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Old 25th May 2005, 06:56 AM   #1
jdybnis is offline jdybnis  United States
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Default speaker to mic level

I'm setting up a speaker measurement rig and I need to figure out how to split a mic-level signal off of the output of a power amp.

The setup I'm using is Soundeasy with an Edirol soundcard. The attached image shows the way Soundeasy is set up normally. The output from the amp feeds back into one of the inputs of the soundcard, and gets used to correct for distortion from the soundcard and amp. In my setup I'm not using a mic preamp. The Edirol soundcard has direct mic inputs with +48V phantom power. So instead of a line-level calibration signal I need a mic-level signal.

The manual from Soundeasy says to get a line-level signal off the power amp with a voltage divider. Since I'm going to feed the signal into a mic input, I'm afraid that the phantom power on the mic input will cause problems for my amplifier. Is that correct, or can I use the circuit as it is?
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Old 26th May 2005, 09:02 AM   #2
jdybnis is offline jdybnis  United States
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* bump *

Anybody?

1) Will the phantom power from the mic input cause a problem when hooked up to the outputs of my power amp through a voltage divider?

2) What is the peak to peak voltage of a mic level signal so I can choose the proper values for the voltage divider?
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Old 26th May 2005, 09:23 AM   #3
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It might be easier just to buy a DI box that has speaker level in at a music shop. They are so cheap these days you can't really make one for the same price, and it will do exactly what you want.
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Old 26th May 2005, 04:10 PM   #4
soongsc is offline soongsc  Taiwan
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The phantom power normally would not interfere with your input, that's why it's call "phantom".

If your mic does not have a mic amp, it is best to bring the signal in through the mic input that is separate from the line input (if the software allows you to doe this). This is because the mic requires about 26db additional gain, but the amp input does not need this. If you bring it in through the line input, it is almost impossible to balance the inputs so that you can have a good input amplitude for both amp and mic.

The voltage divider is to attenuate the amp voltage so that it does not exceed/damage the sound card input, so you need to do some calculation to determine the amount of attenuation you need.
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Old 26th May 2005, 06:24 PM   #5
jdybnis is offline jdybnis  United States
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Thanks. So since the a mic is 26db below line level does that mean I divide the voltage from the amp by an extra 400x (2x for every 6db)?

I'm still concerned that my amp won't like having 48V of DC on the "negative" terminal.
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Old 27th May 2005, 04:39 AM   #6
soongsc is offline soongsc  Taiwan
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Quote:
Originally posted by jdybnis
Thanks. So since the a mic is 26db below line level does that mean I divide the voltage from the amp by an extra 400x (2x for every 6db)?

I'm still concerned that my amp won't like having 48V of DC on the "negative" terminal.
I think it would be better to boost the mic than to attenuate the amp inputs that much this way you get better digital resolution for the signals.

The phantom power is at the positive terminal if I remember correctly.

You need to know the maximum input and output voltages (p2p or rms) of your SC, and the sensitivity of your amp as well.
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