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Old 13th February 2014, 09:48 PM   #1
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Talking Idiot's guide to test equipment part 1 freq response

OK, I'm sure there is a thread on this, or maybe it is so basic I should already know, but I couldn't find one. I have a function generator which I am going to check is accurate and a 'scope. To measure freq response of a preamp, mic head amp, etc not power amp I am assuming I can:

1. use a DMM or VTVM and measure the AC voltage out on the function generator at say 10Hz and 40kHz and the 1kHz output and make sure they are the same;

2. Connect the DMM to the output on the amp;

3. Connect the 1kHz to the amp and measure the voltage out from the amp;

4. Hook up the 'scope and get a good trace;

5. I don't have a sweep generator, so manually move the frequency around and measure the roll off, convert the voltage difference to dB. (Although that is another question.

Or is this way to complicated? Just use a good soundcard, do the same to make sure the function generator is flat, make sure the card is flat then use software? I have a Delta 66.
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Last edited by Blue Jinn; 13th February 2014 at 10:17 PM.
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Old 13th February 2014, 10:21 PM   #2
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Quote:
Originally Posted by Blue Jinn View Post
1. use a DMM or VTVM and measure the AC voltage out on the function generator at say 10Hz and 40kHz and the 1kHz output and make sure they are the same;
You'll likely get a fail right there.


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Originally Posted by Blue Jinn View Post
Or is this way to complicated? Just use a good soundcard, do the same to make sure the function generator is flat, make sure the card is flat then use software? I have a Delta 66.
Bingo. Most software will even compensate for non-flat response of the soundcard, if such is the case (unlikely).
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Old 14th February 2014, 06:21 PM   #3
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Actually, many if not most VTVMs are broad band, although perhaps not RMS reading. Heathkits for example are/were.

Regardless, any decent scope is dead flat in the audio band.

The main issue with soundcards is that they will not measure into the ultrasonic range (if you need that) AND it is very easy to blow up the front end with over voltage. Not so with ur scope in this application.

But the question is what are you measuring? Line level, amplifier level or speakers?

How much accuracy do you need, and what's ur aim?
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Old 14th February 2014, 09:51 PM   #4
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Quote:
Originally Posted by bear View Post
Actually, many if not most VTVMs are broad band, although perhaps not RMS reading. Heathkits for example are/were.
I do have a VTVM which is RMS reading (HP400)

Quote:
Regardless, any decent scope is dead flat in the audio band.

The main issue with soundcards is that they will not measure into the ultrasonic range (if you need that) AND it is very easy to blow up the front end with over voltage. Not so with ur scope in this application.
Good to know! I only need the audio band, although I just acquired a 60Mhz scope.
Quote:
But the question is what are you measuring? Line level, amplifier level or speakers?
Line amp, not power amp. (Preamp, mic head amp mostly) and tape deck. So no more than a few volts.

Quote:
How much accuracy do you need, and what's ur aim?
I will be building a compressor and need to know it's frequency response. I have a handful of pre-amps, and one claims to be "hi-fi." I'm curious as to how hi is that fi. as well. Just needs to be close enough.

Thanks for the response!
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