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  1. #1
    Senior Member
    Join Date
    Mar 2006
    Location
    London
    Posts
    484

    SB3 Spectral analysis shows +9dB at 30Hz on phono and coax outputs

    I connected the various SB3 outputs to my PC via a Tascam US-122L USB audio interface. The PC acted as a spectral analyser, the relevant results are in the image below (each Y division = 3dB). What I do not understand is why the bass response increases by 9dB at 20-30Hz on the phono and coax outputs, but not the 3.5mm output. Points worth noting:

    - Same problem with two different SB3s

    - Same problem with two different pink noise files: one was from Inguz (44KHz / 16bit flac), and the other from www.24bit96.com

    - You can see the problem creeping in at about 500Hz

    - The coax was tested via a Rotel RSP-1068 surround pre-processor.

    I don’t understand this and I can’t help but think I am doing something truly stupid. I would have thought the problem would be audible as I have a pretty decent system, but I have never noticed it although I may well have unwittingly “tuned it out” as I normally apply a cross over at 60Hz for a subwoofer (no filters for this test).

    Any ideas anyone? I am running LMS 7.9.2 on a Raspberry Pi4 (it’s great!).

    Name:  SB3 noise, Upstairs, green=3.5mm left, red=phono L (resize 75%).jpg
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    SB3 -> Quad 909 -> Quad Electrostatic speakers, Quad 405 -> TBI subwoofer
    3 x SB3s + SB Radio
    Rotel RSP1068 surround processor, Quad 99 -> B&W surround

  2. #2
    Senior Member
    Join Date
    Apr 2005
    Location
    Watford, UK
    Posts
    1,902
    I don't know how to interpret the screen shots you've included.
    What are the two plots? Is the top plot the waveform and the bottom plot the spectral analysis?
    What is the difference between the red and green traces?
    If the bottom plot is a frequency analysis, and you're using pink noise, then why is it flat above about 200Hz?
    Pink noise is *supposed* to have a falling spectral content with rising frequency, eg. this digitally generated pink noise in CoolEdit2000:

    Name:  New-1.png
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    As for why the phono & coax outputs are different to the 3.5mm headphone output, my guess is that it's the headphone output that is "wrong", since it passes through an additional headphone amp which is probably not of the highest quality. The coax output in particular should not be any different to the data in the source file, because when using a digital output on a squeezebox all it's doing is converting the TCP/IP data stream into SPDIF without any processing.
    Transporter -> ATC SCM100A

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