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Old 6th August 2015, 08:47 PM   #1
max1979 is offline max1979  Italy
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Default Preamp input selector and common ground(s)

Hi,

I'm building my first preamp based on the well known LDR+DCB1 couple with unbalanced input.
I'm redesign the input selector and a simple (maybe silly) question came into my mind.

90% of the input selector (relays based) I've seen use a single DPDT relay to connect/disconnect one input, both the channels on the same relay. The grounds of the two channels (for all the inputs) are always connected to the common ground, this way we need one relay for each input we want to manage. Is it the best practice?

During the design of my input selector, I decided to use two DPDT relays for each input, one for left and the other for right channel. This way I can really disconnect unselected signal and related ground, closing the door for any unwanted noise. Is it useless? doubling the relays means better noise rejection or is just a waste of components?
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Old 6th August 2015, 08:57 PM   #2
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As a rule, I do not short the unused inputs to ground, and in that case I
don't see how it would matter - you would not be using all the pins on the
DPDT switch.

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Old 6th August 2015, 09:23 PM   #3
max1979 is offline max1979  Italy
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Quote:
Originally Posted by Nelson Pass View Post
As a rule, I do not short the unused inputs to ground, and in that case I
don't see how it would matter - you would not be using all the pins on the
DPDT switch.

So it's correct to use two relays for each input you want to manage, connecting/disconnecting both signal and ground of each channel?
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Old 6th August 2015, 10:13 PM   #4
wayne is offline wayne  United States
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Normally one DPDT relay is used to switch both halves of a balanced signal with the grounds always connected. For single ended you could switch grounds but I don't see why you would want to.
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Old 8th August 2015, 08:37 AM   #5
max1979 is offline max1979  Italy
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Well, I have two doubts.
1) connecting all the grounds of the inputs means my cd reader, my turningtable and my other devices will be force to have a common ground.
2) the noise coming form unselected inputs and related connection cables will always be injected into my preamp

When we talk about input stage of an equipment I don't see any relevant difference between balanced and unbalanced. In both the cases we have to manage two wires bringing the signal and in case of unbalanced connection, the ground is also used as shield.

BUT considering that usually in case of unbalanced connection all the grounds are shortcutted... maybe I'm just making things more complex than needed.
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Old 8th August 2015, 08:44 AM   #6
Zen Mod is offline Zen Mod  Serbia
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don't be silly .

and do whatever way you like .
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