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audio mixing AOp resitor values

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rompelstilchen

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hello everyone

I found this schematic

and I want to understand how to calculate resistors for another OpAmp (TL074C)

I can see that the amp is setup as an inverter (and a non inverter at the same time) but I dont understand the use of R7 and R8 ...

if I consider R5 and R7 as a V divider the - input is up to 0V

so how to calc these for another opamp, what is their use ? is it a question of I/O impedance ?

Multi-channel-audio-mixer-circuit.jpg

thanks
 
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LM3900 is no regular OP, it's a so called "Norton" amplifier. Using it for quality audio applications is a hopeless undertaking (unless you are seeking noise and distortion) and also the shown schematic has some dubious component values. They are possibly drawing faults, as with other circuits.today publications.

Please refer to known working audio circuits.
 

thx for your answer FvM,

- but some components value seem to be recurent among opamp mixer schematics
- there is no web-seal-of-guarantee-to-work afaik
- funny this page says the exact opposite of your post :)

http://www.circuitstoday.com/multi-channel-audio-mixer-using-lm3900

" These amplifiers provide a wide range of voltage inputs and very good response for all audio frequencies. They also provide a large output swing."

Low input biasing current(30nA)

Providing very high open loop gain(70dB)

Low distortion

Good frequency response"


so, here is one I found with tl074, but I would have to use a voltage devider to compensate the fact it needs +- power supply

http://www.next.gr/inside-circuits/4-channel-audio-mixer-with-tl074-l6661.html

- - - Updated - - -

so if the lm3900 is not suited for audio, does anyone know an alternative to these IC for mixing electert mic and line audio ?
 

any alternative to lm3900 ? what do people use to mix audio easily ?
 

what do people use to mix audio easily ?

Multiple AC signals can be merged into a single resistor, assuming you first attenuate them through pot adjustments beforehand (as your post #1 shows).

The final resistor introduces a weighting percentage for each input. Thus you could have 3 incoming signals each 1 VAC, attenuate them all to .33 V, then sum them, you have output at 1 VAC. Amplification is not absolutely necessary.

The concept is easy, however people usually want a mixer to be versatile, therefore it is common to add amplification. Mics need preamps of course.
 

yeah I know about passive mixing,but it does not answer my question, if lm3900 is bad, then what to use ?
 

Hi,

Why not use an (audio) Opamp? There are many mixer (adder) circuits around.

But it's not clear to me where you see the benefit against a passive solution.
Regarding effort, cost, noise, distortion and power requirements you may find no better solution.

Klaus
 

An LM3900 is a very weird quad general purpose opamp. I have never seen one and I have seen thousands of audio circuits that use audio opamps.
A TL071 single, TL072 dual and TL074 quad opamp are designed for audio with low noise, wide frequency response and wide slew rate, very low distortion and very low output impedance (the LM3900 has a high output impedance that is not symmetrical). The TL07x has been used in so many audio products that it is low cost.

There are many newer and better audio opamps today.
 

The circuit you are using is wrong, the none inverting input is virtual ground so the 0.1uF with 1K pot will not pass low frequencies at all.
See this circuit.
Multi-channel-audio-mixer-circuit-2.jpg
 

The circuit you are using is wrong, the none inverting input is virtual ground so the 0.1uF with 1K pot will not pass low frequencies at all.
See this circuit.
The pot is the gain control for the inverting amplifier.
You did not look at the datasheet of the very odd LM3900. It is not an opamp with a normal differential input. You show a normal opamp circuit. Here is an amplifier in its datasheet:
 

Attachments

  • LM3900 amplifier.png
    LM3900 amplifier.png
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to my understanding, using an active IC allows better audio and better i/o impedances
 

Hi,

to my understanding, using an active IC allows better audio

Why better audio?

and better i/o impedances
It depends on the circuit. Driving a 10k resistor is the same as driving a 10k pot.
If you have impedance requirements, you should give us. Our replies can only be as good as your informations.

Klaus
 

... and as good as my understanding of it :)

well the whole mixed signal is going to a ka386 (lm386) but I am not sure about using this ic

so I was talking about general impedance requirements, theory says ampop have max input impedance and min imput impedance(but it's all theoretical)

I am more used to digital electronic

thanks mate
 

An LM386 is a little audio power amplifier, not an opamp. It cannot be made an active mixer but it is fine as a passive mixer when the inputs all have the same resistor values and do not have separate level controls.
If you want an active audio mixer then use a normal audio opamp, not a weird LM3900.
 

I've used the LM3900, but not on an audio circuit.

The LM3900 is a current input (Norton) amplifier. It will amplify the current difference between the (+) and (-) inputs.

The way that R7 and R8 are connected is extremely weird, to say the least.
330 and 470 ohms at that input are way, way too low. For linear operation, the current should be limited to no more than 200 uA.
 

max input impedance and min output, sorry

- - - Updated - - -

no no no no no, I meant the output of the mixer goes to lm386 that is the "power amp"
 

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