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JFET based audio source selector
JFET based audio source selector
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Old 23rd November 2010, 11:14 AM   #21
Mooly is offline Mooly  United Kingdom
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JFET based audio source selector
Originally Posted by Obe1 View Post
Hi Mooly,

Please verify the voltage polarities for me. I thought an n-channel JFET would need 0.0 to say plus (+) 0.3 volts to turn on and a minus (-) 6.0 to (-) 8.0 volts to turn off.
Hi Obe1... I was trying to do a million and one things yesterday... yes they are the wrong way about. +0.3 maximum to turn on fully and -8 for off. Sorry my mistake.

As to how effective this arrangement is... well very is the simple answer. The "offness" due to the shunt arrangement is excellent, far better than a simple mechanical switch. I can apply ridiculous input signals such as a squarewave at say 10 volts peak to peak and there is zero audible breakthrough at any frequency even with the main amp volume turned up.
That's a severe test... you think of amps that when you select a spare input how you can if the volume is up full still hear the other sources due to capacitive coupling across relay contacts or switches etc. The FET's never suffer mechanical degradation either.

You have to remember the FET that passes the signal is always sat at zero volts as it's next to the inverting input on the opamp. This makes it very linear in use as essentially no voltage is developed across it.
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