Value choise

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matthieu68

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Hello, in my learning on audio circuits I aim on several points of course, but I would like to see more clearly some of them already.

This is the choice of resistance values, take for example the input and feedback resistances on an opamp, the resistance values ​​will determine the gain of the opamp but ... We can achieve this with several  values, for example for a unity gain we can use 47k / 47k or then 10k / 10k, then what should be taken into account for the choice of the resistance value?  Douglas' self book talks a lot about Johnson noise which is higher with higher value resistance, so why not always go for small values?

This question is also valid for the choice of values ​​for voltage divider, drain resistors etc ...


Another point concerns the capacitance of the capacitors, they are placed in the audio path to filter the direct current according to the opamps used, I could read that their values ​​do not affect when they are in the audio path (low  voltage)
Is it true ?

The capacitance of the Capacitors has an influence if we put a drain resistor just after that forms a filter, Ok so we calculate what we need not to filter the audible band.

But when should we put a drain resistor?  I see circuits with and some just with capacitors without drain resistance, except I could read that without drain resistance, the capacitors could remain charged and make "plop" During an action of a Switch for example  .

So how do you determine this?


Thanking you
 

ruffrecords

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Like all engineering, circuit design is a trade off. One factor indicates you should choose a lower value of a component and anther indicates a higher one would be better.

With op amp feedback resistors there are several factors to consider. Johnson noise is one but at line levels this is usually insignificant compared to the noise already present with the signal. Another factor is the output drive capability of the op amp. Maybe you want to drive 16 buses from this op amp each one a 10K load to the op amp. That is a total load of 620 ohms. Not many op amps will drive a load that low but an NE5532 will but you also need to account for the feedback resistor because that is also an output load. If the feedback resistor is 10K then the load drops to 588 ohms. You may think this is an extreme example but I have lost count of the number of mixer designs where I have seen a poor little TL072 trying to drive lots of buses and AUX sends. A third factor to consider is the bias current needed by the op amp. FET op amps need practically none but bipolar op amp feedback resistors need to take this into account especially if you want to dc couple stages and therefore need to balance bias currents in order to minimise output offset voltage.

With drain resistors, their purpose is to ensure there is always  0V dc at the output end of the capacitor. Otherwise there may be a residual dc voltage which will cause clicks if that circuit path is switched.

Cheers

Ian
 

matthieu68

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i understand yes thanks for your explanations, and this always brings up the problem i have  :(, i don't understand how the impedance / load in an audio circuit works, maybe you know a link to a pretty clear explanation and  simple for a beginner.

Thank you
 

JohnRoberts

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Be careful about trying to optimize discrete components for just one characteristic (like thermal noise). Lower is better but can be carried to extremes beyond being beneficial. It is worth attempting to hold the benefit of lower Johnson (thermal) noise in perspective.

For managing line level signals with modest (low) gains, the significance of the resistor's noise is well below other more significant noise sources, like op amp input noise.

Of course there are other considerations, but that's why people write books about them.

JR
 

ruffrecords

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matthieu68 said:
i understand yes thanks for your explanations, and this always brings up the problem i have  :(, i don't understand how the impedance / load in an audio circuit works, maybe you know a link to a pretty clear explanation and  simple for a beginner.

Thank you
As a rule you want to transfer as much voltage from one stage to the next. Since the output impedance of the driving stage forms a potential divider with the input of the following stage, you minimise signal loss by making input impedance much higher than output impedance. This is where the 10:1 rule comes from.

With op amps, the effective output impedance is very near zero so you can ignore it. Now all you need to do is decide what to make the input impedance of the next stage. Obviously as high as possible, This also helps with determining the size of coupling capacitor. The coupling capacitor forms a first order high pass filter with the input impedance of the stage it couples to. The response will be 3dB down at the frequency at which the capacitive reactance equals the input impedance. Generally you want this frequency to be well below the lowest frequency you are interested in i.e 20Hz so you might typically aim for one tenth of this or 2Hz since this means the phase shift caused by this network at 20Hz will be minimal This sets a minimum value for the coupling capacitor. There is no harm in making it bigger. Since the distortion caused by an electrolytic capacitor depends on the voltage across it, bigger values make this voltage smaller and hence give lower distortion.

Cheers

ian
 

matthieu68

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Thanks for all his help.

Regarding the impedance, you explain to me the only thing that I understand (the impedance input output)

I especially can not understand the impedance which governs in the same circuit, how should one be concerned about it?  At what values ​​should we keep it and how?

It is quite funny that you mentioned driving buses with a simple tl072, because that is precisely my intention.
In the diagram below you can see the diagram I am drawing, it would be a Tl072 or an opa134, obviously it would only drive 4bus maximum, with 4 send and direct output.

Do you think that the tl072 or the opa134 would not be enough to control this number of outputs?


thank you
 

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ruffrecords

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I am not sure what you mean by "the impedance which governs in the same circuit". Can you expand on this please?

Regarding the TL072, if you look at its data sheet you will see the maximum load recommended for it is 2K. I am sure you can work out yourself what is the worst case load presented by your circuit. Basic ohms law is all you need. Remember all the bus feed resistors go to virtual earth bus amplifiers so you can safely assume they are connected to 0V.

Cheers

Ian
 

matthieu68

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thanks, i had a hard time understanding the load ... i demonize too much some things which are ultimately very simple, now it's ok. for impedance, what i mean is, old systems had to  keep 600ohm impedance in the system.  In modern systems we no longer use the 600ohm, but must look at the impedance in the circuit itself (apart from the input and output impedance)
 
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