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Old 12th September 2009, 03:43 PM   #1
CBRworm is offline CBRworm  United States
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Default Measure THD, etc of power amp with sound card - attenuator?

This may be in the wrong forum, but my goal is to be able to see THD and IM distortion before and after I change components or values in power amps.

I have a few software packages available for my PC which has a good isolated external sound card, but in order for the results to have value I need to build a resistor dividing attenuation network of some sort connected to an appropriate load for my testing.

Since I am trying to measure THD, etc; I want a quiet solution and also don't want to accidentally send +-80 volts into my sound board. I realize that the resistors would need to be sized appropriately for the voltage range I use, but my question is more of what are appropriate types of resistors, and what are good starting sizes. It seems like high tolerance metal film resistors maybe? By size I mean, if I want a 10:1 ratio would it be better to use 10 ohm to ground and 100ohm to source? or 1kohm to ground and 10kohm to source?

My test load will be 8,4,2 ohms. How much additional load do can I put on the circuit to get good signal, but not effect the load the amp sees significantly enough for the output to change. .1 ohm, .01ohm, .001 ohm? I would also prefer to not have to use huge resistors for the dividing network, and would prefer for them not to get smoking hot.

My end goal would be to have a +-1.8 volt signal that is an exact clone of, say, a +-20 volt signal. I need the signal to be as clean as possible to measure.
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Old 12th September 2009, 04:02 PM   #2
rdf is offline rdf  Canada
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Use a 1Kohm 10-turn pot across your load resistor. Done.
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Old 12th September 2009, 04:06 PM   #3
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If you're going for a clean signal, resistors are the way to go.

It might be of interest to test using an actual speaker as well as the dummy load to test the amp's performance into a realistic load.

It should help to know the input impedance of your card. If we say it's 10k, then we can just hook it in series with a 90k resistor to give 1/10 division.

Using a 90 ohm and 10 ohm resistor combination for 1/10 shouldn't throw off the amp loading enough to cause any difference. In any case, unless you know your card's impedance, you might not get perfect 1/10 division (as long as the voltage is in your card's rage it shouldn't be a problem).

If you test using an actual speaker I advise against taking the output voltage from a low value resistor in series with it. This is because even though the amp is controlling the voltage, the speaker responds differently to every signal and your output will be more of a test of the speaker than of the amp.

If you are using a trusted 8 ohm dummy load you can put a .1ohm resistor in series and take your input from across that resistor. Then you will have approximately 1V for every 80V, so your card won't risk being smoked. For a 4 ohm load half this value and for a 2 ohm load 1/4.

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Old 12th September 2009, 04:31 PM   #4
jcx is offline jcx  United States
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its unlikely any soundcard line level front end is much quieter than a 1K Ohm resistor so 100 - 1K is likely a fine target range for the lower divider resistor - using a lower impedance divider only requires inconveniently large size to dissipate the power without adding to measurement accuracy – the same consideration applies to the amplifier’s own global feedback network as well

metal film at <10% of their power rating are good to ppm for audio frequencies much higher than their thermal time constant

an amusing hack is to use a string of identical resistors for the divider – they all see the same Vdrop, and power so if the thermal environments are kept closely similar any common distortion mechanism like TC or VC will ratio out

Last edited by jcx; 12th September 2009 at 05:00 PM.
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Old 12th September 2009, 04:45 PM   #5
rdf is offline rdf  Canada
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The disadvantage of a fixed divider is sacrificing your card's dynamic range for measurements at any level but one. Ideally you want to drive the sound card as close to full permissible level as possible. Seriously, use a pot. I regularly take 96/24 measurements with a generic plastic A/B pot, a 10-turn wire wound will be massive performance overkill.
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Old 12th September 2009, 09:51 PM   #6
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I would do these tests first using a expendable (cheap) sound card. As errors in your test set-up or procedures can take-out the sound card input stage.
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Old 12th September 2009, 10:23 PM   #7
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I use the silicon chip sound card preamp, though it does have the disadvantage that it lowers your measuring resolution somewhat, in that it of course has it's own noise and distortion profile.

One thing that is important though is to make sure you measure your sound card in loop mode (ie line out directly connected to line in) so that you can get a baseline measurement of what the noise and distortion performance of your soundcard is!

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Old 13th September 2009, 05:12 AM   #8
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on the sound card side of the attenuator,(whether you use a pot or resistor string) i would put a pair of opposed (in series wired cathode to cathode) 4.7V 1W zeners to protect the sound card input. most sound cards digitize up to +/- 1V, but can usually handle overloads up to 5V.
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Old 13th September 2009, 01:50 PM   #9
CBRworm is offline CBRworm  United States
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Thanks for the suggestions.

For the initial setup I will definately test with some old expendable sound cards first.

Ultimately I will either end up using an M-audio 1010, which is my regular sound card. Or if the results are similar I will use either my USB Turtle beach, or USB creative Soundblaster. I am not worried about them getting fried. If the results are much better I will use the M-audio, but I don't want to break that one so I may only use it for fine tweaking.

Who knows - I may end up giving up on the software if it isn't great.
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Old 13th September 2009, 05:06 PM   #10
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i use Spectrum Lab. it has a LOT of flexibility and features.
http://freenet-homepage.de/dl4yhf/spectra1.html

it can use sound cards with up to 196k sample rates
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