crystal mic to line in mod

Discussion in 'The Projects Forum' started by transistor_flip_flopper, Apr 10, 2006.

  1. transistor_flip_flopper

    Thread Starter New Member

    Apr 10, 2006
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    Hi, I'm new to this board. I'm working on a project that uses a crystal mic to drive an IC audio amp (LM324). I would like to modify the mic input to accept a high impedance line level send from the aux bus of a mixer. I'm assuming it's a fairly simple mod but it's outside my n00b knowledge of electronics. Any feedback would be much appreciated!

    P.S I can supply URLs of the circuit and IC spec but I'm figuring that may not even be necessary.
     
  2. paultwang

    Well-Known Member

    Mar 8, 2006
    80
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    If it is already at line level, why amplify?

    If the source impedance is very high, you might want to consider a buffer stage (or just use an instrumentation amplifier).

    Could you provide the parameters:
    *Source (the thing you plug into the IN side of the amplifier) impedance
    *Source voltage
    *Desired output voltage

    and have you tried by just plugging it in?
     
  3. transistor_flip_flopper

    Thread Starter New Member

    Apr 10, 2006
    6
    0
    Here's the specifics. I'm building a goofy little kit. A PDF including the circuit is attached (I hope). It's a color organ circuit. It is designed to be driven by an audio input. Either a crystal mic or the speaker outputs of an audio amp (audio transformers in parallel with an 8 ohm speaker load). I cannot use either of these options practically. I need to drive the circuit with a standard high impedance line level send from a PA mixer. The mixer send needs an input impedance of aprox 10K or so.

    "have you tried by just plugging it in"
    I don't know diddly about circuit design but I'm pretty sure a line level signal will totally overdrive an input designed for a crystal mic as they have a very low output.

    Thanks much for taking the time to respond to a beginner! If that PDF does not show up I'll repost.

    I can't seem to get an attachment to work to save my life... Here's a URL
    http://www.gibsonteched.net/media/9935.pdf
     
  4. Gadget

    Distinguished Member

    Jan 10, 2006
    613
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    OK, Major problem here. The entire circuit is at Mains potential...(no isolation from the mains).
    Normal circuits use a mains transformer to isolate the electronics from mains, but this one doesn't in order that it simplifies the Mains switching using SCRs without the need for optocouplers.
    This means that NO connection to any external device can be made without some form of Isolation otherwise either damage to circuitry, or danger of electrocution will occur.
    Note, that the external speaker connections are via isolated audio transformers.
    The Only cheap way to connect this to an aux line would be to use something like a 1:1 isolated audio transformer.

    NB, if you have already tried connecting this without isolation, it might pay to check that you still have an undistorted output from your mixer, and that the LM324 in the "Color Organ" is still working.

    Quite frankly I'm surprised there are no warnings about this in the instructions apart from the small reference in note 1 of the speaker input description.

    It would also be Very prudent to note that EVERY part of this circuit has the potential to have 110 volts on it, so only ever power it up with the covers on and no internal part exposed.
     
  5. kubeek

    AAC Fanatic!

    Sep 20, 2005
    4,590
    777

    That´s unusually DEADLY circuit!
    I am not experienced in american electricity standards, but is the plug really only two-wire with interchangable pins?

    In europe, or I guess everywhere in the sane world, things have to be connected by 3-pin mains cable, or they must have isolation transformer in power supply...

    If you connect it the other way, you get instantly killed, especially with "grounded" metal potentiometers...
     
  6. paultwang

    Well-Known Member

    Mar 8, 2006
    80
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    *Looks like you don't know what you are doing. And the device looks like a fire/electrical hazard.

    *Measure the input impedance by use of the series resistor method.

    *Your mixer is already at line level (you said so yourself). Do not connect it to your device directly.

    *Better yet, redesign the circuit. Bypass voltage amplifier if you have to. Use a low voltage DC power supply.
     
