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Old 26th August 2008, 04:57 AM   #11
okapi is offline okapi  United States
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it would be easy enough to stick a 100K in place of the 909 and the 1K resistors and loose the 100 ohm. would this solve the problem?

can you define VAS for me?

thanks.
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Old 26th August 2008, 07:55 AM   #12
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okapi;
Changing the resistors to 100k and 1k would solve that problem.
The VAS stage is the input Voltage Amplifier Stage (the circuit that amplifies the Voltage and drives the output devices)
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Old 27th August 2008, 05:58 PM   #13
okapi is offline okapi  United States
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below i am summarizing my understanding of resistor value selection for the sound card protection circuit. please correct me if i am wrong as my electronics "knowledge" is self taught.

choosing appropriate resistor values depends on what is driving the sound card protector as well as what the sound card protector is driving. in my case i am using a power amplifier and my computer sound card.

in order to advantage of the full range of the sound card input i want the final voltage to be about 2 V.

since i would like to test my amp at maximum output - 40V - i need to attenuate the signal by a factor of 20.

i can do this with 1900 to 100 resistors, 19000 1000, 190000 10 000 etc. as i increase resistor values i am making the sound card protector easier to drive, however, i am also making it more difficult to drive the downstream sound card which i am guessing has an input impedance of about 10K.

based on this i chose the 1900, 100 ohm resistance values (10 X less than the sound card) with 900/100 thrown in to keep my options open. if i went higher the input impedance of the sound card protector begins to approach that of the sound card which i think is about 10K.

when i suggested using a 100k series resistor and eliminating the 100 ohm resistor i was going back to what was originally suggested earlier in the thread where i think it was proposed to use the sound card load as the shunt resistor.

thanks again for your help
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Old 2nd September 2008, 03:06 PM   #14
Dan2 is offline Dan2  South Africa
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I have a similar project on my "to-do list"

i have an occilloscope program on my computer that uses the sound card input, and also want to make a protection circuit for it.

i want to make something that can cover a wide range of voltages without drawing too much current. so is this circuit the way to go????

as an added precaution, i was thinking of putting another sound card in my computer and using that. if i were to blow that sound card will it stil be able to blow the motherboard?? i will be using a desktop PC.

i was thinking of using opamps for buffers, but they wouldn't offer any kind of protection, would they?
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