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Old 19th May 2015, 09:21 PM   #1
RubenV is offline RubenV  Netherlands
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Join Date: May 2015
Default Building a decent input switcher

Hi,

Thanks for reading! I want to make an input selector since my active speakers have only one input and using my surround processor degrades the sound of my dac (which has a built in preamp). The dac direct connected to the speakers sounds a lot better but switching cables all the time is not convenient for me.

The only requirements are: 2 inputs, one output with the best soundquality possible.

I looked at commercial available equipment like the dodocus (but it degrades the sound) or manley skipjack (too expensive and to many options I don't use).

I do want to keep things rather simple and have something like this in mind:
Click the image to open in full size.

Since I want the input selector to be as transparant as possible I am looking for recommendations of parts I should use.

Does something like a DACT CT3 degrade the sound? Should I use another kind of switch? What kind of bindingposts and internal cables are recommended?
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Old 20th May 2015, 07:47 AM   #2
AndrewT is offline AndrewT  Scotland
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you will need to shield that box to prevent interference affecting all the open loops.

All the seams in the shielded box will need to be in electrical contact, otherwise the interference still leaks through.
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Old 20th May 2015, 09:26 AM   #3
DF96 is offline DF96  England
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I can't see any signal ground connections in that picture. Maybe it uses the chassis as the ground? If so, not the best way to do it. Ground should follow the signal. However, for many purposes you can get away with doing it 'wrong'.
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Old 20th May 2015, 08:52 PM   #4
RubenV is offline RubenV  Netherlands
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Quote:
Originally Posted by AndrewT View Post
you will need to shield that box to prevent interference affecting all the open loops.

All the seams in the shielded box will need to be in electrical contact, otherwise the interference still leaks through.
Thanks I will take this in to account when I start building it. I can welt the side together and use screws for the top or something like that.


Quote:
Originally Posted by DF96 View Post
I can't see any signal ground connections in that picture. Maybe it uses the chassis as the ground? If so, not the best way to do it. Ground should follow the signal. However, for many purposes you can get away with doing it 'wrong'.
How does this grounding works? Can it be done inside, like a grounding wire to the chassis? Or does the chassis as a whole needs grounding?

Any thoughts on the internal wiring (shielding perhaps or a different layout) or the switch itself?
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Old 20th May 2015, 09:01 PM   #5
rayma is offline rayma  United States
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Quote:
Originally Posted by RubenV View Post
I want to make an input selector Since I want the input selector to be as transparant as possible
I am looking for recommendations of parts I should use.
http://glass-ware.stores.yahoo.net/atandsise.html

Last edited by rayma; 20th May 2015 at 09:03 PM.
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Old 21st May 2015, 05:30 AM   #6
AllenB is online now AllenB  Australia
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Quote:
Originally Posted by RubenV View Post
I can welt the side together and use screws for the top or something like that.
Multiple screws through overlapping joints using shakeproof washers that make contact through the paint should work.
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Old 21st May 2015, 09:47 AM   #7
DF96 is offline DF96  England
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Use coax or twisted pair for the wiring, so the ground follows the signal. The sockets should be isolated from the box. The grounds should be connected together near the switch. In addition, there should be a connection from here to the metal box.
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Old 21st May 2015, 10:23 AM   #8
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by AndrewT View Post
..................to prevent interference affecting all the open loops..............
Quote:
Originally Posted by RubenV View Post
...........................Any thoughts on the internal wiring (shielding perhaps or a different layout) or the switch itself?
Quote:
Originally Posted by DF96 View Post
Use coax or twisted pair for the wiring, so the ground follows the signal. .................
Close coupled Flow and Return of the signal wires is very important. Open loops must be avoided.
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Old 25th May 2015, 11:17 AM   #9
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'
If I understand the requirement, it seems a single DPDT (double pole, double throw) switch is all that's needed for switching.

Apparently two low level, low impedance audio signals are to be switched. Essential these are line level signals, not especially susceptible to RF (radio frequency) or other interference. In a living room environment the circuit below could probably be assembled in a plastic box.

Of course a metal enclosure is always better, in which case the enclosure should be grounded as shown. Note the single-point grounding near the output jack.

Additional notes:

1. Mounting nuts should not be relied on for a ground connection. These nuts can loosen slightly over time, causing static, hum ("ground loops"), and other problems. Instead run separate ground conductors as shown. Insulated jacks could be used, as was already suggested.

2. In this application conductors should not be twisted into "twisted pairs." This is because there are no feed-and-return pairs, the left and right channels are different audio signals. Simply keeping conductors as short as possible is sufficient.

3. Shielded stereo patch cables are used for connecting devices. "Patch cables" by definition have a male connector on each end. These are available on eBay and elsewhere in a variety of lengths.
.
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Last edited by bentsnake; 25th May 2015 at 11:23 AM.
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Old 25th May 2015, 01:25 PM   #10
M Gregg is offline M Gregg  England
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There is more to it than this,

It depends how far you want to go, there is not enough current to maintain a good connection across a switch and you might find reliability issues.
The contacts can get dirty and lose contact..

You talked about transparency and then change to a simple switch..

As DF96 pointed out the grounding is as important as the signal live. The signal travels down both lines..

However if you just want a quick change over then go with the switch..
You forgot what you are talking about is basically part of a passive pre-amp ie the selector without the volume pot..

People argue about SS switching and relay switching and then the contact type and metal contacts..

Here is an example of relay input..However you can just go with a switch with silver contacts..its up to you..

Chassis ground and signal ground should be separate and not part of the signal path..
Phono sockets should be isolated from the chassis and the ground for the chassis connected to the output..why because CD players and ipod etc don't have a ground..some people switch both ground and signal..however it depends how far you want to go!

So a passive selector can be passive but powered..relay or SS..or you can just go selector<<as long as you are happy with it!
As simple or complicated as you want..

That's without saying things like whats the point of expensive cables then using different cable inside a selector?
So you see its where do you stop..

Regards
M. Gregg
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File Type: jpg Selec BP.jpg (20.0 KB, 175 views)
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Last edited by M Gregg; 25th May 2015 at 01:46 PM.
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