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-   -   JFET based audio source selector (http://www.diyaudio.com/forums/analog-line-level/177691-jfet-based-audio-source-selector.html)

Obe1 22nd November 2010 02:00 PM

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?

http://www.diyaudio.com/forums/attac...witch-copy.jpg

wakibaki 22nd November 2010 02:13 PM

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.

w

Slipstreem 22nd November 2010 02:17 PM

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.

Mooly 22nd November 2010 04:07 PM

Quote:

Originally Posted by Obe1 (Post 2373510)
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?

http://www.diyaudio.com/forums/attac...witch-copy.jpg

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.

http://www.diyaudio.com/forums/solid...ml#post1560313

Obe1 22nd November 2010 04:15 PM

Thanks Mooly!

I will defiantly read that thread you suggested.

Bye for now,

Obe1

Mooly 22nd November 2010 04:17 PM

Your welcome :)

Mooly 22nd November 2010 04:21 PM

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.

Obe1 22nd November 2010 06:01 PM

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.

Mooly 22nd November 2010 06:12 PM

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

Mooly 22nd November 2010 06:23 PM

1 Attachment(s)
Something like this,


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