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Question about impedence matching inputs into PA system

Discussion in 'General Electronics Discussion' started by dietermoreno, Mar 9, 2013.

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  1. dietermoreno


    Dec 30, 2012
    N00b question about impedence matching inputs into PA.

    Yesterday me and my friends wanted to jam along to a song while we are doing vocals, guitar, and drums, but when I connected my laptop sound card (Sound Blaster Go Pro! USB external sound card) with the sound card impedence selector switch on "audio line out mode", when I connected the sound card 1/8 inch TRS female output to a 1/8 inch TRS male to 1/4 inch TRS female adapter and connected to a 1/4 inch TRS and connected to the channel 2 audio line in input of the PA (vocals were on channel 1), the guitars were heard on the backing track but no backing track vocals were heard even though there were definitely vocals and I tried changing the EQ settings on the PA from minimum to maximum and it didn't do anything other than make the guitars sound rumblier or screeachier (with respect to increasing bass or increasing trebel). The live vocals were heard but no backing vocals were heard.

    I noticed that there is a "CD line in" on the PA, which I have a feeling I'm supposed to use that. The "CD line in" is a red female 1/8 inch mono next to a white female 1/8 inch mono. If I go to Radio Shack and buy the correct adapters and cables to connect to "CD line in" on the PA, will that solve the problem?

    Does the impedence of the 1/4 TRS cable with respect to parasitic capacitance with the ground form a low pass filter at about 1,000 HZ that doesn't allow vocals to pass? What has me suspect this is that the screams were not heard but the shitty autotune high vocals were quitely heard when the gain of the PA was not that high (the backing track was a joke I hate autotune vocals). We tried many different songs for backing tracks and repeatedly the guitars could be heard perfectly but the vocals could not be heard at all.

    I looked at this 1/4 inch to 1/8 inch cable specs like the cables I was using and it said 45 ohms DC resistance and 32 pica F capacitance here here.

    So then I put that into the frequency synthesis equation for RC circuit using this RC filter calculator.

    So 32 pica F is 0.000032 micro F.

    The cut off frequency that the calculator said was 110 MHZ.

    So that means that the cable can not handle frequencies higher than 110 MHZ or otherwise would need a super heterodyne receiver to step down the frequency to a frequency lower than 110MHZ

    but how would a cut off frequency in the MHZ range effect audio frequencies?

    Edit: This site shows that parasitic capacitance of an AC outlet is 3 nano F.

    So then if I put 3 nano F (1 nano F = 0.001 micro F) into the calculator with 45 ohms DC resistance, I get 1.1 MHZ, which is closer to audio frequency, but still no where close.

    So I still don't know what's going on.

    I would have to have more capacitance at the same DC resistance to approach audio frequencies.

    Perhaps there is a grounding issue?

    The capacitance of the Earth is 700 micro F Wikipedia says, and that would yield 5.1 Hz at 45 ohms.

    Perhaps the parasitic capacitance with the earth in the insulated coaxial cable is several hundred times less, say 1 micro F, and that would yield 3538.6 Hz cut off frequency, which is in the audio range.
    Last edited: Mar 9, 2013
  2. dietermoreno


    Dec 30, 2012
    Never mind, retard question answered on UG forums on this thread. connecting a stereo audio line to an instrument line in is a problem with hearing the vocals on the recording that has NOTHING to do with any sort of accidental filtering created by parasitic capacitance and line DC resistance, as already mentioned as the calculations show that parasitic capacitance of 3 nano Farads from a wall outlet only starts to effect frequency response in the MHZ range for a standard 45 ohm DC resistance 1/4 inch coaxial cable.

    That would explain why the bass guitar was heard at the same time that the lead guitar was heard, but the vocals were not heard much at all.

    Its the panning of the vocals.

    Vocals are center panned, and connecting a balanced stereo line to an unbalanced input cancels out the center panned vocals.

    Problem solved.

    That is the reason for not connecting one channel of a stereo audio line to the hot line of the input and the other channel to the ground line of the input.

    So to convert stereo to mono without canceling out the center panned vocals you need a balaun.

    Or just don't convert from stereo to mono and use the stereo input instead of the mono input.

    Or since my PA mixer has 3 channels and if only one vocalist is practicing, use one channel for vocalist and use the other two channels for the audio lines in using a 1/4 inch stereo TRS to 1/4 inch mono Y cable.

    Edit: That UG user is right. Red and white audio line in inputs are RCA, not 1/8 inch (3.5mm in the rest of the world).
    Last edited: Mar 9, 2013
  3. davenn

    davenn Moderator

    Sep 5, 2009
    You can mix the left and right output sound card channels to produce a mono channel for inputting to the mono input on the amp

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