Frequency Response of Board

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Frequency Response of Board

Post by jimt » Thu Aug 17, 2017 7:29 am

I need to know the Frequency Response of the board, for example what is the DC rejection? In my case I need to collect low frequency (<20Hz) sound and I need to know if there is a cutoff frequency inherit to the system.

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Re: Frequency Response of Board

Post by thumbknuckle » Thu Aug 17, 2017 9:03 am

I do not know how much the circuit differs, but I just measured my stereo card yesterday. I'm seeing a roll off starting at 160 Hz and am down about 40dB by 20 Hz. That's just a quick test in and back out with no processing applied. I'll be doing more thorough testing to determine whether the issue is at the input, output, or both.

I have a thread here: viewtopic.php?f=5&t=3049

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Re: Frequency Response of Board

Post by jimt » Wed Aug 23, 2017 12:11 am


Please let us know what you find. If these filters are software enabled/disabled that would be great.

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Re: Frequency Response of Board

Post by flatmax » Wed Aug 23, 2017 10:30 am

Hi there,

Firstly the stereo card has a very large DC blocking capacitor in it, I think you have the HPF active in the mixer which is why you are seeing this 160 Hz thing ... I have updated your thread with my findings which show that there isn't any significant HPF in the audio range on the stereo cards.

The Octo board breaks the differential audio codec lines directly out - so you can create your own analogue buffers if you like.
As far as the RCA breakouts are concerned, you can see them here : ... CA.sch.pdf

Both the output and input lines have a very large 10uF DC blocking cap there. If you want ultra low frequency outputs, then you can experiment with two different approaches :
a] Put another cap. in parallel with the one on the RCA board and you can strongly reduce the low frequency cutoff.
b] Short the two pins of the DC blocking capacitor out essentially taking it out of circuit ... it will now pass everything including DC. This poses problems if you have a different DC node voltage on your electronics which you plug into the RCA connector - or if say you are driving synths which don't want to be held at half rail voltages.

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