Switch both sides of input connections?

[JazzGuitarGimp]

Hi All,

I am contemplating an integrated preamp build - so much so that I've started flushing out a schematic. When I got to the input select switch (this will be a five-position rotary switch), it occurred to me that if I simply switch the signal side of the five sets of RCA input jacks, the shield connections to each of the five connected sources would be always-connected to preamp ground. If there happen to be two or more audio sources connected with 3-prong AC cords, this approach will most certainly cause ground loops, and possibly hum.

Is it common practice to switch both sides of the RCA input jacks? This would require a four-deck, five-position rotary switch for both left and right channels.

Many thanks,
Lou

 

[rayma]

Hi All,

I am contemplating an integrated preamp build - so much so that I've started flushing out a schematic. When I got to the input select switch (this will be a five-position rotary switch), it occurred to me that if I simply switch the signal side of the five sets of RCA input jacks, the shield connections to each of the five connected sources would be always-connected to preamp ground. If there happen to be two or more audio sources connected with 3-prong AC cords, this approach will most certainly cause ground loops, and possibly hum.

Is it common practice to switch both sides of the RCA input jacks? This would require a four-deck, five-position rotary switch for both left and right channels.

Many thanks,
Lou

It's not common, but is still a good idea, and is simple with relays. Some use a single 10 Ohm resistor per channel
from all the input RCA shells (except phono) to ground to reduce ground loop noise. Some even do this with the power amplifier inputs.

 

[JazzGuitarGimp]

Thanks, rayma. It looks like a four-deck, five-position rotary switch would be pushing a hundred bucks. I image I can buy five 2-form-A reed relays for a bunch less than that. Not to mention I can also have panel LED's to indicate which input is selected for just the cost of the LED's, whereas, I'd need a sixth deck on the rotary switch to do it without relays. That would push the already-exorbitant rotary switch up to the next price tier.

Thank you for your advice!
Lou

 

[SRMcGee]

JazzGuitarGimp:

I'm not well-versed in this, but why would you need so many decks? I would think a two deck switch would be adequate -- one for the LEDs and one for all of the relays. In other words, why couldn't you connect two DPDT relays to each position on one deck of the switch? A DPDT relay could handle both signal and ground for a single channel, so two would be needed per source.

Just wonderin',
Scott

 

[JazzGuitarGimp]

Hi Scott,

First, I apologize as I misspoke. I should have said I would need a fifth deck, not a sixth deck - and this assumes I would be doing this without relays AND using LED's to indicate which input is selected. So deck 1 for left signal, deck 2 for left ground, deck 3 for LED's, deck 4 for right ground, and deck 5 for right signal.

However, a SPDT relay could be used to switch each input (both signal and ground) to the input of the line-stage amp. And assuming I would be running the relay coils on DC, and assuming I am not missing something, I don't see any reason why I could not use the same deck to switch the LED's, just by wiring each LED and its series current-limit resistor in parallel with the relay coil (as well as a back-EMF diode). So, just one deck and five relays to do it all.

Cheers,
Lou

 

[AndrewT]

Using relays allows a one deck switch to be used for all the signals and grounds and for indicator leds