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Old 05-13-2019, 08:27 AM   #1
TheDragon
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Default Frequency measurement

I have a old JBL car amplifier that has electronic HP/LP crossover with pots.
Now, the pots don't have any increment markings, just 32-320Hz variable.
I would like to set these at around 100Hz in HP, as close as possible, so the front component speakers don't play that frequency.

I don't have a Oscilloscope but apparently my Uni-T multimeter can read frequencies. So, I sent 100Hz test tones via phone app, measured the output on the amp and it reads 100Hz. I sent 1Khz and 10Khz and it measured correctly, all in Flat position. Great I thought.

I then switched on HP, sent 100Hz, measured and it still reads 100Hz!
I tried a few different below 320Hz and all measured correctly.
Turning the Pot had no effect on the measured output.
If I listened to a bass heavy track, and adjusted the Pot, bass was indeed removed. So audibly it works, but not as measured.
Is the amp only attenuating and not really blocking.

So whats going on? Is my my meter faulty or not suitable for this type of testing?

Incidentally, If I sent various tones from 500Hz -15Khz in the LP setting, the meter cannot read them. Flicking the switch to flat and then it reads them.
Odd.

Any help or advice would be appreciated.
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Old 05-13-2019, 10:24 AM   #2
eccerr0r
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Default Re: Frequency measurement

Not suitable due to automatic gain.
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Old 05-13-2019, 11:13 AM   #3
TheDragon
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Default Re: Frequency measurement

hi.....Automatic gain, which device?
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Old 05-13-2019, 11:50 AM   #4
eccerr0r
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Default Re: Frequency measurement

the multimeter. you can only get a rough idea.
You could measure the voltage and see how much it drops, that's the only other thing you could do to get an idea of what's going on.
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Old 05-14-2019, 01:49 AM   #5
stj
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Default Re: Frequency measurement

you can get phone app's that have a requency display/spectrum analyser.
just remember most phones are optimised for audio-range - not an issue here though.
https://f-droid.org/en/packages/org....efarmer.scope/

and:

https://f-droid.org/en/packages/org....farmer.siggen/
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Old 05-15-2019, 11:19 AM   #6
llonen
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Default Re: Frequency measurement

Basic measurement premise is incorrect, you really a scope to see what is coming out.
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Old 05-15-2019, 03:39 PM   #7
redwire
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Default Re: Frequency measurement

You can just measure Vin and Vout. Use your multimeter on ACV and put in say 200mV and find the -3dB corner frequency where the output dips by 70.7%
Some DMM's are only good to around 1.5kHz before they roll off. Others are good to much higher.
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Old 06-17-2019, 07:21 AM   #8
lax luthier
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Default Re: Frequency measurement

Quote:
Originally Posted by redwire View Post
You can just measure Vin and Vout. Use your multimeter on ACV and put in say 200mV and find the -3dB corner frequency where the output dips by 70.7%
Some DMM's are only good to around 1.5kHz before they roll off. Others are good to much higher.
Did you mean Vout dips TO 70.7 % of Vin?
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Last edited by lax luthier; 06-17-2019 at 07:24 AM.. Reason: Loose nut behind keyboard
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