RE: [OT] Circuit Design Advice ...

[Keith Doxey <ukha.diyha@xxxxxxx>]

Hi Stuart,

That will mix the 4 sources down into one input nicely.

Just one thing to be aware of and that is the volume will change depending
on the number of CONNECTED inputs.

With only one input connected the 1K resistor will have negligable effect
on the signal level.

With two inputs connected but only one outputing audio the other input will
effectively be grounded. This will mean the output is taken from the centre
tap of two 1K resistors giving a voltage drop of 50%. In audio terms this
equates to a 3dB drop in level which is only just noticable

With all 4 inputs connected but only 1 active then the equivelent circuit
would be signal in via a 1K resistor output taken from the junction of 1K
resistor and (3x1k in parallel) 330R. This would reduce the voltage to 25%
or the signal which is a 6dB drop in level.

As long as you dont keep unplugging sources there will be no proble with
sound levels because once set they wont change.

This is not a problem with your design, merely a limitation of a PASSIVE
mixer. The main difference between a passive and active mixer is that the
dot where you join everything together is replaced by an op-amp which
ensures that the other inputs dont interact in the way described above.

What you have is a simple cheap way of connecting multiple inputs to one
source but its not really practical to extend it much further.

HTH

Keith


-----Original Message-----
From:	Stuart Grimshaw [SMTP:stuart@xxxxxxx]
Sent:	23 February 2001 16:02
To:	UKHA List
Subject:	[ukha_d] [OT] Circuit Design Advice ...

I've drawn a circuit diagram (my 1st in 8/9 years!!) and I was wondering if

any of the more experienced electronic engineers/hobbyists would mind
haveing
a look some time over the weekend and passing comment?

It's a circuit to mix the input from 4 different stereo source to 1 stereo
output. If it's not clear on the piccy, the resistor networks are 1k.

http://garner.smgsystems.co.uk/4way_passive_mixer.png

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