• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Attenuators ------ Please Help


2007-09-24 3:22 pm
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.

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.



-- josé k.


Paid Member
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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