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percy 19th September 2005 04:54 PM

Strange Crosstalk result
 
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This is the image of crosstalk result from tests on my power amp (actually a Technics stereo receiver) using RMAA. Notice how after about 5-6Khz the crosstalk increases on the left channel but decreases on the right channel. Why is it happening so ?
I would expect the crosstalk to increases or decrease on BOTH channels not go in opposite directions as is shown in the picture.

percy 19th September 2005 05:01 PM

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btw that was with a 5ohm resistor load on both channels and this is the crosstalk with no load at all -

sam9 19th September 2005 05:31 PM

A- Have you run a basic loop-back to be sure your soundcard and connections are not at fault? I have found that those 3.5mm mico phono plugs that you have to use with soundcards can be a source of priblems -- including crosstalk. Some are better than others -- it seems random.

B- measure the resistance of the two resistors, then of the resistors plus the wiring connected to them. Be sure they are the same for both channels.

C- swap the left/right connections at each stage to be sure the problem remains in the left channel of the amp and does not depend on the test setup connections.

D- be sure the soundcard mixer software has disabled all the effects and "features".

I'm mentioning all this because I've found that wierd results can sometimes be attributable to the set up glitches. I one spent a couple of days looking for high distortion in an amp when the real problem was in the soundcard.

percy 27th September 2005 01:35 AM

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Quote:

Originally posted by sam9

I'm mentioning all this because I've found that wierd results can sometimes be attributable to the set up glitches.

That was probably it. The connections to the resistors from the output terminals were probably goofing up things. I re-did them and the measurements make more sense now. Here is the crosstalk of the amp now -

percy 27th September 2005 01:36 AM

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and here is the crosstalk of the soundcard in a loop back test with the same cables as used in the amp test.

bscally 27th September 2005 03:03 AM

Sound card question
 
well my question is slightly off topic..

What soundcard are you using, although I am very happy with my home sound card I would like to know what the sound card is so that in the future other people can look at your results and compare.

percy 27th September 2005 03:30 AM

Its a Turtle Beach Santa Cruz.

This is a good place for a few sound card comparisons and test measurements - http://www.pcavtech.com/soundcards/compare/index.htm

I have the results of all the tests that RMAA can do, not sure if I should dump them in this thread or not. But I can send a .sav file that can open in RMAA and show all results, or pics, to anyone who is interested.

amplifierguru 27th September 2005 04:40 AM

Hi Percy,

it may well be that the differences displayed on loopback between channels could be exacerbated when operated into the different impedances of the test amplifier.

Your crosstalk is not good - what is the composition? My last commercial amplifier had 91dB channel seperation - at 20KHz, using a common power supply. At 91dB + the composition doesn't really matter.

Cheers,
Greg

sam9 27th September 2005 05:30 AM

I get a little lower X-talk on my SC card with the loop-back, but not much.

I think you have to subtract the loop-back figures from the amplfier's graph. A pain in the neck and not very exact depending on how accuratly you can read the values off with a ruler. I would just look atn the 1k and 10k points. RightMark has no function to do this for you -- you can't even get a data file from the graph.

This assumes cross talk is cummulative. If not, someone let me know please, so I don't get led astray.

amplifierguru 27th September 2005 05:42 AM

Hi Sam,

In an ideal world crosstalk would be simply the level of identical frequency to the test frequency in the other channel. But often the measurement is of everything coming out of the other channel while one is swept so it includes all manner of nasty PS artefacts, hum, noise and, of course, pure crosstalk.

Accordingly it is difficult to come to any conclusion as to addition or subtraction. Add the log reading for complexity.

Cheers,
Greg


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