Audio Precision System One Analog A-Weighting - diyAudio
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Old 8th July 2014, 03:52 PM   #1
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Question Audio Precision System One Analog A-Weighting

Hi there,

I'm using an oldschool AP System One Dual Domain.
Currently I'm doing noise measurements and looking at the FFT of those signals using the Analog Analyzer as an Input and the Digital Analyzer for the FFT of that input signal.

Now I want to apply A-Weighting to those measurements.
So either I use an analog A-Filter or a digital one. Unfortunately the Analog Analyzer doesn't have an A-Weighting Filter it only says in the Drop-Down-Filter-Menu: Slot 1, Slot 2....., External.
I don't want to use an external A-Filter as it probably introduces noise itself and makes my measurement obsolete.

Is there a possibility to do an FFT of the noise signal using the A -A FFT.AT1 test. And then running it through a digital A-Filter? I saw that the Digital A-Filter exists but I have no clue how to use it in my situation.

Thanks a lot,
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Old 9th July 2014, 03:35 AM   #2
RNMarsh is offline RNMarsh  United States
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You can add a pcb card with a passive 'A' filter and plug it in the slot for such things (slot 1, 2 ,3...). I dont know the answere to your question.... though I had the same unit for many years. Maybe use the 400Hz HP filter with an external shunt cap across the 100K input z. That would give a close approximation.

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Old 9th July 2014, 06:42 AM   #3
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I didn't check in the manual, but the System One has BNCs on the front which are labeled as "external filter". It looks like you could insert a filter there.

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Old 9th July 2014, 06:44 AM   #4
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Correct. That's the 'External' setting in the drop-down menu.
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Old 9th July 2014, 06:54 AM   #5
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PS: The external filter is likely after all (or at least the important first) gain stages so won't detoriate noise performance.

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Old 10th July 2014, 03:56 PM   #6
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thank you... the external filter works well.

I've got another tiny question concerning the AP System One:
When I use the Analog Analyzer and let the FFT transform the analog signal, I have very strong variations concerning the low frequencies. The level may jump some dB from measurement to measurement and is therefore not very reliable concerning frequencies lower than 30Hz.
--> can I increase the measuring time of the Analog Analyzer, compute an average RMS value and then plot an FFT?

Thank you!
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