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Old 22nd November 2010, 02:00 PM   #1
Obe1 is offline Obe1  United States
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Default JFET based audio source selector

A gentleman and frequent contributor to this forum has kindly granted me permission to discuss a circuit he developed that uses JFETs to select/switch between audio sources.

The circuit (seen below) only shows one audio source, but the concept can be extended to multiple audio sources by replicating the portion of the circuit within the box marked "Repeat as required".

Using this concept with multiple input sources creates a virtual earth mixer, yet I don't think it was the really intended to be a mixer, but rather just a source selector.

My question is, what would happen if more than one source was selected? Would their be smoke or just the risk of some odd sounding music as the result of the mixing?

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Old 22nd November 2010, 02:13 PM   #2
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No smoke, I think.

This is a complicated circuit with 2 control voltages and an inevitable nonlinearity in the 'ON' jfet, however small. Plus there'll be some added noise. It's not really a good circuit. I'd much rather see a relay used. Primitive and electromechanical as they may be, they do have their advantages.

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Old 22nd November 2010, 02:17 PM   #3
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Agreed. If relays aren't an option then I'd be happier with a 4052 CMOS logic IC handling 4 2-channel inputs than just plain JFETs.
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Old 22nd November 2010, 04:07 PM   #4
Mooly is online now Mooly  United Kingdom
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Quote:
Originally Posted by Obe1 View Post
A gentleman and frequent contributor to this forum has kindly granted me permission to discuss a circuit he developed that uses JFETs to select/switch between audio sources.

The circuit (seen below) only shows one audio source, but the concept can be extended to multiple audio sources by replicating the portion of the circuit within the box marked "Repeat as required".

Using this concept with multiple input sources creates a virtual earth mixer, yet I don't think it was the really intended to be a mixer, but rather just a source selector.

My question is, what would happen if more than one source was selected? Would their be smoke or just the risk of some odd sounding music as the result of the mixing?

Click the image to open in full size.
Easy one first... there would be no smoke if more than one input were selected. The logic I use doesn't allow that though, but if it did it would just act as a mixer.
You might like to read more on it here starting at post #184 and particularly post 201 which explains it a little more and covers the main reasons why I used this method.

My MOSFET amplifier designed for music.
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Old 22nd November 2010, 04:15 PM   #5
Obe1 is offline Obe1  United States
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Thanks Mooly!

I will defiantly read that thread you suggested.

Bye for now,

Obe1
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Old 22nd November 2010, 04:17 PM   #6
Mooly is online now Mooly  United Kingdom
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Your welcome
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Old 22nd November 2010, 04:21 PM   #7
Mooly is online now Mooly  United Kingdom
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If I were doing it again I would this for the PIC controller I think,
FPRC5RX - DIY learning IR decoder

2nd time today I posted that link... it's a brilliant bit of programming and does exactly what the designer says. My PIC skills are pretty much zero I'm afraid but I programmed a PIC using the code in the above link and it's brilliant.
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Old 22nd November 2010, 06:01 PM   #8
Obe1 is offline Obe1  United States
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Hi Mooly,

That PIC controller looks very cool, but I was going for something much more simple. Just a DPDT toggle switch to feed the required voltages to the gates of each pair of JFETs.
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Old 22nd November 2010, 06:12 PM   #9
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Using switches is easy. Just thinking out loud so if I make a daft mistake or comment lol...

I take it your using a split -/+ supply so set up two rails from the main opamp supply of say 8 volts and if possible another of around -0.3 volts. The 8 volt can come from a zener... the current needed is essentially zero and the -0.3 from a germanium diode (as I used) although I can't see why just resistive dividers couldn't be used if the supplies to the opamp are stable. Tie the gates to the -0.3v via say 2.2meg resistors... that put's both FET's off. Then use the switch to overide that by feeding the 8 volts via say 220k
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Last edited by Mooly; 22nd November 2010 at 06:18 PM.
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Old 22nd November 2010, 06:23 PM   #10
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Something like this,
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File Type: jpg FET Switching.jpg (12.2 KB, 426 views)
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