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Old 20th May 2010, 11:47 PM   #1
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Default soundcard protection Z ???

Trying to make a little nose to nose zener thing with an R divider to protect my soundcard input to measure an amp and am a bit hung-up about Z of the divider that ends up in there. was hoping just some passive thing would work. Thinking this is trying to "transfer" V so I want to preserve as much bandwidth as i can by staying 1:10 at least (???)

Namely, if my amp terminals have 8R dummy loads and the soundcard Zin is 10k, the divider should look like min 8*10 ~80R to the amp and the card should see 10k/10 ~1k looking backwards? I mean, bandwidth can be lost from amp to divider and from divider to soundcard, right?

I read in another thread that people were using something like 100k Rseries for direct circuit (not amp output) connection. Also higher values were recommended to not blow output transistors and stuff. This is definitely not my app--I just want to measure low-watt tube stuff.

I just don't understand how 100k doesn't make all the soundcard input all rolled-off. I was thinking that a soundcard with 10k Zin would want to see a k or so so that's about the max series R in the divider that would be usable. I'm missing something embarassingly fundamental but am not to proud to come begging for a steer

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Old 21st May 2010, 12:21 AM   #2
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Location: Kuala Lumpur
Don't depend on the input impedance.
Use a L pad with 10k in series and 560R to ground in parallel with the sound card input, giving about 500R

This gives 20:1 attenuation, protecting to +/- 50V from the amplifier
These relatively low resistances will have a wide and flat bandwidth
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Old 21st May 2010, 01:02 AM   #3
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Thanks so much -- I really appreciate the guidance
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