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I would like to measure thd% on my pc. I use spectra plus software .
If i use a low watt device, there is no problem .
When i use my car cd-player , the output is 17 watts rms.
So i guessed , i should protect my sound car from frying, and i used an attenuator 20db .
Now the question is, when i use an attenuator , i get lower reading of thd%???
If yes , how can i measure an audio device with enough wattage without using an attenuator??
please help me.

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Although I know nothing about the Spectra software or your particular circuit, here is some general information regarding this.

Total Harmonic Distortion is directly related to your amplification circuit. If you need to use an attenuation circuit, you will always have the characteristics of that circuit added into the mix. If you can find a pattern such as the circuit always reduces the THD by 0.2%, then you can compensate with a calculation. What kind of a circuit are you using to accomplish attenuation? Many companies drop down the voltage with a transformer (sound is an AC signal). High end sound equipment uses Balun transformers to change high and low impedance devices to match each other. You can find lots of information on the web regarding this principal. Any time you use op-amp or transistor circuits you are changing this THD spec.

Hope this is helpful.

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Thnx for repluing
i do not something strange to attenuate the signal .
My car cd player has amplified outputs and preouts.
The outputs are 5volts maximum , with no amplification
The amplified outputs deliver 17 watts rms at 4 ohms

So, i guessed that this ratings are to much for my sound card to handle!
So i user a voltage divider (20db) , simply a resistor in series 1000 ohms and a resistor in parallel 100 ohms .
Thats all i used to attenuate the output voltage in order not to "burn" my sound card .

My second question is :
There MUST be some way to measure thd% at high volts and high watts .For example how do we measure thd to an amp with 1000 watts rms ??? Do we attenuate the output? And if the answer is yes , then how much lower ratings do we get?

So when i attenuated my output to the cd-layer the measure was right or wrong?

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THD is a measurement done at a particular output impedance. You are expecting a certain number of watts at 4 ohms. You will see measurements on stereos that will vary such as 0.1% THD into a 4 ohm load or 0.1% THD into an 8 ohm load. If you change the output load, you will change the distortion factor. When you put your resistor divider on the output of the preamp, you put a 100ohm resistor to ground. You just changed the output impedance of your amp. Now it is not 4 ohm or 8 ohm, but instead, it is a 100 ohm impedance. This is what changed your spec for total Harmonic Distortion. Not the amplification factor.


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so i must measure the thd% of the amplified output with 4 ohms load .
How should i do this??
i mean , if i connect to the output the load , where should i connect the measuring cables and where the attenuator circiut?
Do i still need the attenuator circuit?

What about the preouts?
do they need load so i can measure the thd? and if yes , what load should i connect to them . The preouts are for connecting the input stage of an amp , not a speaker...

Another question :
if i am using an acoustic generator and an osciloscope (not to the pc) , the things are easier?
Can an osciloscope measure the harmonic distorsion?Or i only see when the graphic starts to clip?
With an osciloscope i do not need an attenuator?

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I do not work in that industry, but it is my understanding that high end eqipment for measuring such specs use transformers for attenuation. With a transformer, you get an exact replica of the sound wave but at a smaller level. Distortion in the sound wave would also be duplicated on the output. This without adding impedance to the circuit.


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  • 2 weeks later...

i think that the best way is to use the original load to mesure, or a load of the same impedance.
if you put the original speaker to the output, then you have the corrrect results.
now, second problem. if you power the amp from your car, then the voltage output is likely to be 12-14V.
if you use the sound card to mesure, then if i am not mistaking the card olny accepts max. 5V input.
considering that you have 12V, you should divide this voltage with a voltage divider, so you have a smaller voltage.
i think that the best way you can divide the voltage is to divide it to 3. this way, the max voltage it can handle is 15V, and it is easier to do the calculations.
the best way is to use 3 identical resistors, say 1Kohm. put them all in series and connect them in paralel with the speaker.
then use as input the voltage across the resistor that is connected to the ground.
now, here there comes a grounding problem. if you will power then amp from a suply that it is plugged in, then it should have the ground connected to the mains safety ground. the computer also has the ground connected to the mains safety ground.
now, the speakers may connect between the ground of the amp and the output of the amp, or the speaker is connected to 2 outputs of the amp. in this case, if you try to mesure the THD you will short one output of the amp to ground. so this is why it is important to know wich wire coming from the speaker is the ground.
if you cannot test the amp with the original speaker, then use a resistor with the same resistance as the speaker, but remember that is must be able to handle the power that comes from the amp, in this case you could use a 20W resistor. yet, the best result comes with the exact load you will use for the amp, the same speaker.


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