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From: pmr@drufl.UUCP
Newsgroups: net.audio
Subject: A Puzzle...
Message-ID: <628@drufl.UUCP>
Date: Fri, 2-Sep-83 14:27:27 EDT
Article-I.D.: drufl.628
Posted: Fri Sep  2 14:27:27 1983
Date-Received: Sun, 4-Sep-83 13:46:42 EDT
Organization: AT&T Information Systems, Denver
Lines: 46

I would like to explain a phenomenon I observed while repairing
a Dynaco tube preamp and then ask the opinions as to what caused
this phenomenon.

While modifying this Dynaco (I won't get into exactly what), I was
checking out the operation of a switch that selected either the tape
deck input or the output of the phono preamp stage as the input to
the volume control and second stage of gain.  A brief schematic is
shown below.

tape in>-|			volume
	 |			control
	 |
|\	 |	      |---------|		|\
| \	 |	      |		\		| \
|  \	 o switch     |		/		|  \
|   \----o<-----o-----|		\<--------------|   \____>output
|   /	 ^      ^		/		|   /
|  /	 |	|		\		|  /
| /	 A	B		|		| /
|/			      -----		|/
			       ---
phono				-		second
stage						stage

While listening to a record with the switch selecting the phono preamp
stage as shown, I arbitrarily shorted out contacts A and B with a screw
driver.  The results was that 1) the phono became audibly louder, and
2) the quality of the sound became better defined (cleaner).  No voltage
measurements were made at the time this phenomenon was observed.

Additional investigation revealed that the out-of-circuit contact
resistance of AB was about 350 milliohms, the volume control pot was
500K, and the tubes used were 12AX7s.  The switch had no measureable
resistance to ground on any contact out-of-circuit.  All inputs and
outputs of each stage AND the tape input are AC coupled.

The question is

"Why did the lower contact resistance make such a big difference?"

I am interrested in hearing all theories.  Thanks in advance.

	Phil Rastocny
	AT&T Information Systems Laboratories
	drufl!pmr