How To: Measure Amplifier Distortion

To follow this tutorial you will need the following:

• A computer

• A sound card

• ARTA measurement software

• Multi-meter

• A cable or connector plug with an L-pad network

• An 8-Ohm 10watt resistor

A suitable soundcard must have have low noise and distortion. Such a soundcard can be had for about £150. A few examples:

Focusrite Saffire 6
M-Audio Fast Track Mobile
Tascam US-122
Presonus AudioBox


ARTA is a software package that has full functionality, except saving files, free of charge: ARTA Download

An L-Pad network is needed to reduce the output voltage from the amp to an acceptable level for the soundcards input. This is very simple and consists of two resistors as below. It can be built onto a jack or phono plug if you want, and connect to it by crock-clips.

The combination of 47K and 10K is about right for recording the 2.83V test signal to my soundcard, but your soundcard might need a different level of adjustment. You can increase the attenuation by reducing R2.






The 8-ohm resistor should be connected between a the two wires of a speaker cable. This is a dummy load for the amp to drive.




Lets get started.

Connect one output of your soundcard to an input on your amp and select that input. Connect the speaker cable with the resistor across it to the relative amp output.

Start the ARTA program called Steps. Click on the little microphone icon and set your soundcard drivers in the first drop-down list. Use ASIO drivers if they are available. Set the correct input and output channels. Click okay.



Click on the record icon to open the Measurement Setup window. Set the Test Frequency to 1000Hz, and click generate. Your amp should now be outputting a 1KHz test signal, but you won't hear it becase it is only connected to the dummy resistor load.



With the test signal running, connect a multi-meter across the resistor terminals and measure the AC voltage. Adjust the amp level until it reads as close to 2.83V as possible. This is 1watt into 8-ohms.



You can now connect the soundcard input with your little connector with an L-pad attenuator. Take a pair of crock-clips or solder the wires if you wish. Connect the amps positive output at the big resistor terminal to the positive input on the soundcard connector. Do the same with the negative connections.

The level meter in the Measurement Setup window should now show the test signal, and if the L-pad network is right, the level should be somewhere around the middle of the meter or higher without clipping. You might need some tweaking with the resistor values to get this right.



Stop the generate function if it is still running. In the Measurement Setup window of Steps, set the top drop-down box to 'single channel', the response channel to whereever you connected the input, start frequency to 100Hz or whatever you want to test from, stop frequency to 10KHz, or 20KHz, and frequency increment to 1/24th octave. Close the window.



You can now press the record button in the main window to begin the measurement process. This will take a minute or two and you should see the frequency response slowly charted out in green.



When this is done, you can click on the 'D%' button to chart the distortion products in percent. Bravo, you're done! You might like to test the amp at other levels too, like 10 watts and 30watts, but be sure to get the L-pad right not to overload your soundcard.

The D2, D3...D6+ tick-boxes represent 2nd harmonic, 3rd harmonic etc..



(The little blip in the graph is from me touching the connection.)



Interestingly this amp has mostly 2nd harmonic. It should sound a little warm.