15kHz by switching cables and preamps, and for a while it became
fashionable for preamps to provide user-adjustable input capacitance.
Rather than establish a standard, the audio industry has decided to
forget the problem and focus on CD dampers, turntable mats, litz-wound
speaker cables, etc. Excuse me; I editorialize.)

Amplifier and preamp designers are supposed to provide sufficiently low
source (output) impedance to tolerate variation in load capacitance. (The
EIA standard for testing audio preamps requires you to load the outputs
with capacitance to simulate cables.) Thus, changing cables at the
output of a good preamp seems unlikely to make an audible difference.

Cartridge designers have no such obligation.

> Using a function generator, a 'scope, and an HP gain-phase meter (shows
> amplitude and phase differences between the two inputs), I connected the
> function generator to one input of the HP, and the cable under test between
> both inputs.  I also varied the terminating resistance...
> The results?  NO MEASURABLE DIFFERENCES FROM DC TO 250K Hz...I didn't
> vary the output impedance of the source.  Anyone have any suggestions
> for a more thorough test?

Your experiment demonstrates nicely why the cable at the output of a
preamp doesn't matter.  What you're dealing with is a voltage
divider: the source impedance is on top and the load impedance (in
parallel with the cable capacitance) is on the bottom.  If the source
impedance is close to zero, as it is for a function generator (or audio
preamp), the value of the load capacitance has very little effect on
the voltage across the load.

If you repeat the experiment with a higher (and not purely resistive) source
impedance, perhaps by using a real phonograph cartridge playing a test record,
you may well see differences among cables.

-- 
...decwrl!mips!sjc						Steve Correll