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Old 17th January 2012, 02:38 PM   #21
nsx51 is offline nsx51  United States
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Thanks Mooly. I can see the advantage of having the switch out of the signal path. Is there any disadvantage to simply unplugging the headphones when I don't want to use them instead of having another switch to ground them? Please excuse my ignorance, but are the resistors there only for when the signal is shorted, or do they serve a purpose while listening also? In other words, if unplugging the headphones is acceptable, do I still need the resistor while listening?
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Old 17th January 2012, 06:01 PM   #22
Mooly is offline Mooly  United Kingdom
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There is no problem plugging/unplugging apart from wear and tear on sockets etc.

The resistors work like this...

when listening the signal passes through the resistor. It is in series with the audio signal.

The only downside is that it raises the output impedance of the preamp (as seen by the power amp) by adding the value of the resistor chosen to the quoted output impedance of the preamp.

Does that matter ?... I would say the effect should be absolutely minimal for any normal well designed power amp if the resistor is reasonably low in value. It's easy to prove that to yourself. Just take an old interconnect that you can use between pre and power amp and cut it in half. Remake the ground continuity and then just add two resistors (one for each channel) in series with the signal conductor. 1K, 4K7, 10K try them all and see if you can detect any difference.

That's half of what they do...

The other thing is this. When the switch is in the "shorted position" they isolate the preamp output from seeing a dead short... which would remove the signal for everything So with the switch shorted the resistor appears as "load" across the preamp output.

So we have a compromise. The lower the better for the signal to pass through. And the higher the better when it comes to loading the preamp. So what we choose depends on the driving ability of the preamp. Most preamp stages, even if just opamp will drive 600 ohms fully.
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Old 17th January 2012, 07:51 PM   #23
nsx51 is offline nsx51  United States
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Thanks for all the information. Now to build it. The switch you sent a link to, and all other pushbutton switches I've found, are made for pc board mounting. I obviously don't have a pc board. What are my options?
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Old 17th January 2012, 08:33 PM   #24
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Mooly, you da man! I'm all going super cheap on it with compromises all over. I like your design. I agree with the added resistance not likely to be an issue. Very few power amps have input resistance below tens of thousands of Ohms, so this looks great.
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Old 17th January 2012, 10:34 PM   #25
nsx51 is offline nsx51  United States
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My Magnum Dynalab tuner has toggle switches on the front panel, and they're nice. Won't need a pc board, but I'm open to suggestions.
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Old 18th January 2012, 06:58 AM   #26
Mooly is offline Mooly  United Kingdom
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How about a single rotary switch ?

Something like,
LORLIN|CK1031|SWITCH, 3POLE, 4 POS, METRIC | Farnell United Kingdom

Switches | Farnell United Kingdom | Results

You need at least two pole and 3 way. All these Lorlin switches have adjustable stops so a 2 pole 6 way or a 4 pole 3way etc are all good to use.
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