[sdiy] Toggle switch for mic

Steve Lenham steve at bendentech.co.uk
Tue Sep 22 21:28:46 CEST 2020

On 22/09/2020 17:57, ColinMuirDorward wrote:
> Thanks for this tip.
>     except it really really needs to have gold-plated contacts if you want
>     it to be reliable in this sort of application.
>     Using the same general-purpose silver-contact toggle that you'd choose
>     to switch 12V for a motor is something you would almost certainly
>     regret!
> Is this because the current is very low in this application? Curious 
> about why.

Yes, basically. Silver is a great conductor when it is clean, but tends 
to build up an insulating tarnish as time goes by, especially if the 
atmosphere isn't perfect. This doesn't matter if you are switching a bit 
of power, because the slight arcing as the switch makes and breaks burns 
through the tarnish and the process has to start again from scratch. But 
with small signals, there are not enough amps/volts for this to happen, 
so the contact resistance builds and builds until the switch no longer 
makes properly.

Gold doesn't tarnish in this way, so is great for small signals. BUT - 
it's no good for switching power as the same process that burns off the 
tarnish from the silver will also burn off the gold! It's horses for 
courses. Gold-contact toggle switches usually have low ratings like "30V 
0.4VA maximum", while silver-contact ones will be rated at 125/250V and 
1A or more.

Just for the sake of completeness, I learnt recently that there is a 
halfway house in the form of switches with gold-over-silver contacts. 
These can be used for small signals or power, but if you use them for 
power then the thin gold layer burns off and leaves you with just the 
silver. You then can't go back and use the same switch for small 
signals. This might sound daft but it lets you stock just a single 
switch for both purposes and also to use a double-pole switch to 
reliably switch one power signal and one low-level one. I specced this 
type of switch for a mic preamp I recently designed, so that a single 
part could switch both low-level audio and 48V phantom power.


Steve L.
Benden Sound Technology

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