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Old 3rd October 2009, 02:08 AM   #1
neazoi is offline neazoi  Greece
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Default Passive preamp, multiple inputs?

Hi, I am building a super high quality passive preamp.
For the attenuator I chose this circuit because I cannot find a better one.
http://diyaudioprojects.com/Solid/DI...ve-Attenuator/

I need to find a contact-less way (not any kind of mechanical contacts) to make a preselector, in order to connect more than one input devices to it (CD, MD etc)

The easiest way I thought, is to connect all input RCAs in parallel and chose myself manualy which device will operate (for example, turn the MD off then turn the CD on), without using a preselector at all. (any other ways you could think of?)

But what about the impedance or any other problems this could cause?
When the unwanted devices are switched off, this should not cause any problems, is that right? i.e the preamplifier will "see" in its input only the connected (and switched on) input device and not the connected (but switched off) input devices.

The scenario is to have all of the inputs connected to my machines (CD, MD, etc) permanently, and to switch on the CD, when I want to listen to a CD.

I think the preamplifier will "see" the rest mahines like left unconnected and so there will not be any impedance problems, but I need your advice on it..

Are my thoughts right?
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Old 8th October 2009, 08:55 PM   #2
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Default Relays? Trigger?

Do you mean you do not want mechanical switches? So use relays, I wouldn't even think going the way of connecting all inputs together (even if all but one are switched off). If you want input components to be automatically switched on on preamp, add trigger circuits to relays. But what's wrong with mechanical switching, I am curious.
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het staeler audio
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Old 9th October 2009, 07:14 PM   #3
Javin5 is offline Javin5  Switzerland
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Quote:
Originally Posted by neazoi View Post
The easiest way I thought, is to connect all input RCAs in parallel and chose myself manualy which device will operate (for example, turn the MD off then turn the CD on), without using a preselector at all. (any other ways you could think of?)

But what about the impedance or any other problems this could cause?
When the unwanted devices are switched off, this should not cause any problems, is that right? i.e the preamplifier will "see" in its input only the connected (and switched on) input device and not the connected (but switched off) input devices.

The scenario is to have all of the inputs connected to my machines (CD, MD, etc) permanently, and to switch on the CD, when I want to listen to a CD.

I think the preamplifier will "see" the rest mahines like left unconnected and so there will not be any impedance problems, but I need your advice on it..

Are my thoughts right?
No. The outputs of the switched-off equipment are pretty much undefined, depending on the output circuits used. You really don't know what you have connected in parallel to your active source and how much these switched off devices are loading your active source. Some of these may be highly nonlinear and some may even have relays which shortcircuit the output. Really a horrible idea Go for relais, as lukasfrancl suggested..

Kurt
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Old 14th October 2009, 07:27 AM   #4
osscar is offline osscar  Latvia
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Location: Riga, LV
My yesterday completed passive volume regulator input/output selector.
Stepped attenuator from e-bay, with smd resistors- will be changed probably in future with something better - like Magura`s DIY stepped attenuator.
Hammond black anodized enclosure with little modding .
Black anodized knob.
Two toogle switches: input: CD/RADIO; output: AMP1/AMP2

Click the image to open in full size.
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Old 14th October 2009, 07:42 AM   #5
neazoi is offline neazoi  Greece
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Quote:
Originally Posted by lukasfrancl View Post
Do you mean you do not want mechanical switches? So use relays, I wouldn't even think going the way of connecting all inputs together (even if all but one are switched off). If you want input components to be automatically switched on on preamp, add trigger circuits to relays. But what's wrong with mechanical switching, I am curious.
--
L_u_k_a_s
het staeler audio
The lightspeed attenuator creator says that under certain conditions the contact point may bewave like a contact pin diode, rectifying the AC. I know this seems to be minor though...
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