Simple audio mixer using FET

[ectoplasm]

I want to build a simple audio mixer (adder) (from stereo line inputs)
of good quality and low energy consumption. I found this simple
design:

http://www.aaroncake.net/circuits/mixer1.asp

and I also found this one that also uses a transistor:

http://www-fa.upc.es/websfa/lab/111/audio/audmixer.txt

Q1: would the simple design work well, or is it just a theoretical
example? Otherwise, why would the second design add a transistor? (Q2)

The second design's webpage says:
"If two of these circuits are made in the same enclosure for
stereo, then there can be a single power supply to run both of them.
There should be a resistor in series with the incoming 9V+ lead so as
to minimize crosstalk, and reduce the PLOP when it's powered on."

Q3: How big should this resistor be (Ohm)?
Q4: So each of the two stereo's mixers circuits should have their own
resistor in series with / connecting to the battery?

Q5: The second design uses a lot more power because the transistor
draws a current while it's not used. Battery power wouldn't be
feasible, right?

 

[Eeyore]

I want to build a simple audio mixer (adder) (from stereo line inputs)
of good quality and low energy consumption. I found this simple
design:

http://www.aaroncake.net/circuits/mixer1.asp

and I also found this one that also uses a transistor:

http://www-fa.upc.es/websfa/lab/111/audio/audmixer.txt

They're both fairly rubbish. They remind me of the stuf you could buy about 40
years ago.

Q1: would the simple design work well, or is it just a theoretical
example?

It'll work.

Otherwise, why would the second design add a transistor? (Q2)

The second one has a lower impedance emitter follower output.

The second design's webpage says:
"If two of these circuits are made in the same enclosure for
stereo, then there can be a single power supply to run both of them.
There should be a resistor in series with the incoming 9V+ lead so as
to minimize crosstalk, and reduce the PLOP when it's powered on."

Q3: How big should this resistor be (Ohm)?

A couple of hundred ohms maybe.

Q4: So each of the two stereo's mixers circuits should have their own
resistor in series with / connecting to the battery?

In series with the positive yes.

Q5: The second design uses a lot more power because the transistor
draws a current while it's not used. Battery power wouldn't be
feasible, right?

Both circuits draw power with no signal.

Graham

 

[ectoplasm]

They're both fairly rubbish. They remind me of the stuf you could buy about 40

years ago.

You sound critical.

So, how about a circuit using opamps? I found these:

design 1:
http://www.mitedu.freeserve.co.uk/Circuits/Audio/6ipmix.htm
(ignore those mic input stages)

design 2:
http://europa.spaceports.com/~fishbake/mixer/mixr.jpg

Which of these designs do you think is better? Or what else do you
swear by?
Remember, it's just to mix some line inputs, e.g. computer and DVD
player, TV, etc...

Thanks,
E.

 

[Eeyore]

You sound critical.

You bet.

So, how about a circuit using opamps? I found these:

design 1:
http://www.mitedu.freeserve.co.uk/Circuits/Audio/6ipmix.htm
(ignore those mic input stages)

The 'designer' reckons those are acceptable mic amps !!! ?

The remainder is something of an improvement though. It could easily be made to run
from a single supply too.

design 2:
http://europa.spaceports.com/~fishbake/mixer/mixr.jpg

Jesus Christ ! It seems like any idiot believes he can design an audio mixer without
even knowing the basics of how to use an op-amp !

Which of these designs do you think is better? Or what else do you
swear by?

I design my own ! Well, actually they tend to go into volume manufacture.

Remember, it's just to mix some line inputs, e.g. computer and DVD
player, TV, etc...

You might as well make one that's worth using though !

Can you get the binary groups ? As you seem to be using google maybe not ? I'll
sketch something for you.

Graham

 

[ectoplasm]

Can you get the binary groups ? As you seem to be using google maybe not ? I'll

sketch something for you.

Wow, you would? That would be great.

I'm not in binary groups, but could you please send (cc) it to my e-
mail address?

Thanks,
e_c_t_o

(my e-mail address, click to see it in my profile or use above written
name, including the underscores, <at> hotmail.com)

 

[Jan Panteltje]

I want to build a simple audio mixer (adder) (from stereo line inputs)
of good quality and low energy consumption. I found this simple
design:

http://www.aaroncake.net/circuits/mixer1.asp

and I also found this one that also uses a transistor:

http://www-fa.upc.es/websfa/lab/111/audio/audmixer.txt

Q1: would the simple design work well, or is it just a theoretical
example? Otherwise, why would the second design add a transistor? (Q2)

Those are not very good mixers.

Perhaps it would be simpler if you just used an opamp inverting input,
use symmetric supply to make life easier.
Advantages:
1) good linearity (opamp)
2) good noise (opamp)
3) few caps (freq response cost)
4) if quad opamp more available for tone control etc.

R1 should be much higher in value then the pots, or use buffers (balanced input).


------R2----
pots | |
[]<--R1 ------------ - |
[]<--R1 ---| opamp ------------ out
[]<--R1 ---| - +
[]<--R1 ---| |
|
///

 

[Jan Panteltje]

You sound critical.

So, how about a circuit using opamps? I found these:

design 1:
http://www.mitedu.freeserve.co.uk/Circuits/Audio/6ipmix.htm
(ignore those mic input stages)

Yea, but the 1u caps before the pots and the 10u cap in the output is not needed
Only the input caps are needed (in case connected to something with DC).

 

[Tim Williams]

What's wrong with two resistors? Zero power consumption!

Tim

 

[ectoplasm]

What's wrong with two resistors? Zero power consumption!

You mean a passive mixer, right?

I am doing it now... it's a solution. Input signals get a bit weaker
in the mixed signal, but it's ok for me.