How to measure Distrotion in power Amp ??

[papanohn]

Hello everybody.
I'm a new diy from thailand.I decide to make a poweramp by my self. I don't know about how to measure distrotion ,THD ,TIM
I have Ocscilloscope and function generator. and DVM.

Help me please..
Thank you .. everybody

 

[maylar]

You can't measure distortion with those tools. You need a distortion analyzer.

 

[Nelson Pass]

You can make a distortion analyzer, though.

 

[jcx]

it is difficult to measure distortion with a analog 'scope but high end digital scopes do have fft/spectrum analysis options

nulling/differencing and notch filtering can allow you to look at "distortion residuals" with a analog scope but its not easy or flexible

today's cheap option for audio measurements is the PC sound card and spectrum analysis software - freeware such as RightMark RMA gets you started with almost any PC (a handfull of attenuators, decoupling caps and possibly analog filters/buffers to interface high power amp V to the soundcard input/output are recommended )

M-Audio Transist is recomended as a cheap USB external sound card with a 24/96 codec that has great performance for ~US$ 80

check out tools and resources threads at headwize and head-fi

 

[papanohn]

Thank you for all.

 

[jackinnj]

You can make a distortion analyzer, though.

TI in particular, has a good "how to" guide for building an audio notch filter -- search in their development tools (perhaps later I will find where it is bookmarked -- but I am not on my work computer.)

You also need a low distortion oscillator -- if this is your first gambit consider an oscillator for only one frequency -- 1kHz. Here's a simple idea which will work -- you use a basic crystal controlled oscillator -- 32.678 kHz crystal and divide down by 32 to get a ~ 1 kHz square wave (or a 1 MHz crystal and divide by 1000). Follow this by a couple of op-amp low pass filters and you will get a reasonably low-distortion sine-wave oscillator. Once again, TI has good free-ware for designing low pass filters. Search for "Filter-Pro" on their website.

If you have access to the Linear Technology or Maxim higher order switched capacitor filters these can be used -- they aren't that expensive -- an 8th order low pass filter from Linear is about $2.00.

Jack