Party Line Intercom

Discussion in 'The Projects Forum' started by mark63534, Oct 15, 2013.

  1. mark63534

    Thread Starter New Member

    Nov 24, 2011
    I would like to create this circuit:

    Link to the Circuit

    however, I have a bunch of questions:

    how does one create a rail (basic circuit 101, right? I assume this can be created by basically soldering the power line to a open uninsulated copper wire of some sort...)

    where the microphone is, I plan on using an electret microphone (despite the author saying not to use VOIP headsets, I plan on doing so). is it ok for me to use both the ring and tip in the trs format for "1" and "2" as sleeve? (i.e. combining both tip and ring to form one line versus 2)

    I have a little problem completely understanding how the microphone amplifier output can be completely nulled. I understand the usage of the DPDT switch, however, I feel that I can't picture not hearing myself thru the headphones despite the switch being there.

    lastly, the "resistor across the wiper" - they said that value (10k) can be improved. If i were to use 50k pots or something around that, what value would you recommend for the resistor?

    p.s. i plan on using a 3 pin xlr (not using the program audio, using the microphone output as a second input to null it out [see question 3]) so, does that work out too?

    Thanks for all the help, if anyone replies lol. I feel like what I just posted was too much

    *also attached the pdf
    Last edited: Nov 5, 2013
  2. mark63534

    Thread Starter New Member

    Nov 24, 2011
    so, no one has any idea what i'm trying to do here... ?
  3. Dodgydave

    AAC Fanatic!

    Jun 22, 2012
    your link dont work....
  4. SgtWookie


    Jul 17, 2007
    Edited to fix the link
    Electronic schematics were frequently drawn with the power supplies and ground shown as horizontal lines on the paper, and then the components were added vertically between the two, resulting in a pattern resembling railroad tracks; the lines representing power/ground looked like the railroad rails. This technique is antiquated now, but the "rails" terminology lingers on; electronics people generally know what you're referring to when you say "rail".

    I don't know why you're planning on using VOIP headsets when they're specifically mentioned to NOT use?

    Headphone: the idea in the schematic is to use a standard stereo headphone that has separate left and right speakers. Yes, you could short tip and ring together if you needed to do so, or if you were using (a) mono headphone(s).

    The mic switch completely disconnects the LM386 power amplifier audio input from the microphone, and grounds it. The LM741 is probably the least desirable opamp to use, as it is electrically noisy, slow, and has poor crossover distortion characteristics amongst other ills. An OPA134 would be a vastly superior choice.

    100k should work OK, just to keep the input from "floating" when bad spots develop on the pot.

    What do you mean, to "null it out"? This really doesn't make sense.

    One of the big items that delayed responses to your thread is that the link to the PDF didn't work just by clicking on it; and another is the reference to an external website. I corrected the link above, and also have attached the .PDF in case it disappears from the external website. It usually helps considerably to attach a schematic in .PNG format, as that doesn't entail downloading anything to view it; so more people will be inclined to see what's up.

    It's usually best to err on the "too much information" side rather than "not enough", as the latter causes the thread to drag on for many pages just trying to extract some basic information from the O.P. (original poster).
  5. THE_RB

    AAC Fanatic!

    Feb 11, 2008
    For "party line" intercoms they can use a single wire (and gnd) for all the intercoms signals. Every intercom's mic inserts sound onto the party line. Every intercom amplifies the party line signal and sends it out to its speaker *except* that each intercom cancels it's own mic signal from the party line (so you don't hear yourself speak, only the other people).

    Another way to say it is that it's mic still sends the signal down the party line but that signal is cancelled locally on that intercom's amp.
  6. mark63534

    Thread Starter New Member

    Nov 24, 2011
    now I understand ! well, kind of.

    nice. so I should assume that the "rail" concept can still be used by means of a bare wire... just as long as it's not touching anything. I can't make a "professional rail wire", so i'll just have to do it up ghetto. lol

    I am cheap. I cannot just use those professional 4-pin XLR headsets (as much as I want to) because they cost a lot of money. So, I really don't have a problem starting out with an electret microphone. In the end, 4 VOIP headsets can pretty much equal one of those professional headsets. Unfortunately, the VOIP headsets can be flimsy, but I'm not planning on getting the really flimsy ones. I feel like a good VOIP headset can almost rival a professional microphone headset.

    If I forgot to clarify, when it comes to shorting the ring and tip together, I was talking about the electret microphone when it comes to powering microphones (see link). I once did a test on my own microphone port on my computer, and when I shorted both together, it did not make a difference (like as if the ring and tip were already connected). So, I'm just trying to verify that so I can correctly interface it with this circuit (because in the circuit, it only gives me two lines to terminate it to [line and ground]).

    Now I understand the use of the DPDT switch. I also understand why the LM741 is not desirable to use (hence, the author suggesting to use the NE5534). But why do you prefer the OPA134 (and does it have the same or similar pinout)?

    I shall use 100k then. I was planning on putting together a 50k pot with a 50k resistor (just cause in the pdf, he placed a 10k pot with a 10k resistor [but then again, keep in mind that his values were not fixed])

    When I said "null it out", I was pertaining to "null out" or remove the microphone's output from the headphones. Based on the PDF, the author says this can be done by adding the line of the microphone output again in the second input (which was going to be empty, because I don't need the program audio in the circuit). Basically, the person talking into the mic would not be able to hear themselves, which is my goal.

    When I said to use the 3 pin xlr, I meant to say:

    • Pin 1 - Ground
    • Pin 2 - Party Line
    • Pin 3 - Power Line
    and connect all the xlr's in a junction box. Of course, I would have the power come in to the junction box from a separate wall wart of some sort (that won't blow my circuits, less than 12v to be safe...).

    A stereo output for the headphones would be nice, but I feel like mono would suffice for the most part.

    p.s. I know this is pushing it, but I was planning on using cat5 for my xlr lines (because I don't know anyone who would sacrifice 4 or 5 100'-200' foot good microphone cables). That cat5 that I plan to use is Unshielded. Any takes on that (except for the fact of using a balun at both ends of the cable...)?

    I placed that url in there to show an example of cat5 and xlr together, btw, fyi.

    My bad for messing up that link. I'm still trying to fix it. How did you make that work... :/

    very sorry about that.
    Last edited: Nov 5, 2013