  7. transistor_flip_flopper

    Thread Starter New Member

    Apr 10, 2006
    6
    0
    Well, this is a learning experience if I do say so. The kit is from hobbytron, a fairly high visiblity web retailer so it surprizes me that it is so, um... dangerous? I have not powered up the circuit at all yet as I am waiting on a component that was missing from the original kit. Dumb luck I guess. The transformer isolated input route sounds like a workable way to go. I'm not sure about the 1:1 part though as I still think the high gain crystal mic input would be overdriven by anything close to a line level signal. Would an 1:1 isolation transformer and a correctly placed potentiometer perhaps do the trick?

    As paultwang said correctly, I don't know what I'm doing... But I have a healthy respect for 110V and do know enough about electricty (notice I said electricty as opposed to electronics) to wire this thing using a 3 conductor grounded power cord, grounded case, and outputs. The crappy little two conductor cord and sockets that were supplied will be thrown away. That was always the plan.

    Thank you all for the good feedback and well received warnings. If anyone is willing to give a bit of guidance on how to wire that pot/isolation transformer to the input of this thing that would be way cool.

    At the risk of blathering on even more... is there any way for me to rejig the transformer isolated inputs that are already there to accept a line level signal?

    Thanks again!
     
  8. paultwang

    Well-Known Member

    Mar 8, 2006
    80
    0
    Your device and all interconnected things should have chassis grounded to earth. Include a ground fault interruptor for additional personal safety too, just in case the fault current doesn't trip the breaker.
     
  9. transistor_flip_flopper

    Thread Starter New Member

    Apr 10, 2006
    6
    0
    Update from beginner guy:

    After thinking about all this I am going back to a plan I had previously considered. I'm going to scavenge an audio amp out of an old set of "powered" computer speakers and connect the outputs to the transformer coupled inputs with an 8 ohm dummy load in parallel. This will solve the isolation problem by using the supplied transformers in addition to a buffer amp with a known/correct input impedance. I will still also build the thing using a fully grounded configuration.

    If anyone thinks this is a bad idea for any reason please be kind and let me know?

    Again, thank you all for the very helpful feedback.

    I wish I had more to offer this board in return, but unless anyone needs advise with sound re-enforcement or digital audio editing I'm afraid most subjects here are out of my league.
     
  10. kubeek

    AAC Fanatic!

    Sep 20, 2005
    4,590
    777

    You should also pay attention to the main swithch, the SPDT should not look like the one on the picture, use some well-insulated type for mains voltage, and also for the sum of the currents that could go through the lamps.
     
  11. Gadget

    Distinguished Member

    Jan 10, 2006
    613
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    I have seen this sort of kit design before for this sort of device. Most have an internal mic and no external connections, a double insulated case, and plastic shafted controls. Normally experiance with 230v (or 110 in your case) is expected, and there are lots of warnings in the instructions and on the case.
    In fact I have built one, a "Dick Smith K3161" and it works fine, But modifications and external connectors are a No No.
    It is unusual that your kit actually has ext speaker inputs, even though they have been isolated with transformers.
    I suggest once finished, that those speaker input connectors be wired together, the 2 mains pins connected together, then an insulation resistance meter (500v megger) connected from the 2 mains pins across to the 4 speaker inputs. Here in NZ the regs say the resistance at 500v must be more than 1M-ohm, but I would be rechecking the circuit if I saw much less than 20 or 30 M.
    The Earth Leakage Breaker /Ground Fault Interuptor/Residual Current Device/Core Balance Relay (or whatever you wanna call it) is also an excellent safety suggestion.
     
  12. transistor_flip_flopper

    Thread Starter New Member

    Apr 10, 2006
    6
    0
    First off, my humble thanks to all for the help.

    Gadget.
    I'm not going to be modding the supplied circuit in any way. I'm connecting an amplifed audio source to the transformer coupled (isolated...) inputs as per the suppliers instructions. The only "external" input will be a line level send to that audio amp. The audio amp will be housed in the same chassis but will have it's own transformer isolated power supply. I will strongly consider adding a ground fault interruptor to the chassis.
    Your (and other poster's ) concern for my safety has not gone unappreciated. I do have a good understanding and useful experience with 110V wiring. The component that was missing from the kit arrived today so I should be able to get this thing put together and test it out. I'll check back and let y'all know how (if?) it works.

    I'LL BE CAREFUL... ;-) No joke.

    Thanks again!
     
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