Good morning.
you have a link regarding THD measurements with a sound card that I can’t find a site with a detailed explanation in particular about the connection of the sound card to the output of the speaker or rather 8 ohm resistance. It is to measure the THD on my F5t v2 amp that I have just finished to set the potentiometer 3.My sound card is an Asus essence Stx2.
you have a link regarding THD measurements with a sound card that I can’t find a site with a detailed explanation in particular about the connection of the sound card to the output of the speaker or rather 8 ohm resistance. It is to measure the THD on my F5t v2 amp that I have just finished to set the potentiometer 3.My sound card is an Asus essence Stx2.
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What is the nature of the problem you are facing?
The STX2 is a difficult card to measure with, primarily because the mixer is designed without any actual logic. It is almost impossible to isolate the input from the output - which means the moment you try to record something, it usually shows up on the main output as well (which you are playing back from).
I do not recommend Asus cards for measurements. In almost all cases for other cards you will typically connect the main output to the amplifier input, and the 8 ohm load is connected back to the line input. You should
1. Watch the ground connections, you can destroy the amplifier, the computer, or the soundcard in any combination if you connect the output lead of the amp to the ground terminal perchance
2. Have back-to back zeners across the soundcard input for protection, and a trimmer of some sort for adjustment of attenuation. A capacitor to prevent DC is also helpful, use a large bipolar cap and a small bypass for accurate HF measurement
3. Note that you will get a spike around mains and harmonics as you have a ground loop inherent to the connection. For P3 adjustment it is sufficient, for accurate measurements you need to eliminate the ground loop, which usually means a second converter at one end or the other, using either optical/ADAT or transformer-isolated SPDIF (which is all the good ones).
The STX2 is a difficult card to measure with, primarily because the mixer is designed without any actual logic. It is almost impossible to isolate the input from the output - which means the moment you try to record something, it usually shows up on the main output as well (which you are playing back from).
I do not recommend Asus cards for measurements. In almost all cases for other cards you will typically connect the main output to the amplifier input, and the 8 ohm load is connected back to the line input. You should
1. Watch the ground connections, you can destroy the amplifier, the computer, or the soundcard in any combination if you connect the output lead of the amp to the ground terminal perchance
2. Have back-to back zeners across the soundcard input for protection, and a trimmer of some sort for adjustment of attenuation. A capacitor to prevent DC is also helpful, use a large bipolar cap and a small bypass for accurate HF measurement
3. Note that you will get a spike around mains and harmonics as you have a ground loop inherent to the connection. For P3 adjustment it is sufficient, for accurate measurements you need to eliminate the ground loop, which usually means a second converter at one end or the other, using either optical/ADAT or transformer-isolated SPDIF (which is all the good ones).
Howto - Distortion Measurements with REW
In general, pc chassis mounted sound cards are not ideal due to inherent connection to noisy digital environment. Get a basic $38 to $100 external USB sound card. They work better and are quieter, with better ability to isolate your amp and prevent it from frying your expensive sound card.
Use a voltage divider attenuator to ensure voltage sensed on dummy load does not exceed max front end voltage rating of sound card. Focusrite Solo can take about 22Vpp max on input. A 10:1 divider will let you take 220Vpp or 77Vrms. For basic 1w into 8ohms measurements, no divider is needed if you can ensure your amp never has a hiccup and lets loose full swing on the output to circa 25 or 50v etc.
In general, pc chassis mounted sound cards are not ideal due to inherent connection to noisy digital environment. Get a basic $38 to $100 external USB sound card. They work better and are quieter, with better ability to isolate your amp and prevent it from frying your expensive sound card.
Use a voltage divider attenuator to ensure voltage sensed on dummy load does not exceed max front end voltage rating of sound card. Focusrite Solo can take about 22Vpp max on input. A 10:1 divider will let you take 220Vpp or 77Vrms. For basic 1w into 8ohms measurements, no divider is needed if you can ensure your amp never has a hiccup and lets loose full swing on the output to circa 25 or 50v etc.
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