2 questions: Measuring THD & Making your own resistor??

[loudsubz]

First question for anyone out there that likes to do your own testing, what method do you use for finding out the THD of amplifiers? Is there a specific piece of sftware or is there an actual electronic device to do so? I tried using YMEC DFFS3 the other day but I was having issues with it.

2nd question:

Can you make your own high power resistors from common materials? Ive always wanted to do my own tests but I always find that power resistors in 4 and 8 ohm are very expensive. Is there another way to do this? My teacher was telling use multiple 1w resistors in parallel and sink them into a cold cup of water to disipate the heat and he said that should work..

 

[Lefty]

First question for anyone out there that likes to do your own testing, what method do you use for finding out the THD of amplifiers? Is there a specific piece of sftware or is there an actual electronic device to do so? I tried using YMEC DFFS3 the other day but I was having issues with it.

2nd question:

Can you make your own high power resistors from common materials? Ive always wanted to do my own tests but I always find that power resistors in 4 and 8 ohm are very expensive. Is there another way to do this? My teacher was telling use multiple 1w resistors in parallel and sink them into a cold cup of water to disipate the heat and he said that should work..

#1. THD measurements do require special test equipment. A distortion meter (or analyzer) is required as well as a signal generator and a scope to determine power output clipping point. A software program the drives the sound card line output could be used as a signal source however I don't now how percise they are for setting the amplitude output, they are fine for their frequency accuracy.

#2. Yes you can fab your own power resistors. using mutipule resistors in series/parallel configurations can be done, but they are best put in a transformer oil bath rather then water which can eiasly become conductive with containemts.

A real easy resistance 'dummy' load can be made with just long lengths of wire. Every wire gage has so many ohms per hundred ft, say 1 ohm per 100ft on gauge 12 (you need to accuratly measure this with a know accurate ohm meter) will dissappate the heat in a even manner over it's complete length. Just don't wind up the length into a coil as that will create induction which you don't need. I use to troubshoot large industrial regulated 5 & 12 volt DC power supplies that were rated to 30 amps or more. I found that a long length of wire of the right gauge would make a fine load that allowed troubleshooting and testing right up to their max ratings. It the insulation gets too hot (warm is ok) you know you need to go to a thicker gauge and that it will therefore have to be longer to have the ohm value.

Lefty

 

[drspiff]

I've made up resistances out of nichrome wire like you find in electric heating circuits. As long as you don't get the wire glowing, the resistance should stay reasonably close. And the advantage is that you don't need a dirigible hanger to layout enough wire to get to 4 or 8 ohms.