Audio relay

[harps]

hello : )

wondering if anyone has experience with audio and relays?
i am hoping to route audio via a mechanical relay ( DPDT) to switch between two inputs,routing one or the other input to a single out.

The audio is balanced ( hot cold + earth) i have two ideas in the pics below.

i would like to use just one relay to save money and space on the board.

I wonder if i need a resistor to terminate the input not in use to stop interference passing down the audio cables? ( perhaps i need to use two relays? as in the second picture? i hope this makes sense.

any help would be great. : )

 

[MikeMl]

What is the audio level? What are the audio sources? What is the output connected to?

 

[harps]

hi : )

thanks for your reply.

The two signals connected to in1 and in2 will both be either line or mic ( but never a miss-match) always a pair of the same type either line or a pair of mic level signals . the output of the relay will be going to a mixer with selectable mic/line inputs.

in one instance it could be 2 x mono balanced audio line level signals from two soundcards.
and the relay could switch between the two devices when 5v is applied,with the out routed to a mixer line input.

 

[MikeMl]

At line level, the single relay will work fine. Using electret (amplified) mics, it should work fine. Sorry to burst your bubble, but the inputs to computer sound cards are single ended, meaning that the sleeve of the input jack is just tied to the computer ground, and this is an unbalanced input; there is no need to switch both sides of the audio line.

I have built interfaces to Computer Soundcards, and the biggest issue you are likely to encounter are ground-loops between your external equipment and the computer ground (the switching power supply inside the computer is the culprit). I had to use audio isolation transformers between the external equipment and the computer both in the Line-IN and Line-OUT connections to get rid of the ground loops. Hams make extensive use of Sound card INPUT/OUTPUTS to interface their HF radios for various digital communications, and audio-coupling transformers are essential. Google "RigBlaster" for an example of a commercial product which addresses this issue. Using an isolation transformer makes for a balanced input, where neither side of the line is grounded.

 

[harps]

hi, i think you see my diagram the opposite way round. my faultm i should have been more clear and show the direction the signals are routed maybe with arrows.

i'm routing/switching 2 signals to one output.

2 x input going into the relay and 1 x ouput from the relay goes to an external mixer

sorry about the confusion

all signals are balanced line level (hot cold and earth) there is no unbalanced gear in use.

: )

 

[MikeMl]

...
all signals are balanced line level (hot cold and earth) there is no unbalanced gear in use.

Not if the signals are coming out a Soundcard!

Not unless the Mixer has truly differential +- inputs!

 

[harps]

my sound card has balanced outs and my mixer has balanced ins.

 

[Sceadwian]

Mike, they do make sound cards that have balanced outputs.

Is the relay noise going to be acceptable? There's no way you'd want to switch the inputs if you were actively outputting sound to a PA system, the contact bouncing could easily blow speakers. If this is just for simple multiplexing of a signal and you're not worried about live use then it should work fine, but you'll always hear clicks and pops when the relay switches.