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Old 12th February 2008, 07:41 AM   #1
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Default Attenuators ------ Please Help

right then, so i want to use a pc with a very good sound card to measure frequency response and distortion.
however i need an attenuator to do this, any ideas / what do people hear use?
i have seen many attenuators on the market, mostly for guitar players to allow a cranked up sound at low levels.
my output impedance is 8ohms but i am not sure of the input impedance of my sound card though i told the range between different sound cards is massive and that there seems to be no real standard.
so basically i need to convert a 5-10W amplifier output to a line level to run in to my sound card

any ideas?

all help greatly appreciated.

cheers
ed
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Old 12th February 2008, 10:47 AM   #2
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Be careful when using your sound card to do measurements. Because most sound cards have minimal input protection, they can be easily overdriven to the point of destruction!

Use a voltage divider as well as a clipper. This circuit, courtesy of Gary Pimm, uses a standard red LED and 1N4148 diodes - you'll need one for each channel. The LEDs will light up indicating when you are overdriving the inputs.

Click the image to open in full size.

Enjoy,

-- josť k.
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Old 12th February 2008, 12:01 PM   #3
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SO WHAT VOLTAGE WILL THIS CIRCUIT CLIP AT . WOULD IT BE 2.3/2.4V (THE DROP ACROSS THE 2 DIODES)
ALSO TO USE THIS FROM AN AMP WITH AN 8 OHM AMP WOULD I RUN THE OUTPUT OF THE AMP TO AN 8 OHM RESISTOR. THEN IN TO THE DIVIDER/CLIPPER CIRCUIT.

CHEERS
ED
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Old 12th February 2008, 03:21 PM   #4
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Quote:
Originally posted by soundbadger
SO WHAT VOLTAGE WILL THIS CIRCUIT CLIP AT . WOULD IT BE 2.3/2.4V (THE DROP ACROSS THE 2 DIODES)
ALSO TO USE THIS FROM AN AMP WITH AN 8 OHM AMP WOULD I RUN THE OUTPUT OF THE AMP TO AN 8 OHM RESISTOR. THEN IN TO THE DIVIDER/CLIPPER CIRCUIT.

CHEERS
ED

Ed, why are you SHOUTING?

Jan Didden
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Old 12th February 2008, 03:34 PM   #5
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no reason just had caps on.
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Old 13th February 2008, 04:19 PM   #6
kevinkr is offline kevinkr  United States
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Might be a good idea to attenuate the signal before you feed it into the sound card input - I'd recommend by about 20dB or so for measuring the output of your amplifier. Incidentally many sound cards accept as little as 700mVrms before becoming non-linear so watch out about over- driving.

Use whatever load resistance is required at the output. Note also that this attenuator is only suitable for use with low impedance power amplifier outputs. I use a box which provides both attenuation and gain and has an input impedance of 100K. (Gain provided by 5532 op-amps.)
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