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Old 8th October 2006, 01:53 AM   #21
anatech is offline anatech  Canada
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Hi keantoken,
You're right that this will involve capacitors also. This is because the input of the sound card has capacitance, as do the cables. This will create a low pass filter. A capacitor in parallel with the dropping resistor will correct the loss of highs in the same way an oscilloscope probe is adjusted. Look oscilloscope probes up on the internet to get the concept.

Your idea of using a 7.5 ohm dropping resistor and a 0.5 ohm sensing resistor will work for a range of voltage levels. I was thinking more along the lines of an 8 ohm load resistor (or 4 ohms if you like). The "probe" would then be a higher resistance element. The input impedance of most sound cards seems to be around 10 K very approximately with some value of capacitance. The cable running to and from the sound card will add it's own capacitance to the total load.

-Chris
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Old 8th October 2006, 04:12 AM   #22
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My sound card is a Creative SoundBlaster AWE64 Gold. I know that it is old, but it still sounds very good and has very good frequency response. I recently installed a newer and more generic (or less distinguished) soundblaster but it doesn't look that impressive. I will test both with RMAA and see what the difference is. My current sound card has RCA phono outputs, and I have an RCA to headphone converter. I was wondering if the line-in impedance is the same as the phono impedance. If I used this cable to test my sound card with RMAA, would it do anything harmful?
I didn't think about cable capacitances... Haven't had much experience with probes and cables so it slipped my mind...

Do I ever sound like I know everything that people tell me, when in reality it looks like I know nothing and I am just trying to make everybody think that I do? I have gathered an uncanny amount of knowledge through the years, but my lack of experience makes it hard to recall the things that I have read. Then when someone tells me about these things I finally remember... It's horrible...
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Old 8th October 2006, 08:21 AM   #23
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Okay, got that model. Turns out there is very little difference between the FFT's now *sigh*. My imbalance was 2% too... Did anyone know that you can write wave (sound) files with LTSpice? You can save the output of an amp to a wave file and analyze it or listen to it later. Look up .wav in the help file or PDF. Could I use this with RMAA to get an accurate distortion figure, so that I can accurately find out if my idea works?
This is a pretty cool feature...
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Old 8th October 2006, 10:38 AM   #24
MikeB is offline MikeB  Germany
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Keantoken, you really should not care much about 2% unbalance, unless you match also all resistors and bjts <1% in real world.
Additionally, these 2% will be swamped out by thermal variations.
Are you sure you have checked things correctly ? I get a 30% unbalance using 200ohm for one of the 150ohm.
Try the wilson current mirror, as shown in the "symasym - the sequel" thread. It will reduce unbalance below 0.1%. (openloop only, it will keep unbalanced to compensate DC-offsets via nfb, unless you tightly match the drivers)

Chris, within audio bandwidth there should be no need to compensate the voltage divider. Simply use not too high resistances. Using 1K to 100ohm for example gives a divider of 1:11 with an outputimpedance of 100ohms. The 1k will dissipate ~60mw to give output of 1v peak.
Keantoken, you would simply put this divider in paralell to the 8ohm dummyload.

Mike
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Old 8th October 2006, 01:32 PM   #25
clem_o is offline clem_o  Philippines
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keantoken,

Spice is a pretty good simulator, but you will have to build up the knowledge of distinguishing between what it tells you and what the real-world circuit will do. Just keep in mind it is just a tool, and a good one for developing, however ultimately it's still the physical circuit on the bench (and iterations of it) that will really count... Bob Pease of NS totally hates these simulators...

Cheers
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Old 8th October 2006, 01:36 PM   #26
clem_o is offline clem_o  Philippines
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Hi MikeB,

I think Chris prefers the higher impedance voltage dividers for the sound card to ensure that the audio card never gets too high a current into its inputs - even if some mistake is made (i.e. the bottom resistor of the divider wasn't properly grounded, or someone plugs in the RCA jacks with the amp turned on)...

Cheers!

clem
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Old 8th October 2006, 06:21 PM   #27
anatech is offline anatech  Canada
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Hi Clem,
Yes, thank you. That is what I intended. I do come from an instermentation back ground as well and know that mistakes are only a matter of time.

I must say that it's getting scary how you can figure out my motives!

-Chris
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Old 8th October 2006, 06:24 PM   #28
PMA is offline PMA  Europe
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God bless the Porridge ..
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Old 9th October 2006, 12:03 AM   #29
clem_o is offline clem_o  Philippines
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Quote:
Originally posted by anatech
I must say that it's getting scary how you can figure out my motives!
Hi Chris,

Hehe, don't worry about it, this one wasn't too hard. Remember I'm at a uni, too many kids blowing up things here... !!

:-)

Cheers,

Clem
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Old 9th October 2006, 01:58 AM   #30
anatech is offline anatech  Canada
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Hi Clem,
I want pictures.

-Chris
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