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Old 16th July 2009, 05:43 PM   #1
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Default preamp question

i am about to make a preamplifier

there is a question about the input switch :

is there any point when the input selector switch is placed in position A, which is going to be a cd for example to find a way to terminate someway all other unused inputs ???

supposing that you listen music from the cd then your tuner might be also on so signal from the tuner is going to be present on the switch very very close to the cd player

so all the above makes any sense ????

dont want to mess up with any relays

thanks
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Old 16th July 2009, 06:22 PM   #2
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you have the option of
1.) selecting the source hot lead.
2.) selecting both the source hot and cold leads.
3.) selecting 1.) and shorting the unused hot lead to ground.
4.) selecting 2.) and shorting both the hot and cold leads to ground or to themselves.
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Old 16th July 2009, 09:47 PM   #3
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Quote:
Originally posted by AndrewT
you have the option of
1.) selecting the source hot lead.
2.) selecting both the source hot and cold leads.
3.) selecting 1.) and shorting the unused hot lead to ground.
4.) selecting 2.) and shorting both the hot and cold leads to ground or to themselves.
I apologize in advance for these newbie type questions but that is what I am, a newbie...

Which is the best option? I assume 3 and 4 are shorting selector switches while 1 and 2 are non-shorting. I've read information on selector switches talking about poles and decks. How do those relate to the above?
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Old 17th July 2009, 05:51 AM   #4
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Quote:
1.) selecting the source hot lead.
2.) selecting both the source hot and cold leads.
3.) selecting 1.) and shorting the unused hot lead to ground.
4.) selecting 2.) and shorting both the hot and cold leads to ground or to themselves.
Method 2 is the optimal way to prevent crosstalk between sources, even though this crosstalk is normally very small.
A simple test can determine how big the problem is;
Set the selector switch to CD (player On but not playing) and switch On the tuner (tuned in to a good signal) and turn up the volume. Can you hear the signal from the tuner in the background? Same test goes for crosstalk between channels for each source.

However another point is noise. Having several sources connected to your preamp (and only switching the hot leads) the cold leads from these can introduce noise as all sources cold leads still are connected together.

For method 2 and 3. I don't like to shorten hot and cold outputs from any device. I often use high power output circuits, and even though they won't blow up, they do get very hot when output is shorted.

Yes! You can just turn Off the sources not used (takes care of most of the problems), however I think some of you shares the same problems as I have;
experimenting with new circuits often causes the living room setup to look like a local branch of Houston Control
Therefore my setup is made "wife safe". One switch turns on everything (my energi supplier just love me), and my wife is instructed only to touch the input selector and volume knob
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Old 18th July 2009, 12:28 AM   #5
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I built my last (and current) preamp using method 1 with zero noise or crosstalk problems.

I suspect the noise/crosstalk issue is more common with consumer gear with higher Z-ins/Z-outs and, specific to receivers and AV processors, is their higher gain plus their complexity. Their complexity gives more sources for spurious noise and they often have long wire runs, big wire bundles, and their high level of integration.

I think if your preamp design takes into consideration layout, grounding, parts placement, and appropriate system impedances, you won't have any problems.

Enjoy,
Paul
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