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Old 9th December 2012, 11:03 AM   #1
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Red face Request for Guides/Explanation Regarding Audio Mixer Circuits

Hi all,

I am very new to this forum as well as DIY electronic circuits.
I am not an electrical engineering student nor I have sufficient background (I guess ).

I have a few questions regarding the building of an audio mixer.
Basically, I am interested to make an audio mixer to combine two audio sources which is from my laptop and my phone into one input to my multimedia speakers.

After hours of Googling around, I found various different schematics which I admit I find it hard to figure out what the components do.
So, it would be good if someone experience enough could explain them to me.

For instance, this schematic below:

Click the image to open in full size.
(Op-amp Summing Amplifier (Voltage Adder))

May I know what are the uses of the capacitors?
May I assume the capacitors before the amplifier have similar functions and their functions are different from the one after the amplifier?

I do understand that values of R1, R2, R3 and Rf influences the gain of the circuit.
Besides that, the output of the circuit is inverted does it matter if the output is not inverted back?

Furthermore, if I am interested in only unity gain (say R1=R2=R3=Rf), do I still require the amplifier for the mixer to work?

Lastly, are there any websites/readings regarding audio sources and signals. I believe with more understandings regarding audio signals I will be able to understand and design circuits more easily.

Thank you (for going through such long post of mine )!
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Old 9th December 2012, 11:21 AM   #2
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The capacitors stop any DC voltages that might be present. It's normal for all audio amplifiers not to amplify DC signals... it's something we don't want.

So the ones after the pots prevent DC from any source components entering the amplifier/mixer. They also serve another important function. Any DC that is present on a variable resistor can make the operation of the control noisy as it is turned if that current passes through the wiper of the control

The cap after the opamp stops any slight DC voltage (what we call an offset) from being applied to the next stage that the amplifier couples to.

The opamp is a "virtual earth" amplifier. This means that the point at which the signals are "summed" (the - input) always appears to be a zero point with no signal present.

You need to read and understand how an inverting opamp circuit works to really understand how the mixer works.

The mixer (by virtue of bringing the summing point to zero) ensure that there is no interaction between the inputs. If you omitted the opamp then the mixing would be at the mercy of the individual source components properties such as their output impedance.

That the circuit inverts is not a disadvantage although there is thought that absolute phase is important. Try it yourself. If you swap the speaker polarity of both speakers in a stereo setup you have inverted the phase exactly the same as this circuit does. Can you hear any difference ?
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Old 9th December 2012, 11:30 AM   #3
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Hi, I sincerely appreciate your swift reply!
It did help me clear most of the doubts especially regarding the caps.

Yup, I have studied op-amp circuits (basics) in school and university.
Therefore, I understand how the basic op-amp circuit work (those without capacitors and such).

So, are there any recommended amps to be used in my case here?
How do different amps affect the circuit here?
In addition, are there any recommended capacitance for the capacitors as well as the resistance for the resistors?
Similarly, how do these affect the circuit?

As far as I know, if I use the same resistors across R1, R2, R3 and Rf I will get a unity gain circuit. So, can I use any resistors (like 1k ohm)?

Thanks again!
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Old 9th December 2012, 12:11 PM   #4
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Opamps first. You need to begin with something that you know will work without running into stability issues.

For learning the ancient 741 takes some beating and will still turn in a "half decent" audio performance. Also consider the TL071 or TL081 FET opamps. All three are compatable and can be swapped and changed as wished.

You do realise that the circuit as drawn runs a "split" supply consisting of a positive and negative rail. For example two 9 volt batteries with a centre tap that becomes "ground".

The value of the caps affect the low frequency response. If they are to small in value then the bass frequecncies are attenuated. Each cap after the pot works in conjunction with the following resistor to form a "first order" high pass filter.

calculators for electronic circuit design

Although the resistors can theoretically be any value in practice the lower limit is determined by the minimum load the opamp can drive. So I would keep Rf at say 10K and scale the other to what you need. Go too high such as 1Meg for Rf and problems such as DC offset (for the 741 type opamp) and problems with noise and poor high frequency response appear. With high values, stray capacitance becomes a factor. So stick to around 10K give or take.
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Old 9th December 2012, 12:15 PM   #5
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hey guys im new with this forum. i need a circuit diagram for my konzert k10 amp. some of its components value were erased due to hard blow on short circuits. tnx in advance
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Old 9th December 2012, 12:20 PM   #6
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Quote:
Originally Posted by konzer10k View Post
hey guys im new with this forum. i need a circuit diagram for my konzert k10 amp. some of its components value were erased due to hard blow on short circuits. tnx in advance
I would suggest you start a new thread in the appropriate forum
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Old 9th December 2012, 12:27 PM   #7
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okay thank you.
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Old 9th December 2012, 12:32 PM   #8
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Quote:
Originally Posted by Mooly View Post
Opamps first. You need to begin with something that you know will work without running into stability issues.

