Keyboard mixer

[Buffalo Tom]

I need a keyboard mixer for three keyboards. One mono and two stereo. Here is what I came up with. The switch for input 3 is a dpdt allows input 3 to be mixed togheter with left+right channel 1. The potentiometer is stereo to control both left and right channel. Potentiometer at center position is 0dB and then turning the pot left or right attenuates input 1 or 2 depending which way I turn the pot. Ive only breadboard the left part of the circuit so far and it works satisfying. Waiting for the stereo pot to arrive before I can try the complete circuit. The power supply is +/-15V with a dc/dc converter.

I have a question about the input impedance. I believe my circuit has a 100K input impedance. Is that good for keyboards or should I change anything?

Is there anything else in my schematic that can be improved?

[Steben]

NE5532 is about max 300k input impedance.
I would lower the 100k to 47k or 33k... but that is me.
What are the left and right inputs coming from?

I've seen many keyboards <<10k output impedance. What keyboards are we talking about?

[Buffalo Tom]

NE5532 is about max 300k input impedance.
I would lower the 100k to 47k or 33k... but that is me.
What are the left and right inputs coming from?

I've seen many keyboards <<10k output impedance. What keyboards are we talking about?

MAG C3 Organ (Hammond) and Cecilia Magna (pipe organ) and input 3 is the bass channel on the MAG organ. So lower the 100K resistors to 47K would give me a 47K input impedance instead? Is that more suitable for this mixer? Thanks

[antonis]

NE5532 is about max 300k input impedance.

Is it so..??

300k max input impedance is considered for open-loop differential mode..!!
(the effective resistance between inverting & non-inverting inputs..)

Here we have close-loop common mode configuration, hence very high input impedance due to practically zero voltage difference between inputs..
(negative feedback takes care of it..)
If you want to estimate it, you'll have to do it with bias current specification..

P.S.
Reading datasheets is a good thing..
Understanding them is a better thing..

[Steben]

NE5532 is about max 300k input impedance.

Is it so..??

300k max input impedance is considered for open-loop differential mode..!!
(the effective resistance between inverting & non-inverting inputs..)

Here we have close-loop common mode configuration, hence very high input impedance due to practically zero voltage difference between inputs..
(negative feedback takes care of it..)
If you want to estimate it, you'll have to do it with bias current specification..

P.S.
Reading datasheets is a good thing..
Understanding them is a better thing..

Very well. Yet the choice for another resistor remains. I just wanted to point the impedance is quite high anyway.

Do you still think noise consideration is futile in this one?
Signal out of keyboard can be high I guess but just as much low.

[antonis]

Do you still think noise consideration is futile in this one?

Not at all..
But we firstly have to take into account "balance" between voltage & current noise..

[anotherjim]

Modern keyboards with digital workings can have surprisingly weak line output levels. I suppose it's because they often have 3.3v circuits. But those Diversi organs could actually be an embedded pc with a proper audio interface giving pro balanced line levels, so what does the manual specs page say? I couldn't find any on a quick search.
And yes, most keyboard amps and line mixers are 10k input impedance. You will do no harm going higher than 10k.

[Buffalo Tom]

Thanks for your help. I will keep the 100K input Z for now. Don't know the output impedance for the keyboards and cant find the manual either. I have sent a mail to the manufacturer. Another thing I must add to this mixer is a master volume for mono input (3). Is this a suitable approach for a master volume on channel 3?

Version 2