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)?