For learning the ancient 741 takes some beating and will still turn in a "half decent" audio performance. Also consider the TL071 or TL081 FET opamps. All three are compatable and can be swapped and changed as wished.

You do realise that the circuit as drawn runs a "split" supply consisting of a positive and negative rail. For example two 9 volt batteries with a centre tap that becomes "ground".

The value of the caps affect the low frequency response. If they are to small in value then the bass frequecncies are attenuated. Each cap after the pot works in conjunction with the following resistor to form a "first order" high pass filter.

calculators for electronic circuit design

Although the resistors can theoretically be any value in practice the lower limit is determined by the minimum load the opamp can drive. So I would keep Rf at say 10K and scale the other to what you need. Go too high such as 1Meg for Rf and problems such as DC offset (for the 741 type opamp) and problems with noise and poor high frequency response appear. With high values, stray capacitance becomes a factor. So stick to around 10K give or take.
Do you mind explaining more on the "split" supply part?
Do you mean the potentiometer there?

For the low frequency response, is it safe to say that:
1. First, I have to find out what resistance am I going to use (say around 10k)
2. Then, I use the calculator to calculate the lowest frequency which the circuit is needed to handle? Say 40Hz which is the lowest my speakers will respond to.

Last edited by 2ndIdentity; 9th December 2012 at 12:37 PM.
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Old 9th December 2012, 01:08 PM   #9
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The split power refers to the opamp power supply. For that circuit to work (and it is the normal and correct method), it needs the opamp (assuming the device numbers mentioned) to have a positive supply on pin 7 with respect to "ground" (thats your zero volt line) and a minus supply on pin 4 with respect to ground.

To power the circuit from a single rail, for example from one battery, would require additional components to derive a "mid point" reference level.

Its a subtle thing to grasp. On a single supply the opamp output would have to be biased to half the supply voltage in oder that the output could go "equally high" and "equally low". ON a single 9 volt battery that means the opamp output would be arranged to sit midway at 4.5 volts and then it could go toward 9 volts or down toward zero volts.

The split supply is a recognised and standard way of powering opamps but for battery use single rail operation can be more convenient.

If your speakers go to 40 Hz then you need the reponse to go lower than that by some margin in order that the 40Hz level is un-attenuated. A typical figure would be around 5 Hz.
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Old 9th December 2012, 01:08 PM   #10
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If you look at op amp datasheets (like on datasheetcatalog.com) you will find they have a drawing that shows how close you can get the input to the supply voltage before they stop working. Plotted against the supply voltage, usually. This means the ground symbol in your drawing post one, is 1/2 way in between the voltages you put on pins 4 (-v) and 8 (+v) of your DIP package op amp.
So the split supply doesn't use a lot of current in the middle, the ground place. Rather than monkey with replacing 9v batteries all the time (at $8 the pair, starting last week), I found a way with two 5 Watt 8v zener diodes and a couple of resistors to turn a DC wall transformer (18 VDC) into a split supply. With only one op amp you could probably get away with cheaper 3 W zener diodes. Schematic is in the last post of Improving a "Disco mixer" to mid-fi performance If you have a 24v wall supply from the charity resale shop, you can use 11v zeners, With a 32 V printer/fax/copier wall transformer you can use +-15 v zeners. The maximum input signal you can mix is roughly +V - (-V) - 4, so my +-8v mixer handles a 9 Vac radio output just fine.
Copying other people schematics is the easiest way to "calculate" the input and output capacitors. I've used .22 uf on the input with 10kohm input impedance, but .1 is cheaper if you have to buy them. 10 uf on the output is usual.
741 sounds vile, TL071 sound nice but latches up (stops working) if you get the input too close to the supply voltage even once. Note the power supply decoupling capacitors (+ to - to ground) are not on your schematic but are necessary for anything faster (slew rate) than a 741 or 4558. 4558 has mediocre sound, NE5532 is better with no latchup and no capacitor required around the feedback resistor. But 5532 comes in the wrong package a lot, have to carefully check you're getting 8 pin dip package.
If you're going to build something, 8 pin DIP package is nice, watch what package you order. In the US mcmelectronics has some nice cheap ($2) dip project boards, there is something similar in France but I lost the link when I had to reload the operating system. Vector board is $10 a slice.
A steel box is useful to keep hum from the transformer away from the op amp, and chokes on the power coming in keep lamp dimmer/switcher power supply/CB radio trash coming in the power lines. A 100 pf cap to ground on each input keeps radio trash from coming in that way.
Have fun.
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Last edited by indianajo; 9th December 2012 at 01:13 PM.
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