[net.audio] A Puzzle...

pmr@drufl.UUCP (09/02/83)

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

ark@rabbit.UUCP (09/05/83)

If you are feeding an amplifier stage with high input impedance,
putting a fraction of an ohm in series with the input cannot have
any significant effect.  Therefore, there must be something else going
on which you missed.  Perhaps the contact is intermittent?  Perhaps
there is something else there which you shorted out?  Perhaps there
is some kind of crud on the contact that is making it act as something other than
a simple resistance?  I have heard of dirty switch contacts acting
as diodes, for instance.  What happens when you try cleaning the
switch contact (there are silicone sprays readily available for this)?