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Want to do something with Audio without background in electronics
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soFPG:
Hi lovely EEVblog community,

my mind comes up with new project ideas all the time. Usually they stick around for a while but eventually are replaced with other ideas. This idea though comes up every now and then and doesn't want to go away.

What it's all about:

* design a PCB which is able to measure the impedance of a loudspeaker chassis (usually 4 - 8 Ohm resistance) from 20Hz to 20kHz
For me who doesn't have a background in electronic engineering and circuit design, this is not as easy as it might be for others who already have the necessary knowledge (I am more a software guy).
Nevertheless, I want to describe possible solutions for this project which I came up with:

* I am probably going to need a DAC which creates a sine wave from 20Hz to 20kHz
* this sine wave probably needs to be fed into an operational amplifier (or any amplifier) whose output is connected to a speaker
* then there must be a resistor with precisely known value in series to the second connector of the speaker
* somewhere inbetween the resistor and the loudspeaker I have to measure the voltage with an ADC
I have some questions:


* How much output power does the sine wave has to have and at which voltage to be able to drive a 4/8 Ohm speaker properly?
* How do I measure negative voltage (half period of the sine wave which is below 0)? Do I have to measure it at all or is there some workaround?
* Are there any specific things I have to pay attention to while designing the PCB?
* Do I need 24-bits of resolution or are 16-bits more than enough (again, I am only talking about Impedance not about sound quality)?
I want to use an ESP8266 for that because it has a built in I2S interface which can be used to interface with the ADC/DAC. Also, It would be pretty neat to have a webinterface where the user (most likely I am going to be the only user) can see the curve (Impedance over Frequency) and save it to his computer.

Any help / ideas / solutions are very much appreciated!

Thank you so much
Messtechniker:
For this I went the sound card route.
All you need:
- A stereo sound card. Even a cheap one will do because
  any frequency response anomalies can be compensated for
- A small power amp (10 W will easily do)
- A 50 Ohm 5 W or so resistor
- A measurement program like "Arta" or "Audio Tester", for example.

Will do nicely. No need to bother making a pcb.


soFPG:
Thanks for your reply.

I really appreciate your idea but (unfortunately) I am more interested in the way of how to solve such a problem than in the real impedance values at the end.
bob91343:
Read up on impedance bridges.  That is perhaps the best solution.  There are many types and some more suited than others.

You indeed will need a source of signal, either from a signal generator or home brew.  The latter is like reinventing the wheel but it's entirely feasible.  For this you  need to investigate oscillator circuits.

You will need a standard of impedance, one you know accurately, such as a resistor and a capacitor.

If you don't need great precision, an oscilloscope may be a way to measure phase angle and, thus, impedance.

You can build what's called an Octopus, which will put a graph of impedance on the screen of a scope, which you then calibrate.
soFPG:

--- Quote from: bob91343 on January 12, 2020, 06:17:48 pm ---You indeed will need a source of signal, either from a signal generator or home brew.

--- End quote ---

Do you think a microcontroller could do that? I mean sending the digital values of a sine wave to the DAC?
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