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Thread: ECM8000 Mods

  1. #131
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    Originally posted by Rossi
    Another difficulty is the Linkwitz mod. It's virtually impossible to solder a wire to the housing of the capsule as it is made of aluminum. I'd be happy for suggestions how to solve that problem.
    I use this:

    http://www.web-tronics.com/2200-mtp.html

    It's a silver ink pen. More than enough conductance. That path is a _very_ low current path. I coat the trace with silicone or E6000 after the ink dries. Beware of closing off the vent hole.

    Bob

  2. #132
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    Originally posted by Marik
    Naw DJL, the noise of this capsule is an electronic one and comes from the internal FET. That's why I want to open it and see if will be able to replace the FET.
    I have reason to believe that it is mostly Johnson noise from the acoustic damping resistance of air and from that of the pressure equalization vent. I'll be really curious to see if your effort results in a reduction.


    Bob

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    <It's a silver ink pen. More than enough conductance. That path is a _very_ low current path. I coat the trace with silicone or E6000 after the ink dries. Beware of closing off the vent hole.>

    Bob,

    I was thinking of using it, however prices on this stuff are high.

    <I have reason to believe that it is mostly Johnson noise from the acoustic damping resistance of air and from that of the pressure equalization vent. I'll be really curious to see if your effort results in a reduction.>

    As I mentioned before, after a few attemps I gave up playing with capsule itself. I use WM61A capsule instead of stock WM60. It has 4 db lower noise. Better filtering reduces the noise compare to the stock mic even further.
    I believed that besides of acoustic dumping resistance noise, the FET contributes into overall noise in significant way. If you have other thoughts please share.

    Canopus,

    <I don't know if you recorded anything previously that you could rerecord as a before-after test, but even just something on its own would be cool.>

    I have a stock unmodified one, and yes, I will record something and post it.

    <How much $ do you think this cost you overall?>

    So far to me it cost at least 50 hours of trying different things, soldering, listening, recording, comparing etc. Plus some scandals and accusations like 'your mics are more important than me' etc. And.... parts cost.

  4. #134
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    Dont' worry too much about accusations like 'your mics are more important than me'. No matter what your hobby is, a woman will always think it is more important than her. But remember that their hobbies ARE more important than you and they will tell you so without hesitation.

  5. #135
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    Hi - sorry if this appears twice after all - but it seems like
    the first time posting something went wrong.


    <<And I like the twist with the ground of the capsule at a mid-tap.>>

    <Normally, to make the capsule's FET to swing a lot of voltage,
    you would need to use +/- voltage (+ to drain, and - to source through bias resistor) and to connect the capsule's body (internally connected to diaphragm) to ground. Since we have only + polarity of the phantom supply, we fake it. This way diaphragm in reference to the FET is still at
    the same relationship.>

    So if I understand it correctly the capsule's body is lifted because the source has been lifted as well, OK ?


    <Yes it is a dual matched FET. It got to the place of 2A1349 (same package).
    You could use matched 2SK170 here, as well.>

    Nice, that could be convenient.


    <<#1 could it be that the 1M biasing resistor should also be halfway the supply i.s.o. ground?>>

    <No, I use this resistor as a gate leak one, so the buffer is self-biased.
    If you connect it as a bias resistor the stage will draw too much current and the supply voltage will drop considerably.>

    I'm not sure how to understand the self biasing in combination with a current source.

    Doesn't the lower JFET (which is configured as a current source) determine the current draw of the 2-JFET buffer ?
    Assuming the lower JFET is set for say some 3 mA (just a rough guess from an average -GR-JFET), this sounds like the upper JFET prevents the lower JFET from fully reaching that estimated 3 mA ? As a consequence the output impedance of the current source can suffer since the lower JFET isn't in saturation.

    To prevent too big of a current: another approach could be to change the dimensioning of the current source so that it does develop its full current (in combination with a higher bias-voltage for the upper JFET.


    <(know that, done that)>

    Right ! Better than theory ! :-)

    I must add that I don't know how sonics will change (for better or worse) and if all this probably neater biasing really matters, but the benefit from using a current source i.s.o. a resistor might improve things further.
    So again, in the end the sound matters, however it is achieved.

    Bye,

    Peter

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    Originally posted by Marik
    <It's a silver ink pen. More than enough conductance. That path is a _very_ low current path. I coat the trace with silicone or E6000 after the ink dries. Beware of closing off the vent hole.>

    Bob,

    I was thinking of using it, however prices on this stuff are high.


    I believed that besides of acoustic dumping resistance noise, the FET contributes into overall noise in significant way. If you have other thoughts please share.

    I think it can be found at a lower cost but I never considered $13 or $14 worth worrying about. It's very useful stuff and if stored upright in a fridge will last a long time.

    RE the noise, a friend of mine took one apart and just hung an equivalent capacitor across the FET in place of the diaphragm to measure the pure FET contribution to a WM61A and it was considerably less tha half of what it was before the modification. The difference can only be atributable to acoustic Johnson noise.

    Another way of viewing acoustic Johnson noise is thermally agitated air molecules impinging on the diaphragm. The level of this is related to temperature and the acoustic resistances involved in the device.

    The following paper was the first quantitative treatment of it I found:
    http://scitation.aip.org/getabs/serv...71077874424341

    and Dick Campbell sent me a response to that paper that appeared in a subsequent issue of the journal that specifically called it Johnson noise and did a further analysis of it, strongly implicating the pressure equalization port. I don't have a link to that response, unfortunately.


    Bob

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    I believe the bias in microphone FET's such as are used in the WM60 or WM61 is done via the leakage current through an integrated diode connected from source to gate and that the bias voltage is essentially zero regardless of the voltage on the can.

    Any voltage between the can and the gate/backplate is effectively ignored so that the can could be left at circuit ground for induced noise shielding.


    Bob

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    Originally posted by Marik
    ...So far to me it cost at least 50 hours of trying different things, soldering, listening, recording, comparing etc. Plus some scandals and accusations like 'your mics are more important than me' etc. And.... parts cost.
    Ha! You think it's bad now, wait until you have kids! Not that they're not worth it, but they greatly limit your time, which is why you're doing all this and I'm not helping.

    Anyway, wives/girlfriends never seem to see the other side of things. You've made a great pioneering contribution here, one that will probably stand for years to come. And I, for one, am very grateful!

  9. #139
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    Originally posted by arcanemethods
    I believe the bias in microphone FET's such as are used in the WM60 or WM61 is done via the leakage current through an integrated diode connected from source to gate and that the bias voltage is essentially zero regardless of the voltage on the can.
    Bob,

    Are you talking about conventional connection? Then yes, it is self biased, and is done through gate to source forward current, which is very small. However, I am using it as 'cathode follower', and believe that source resistor (which is BTW another source of the noise--I am using here Vishay 0.1%) sets the current--
    tell me if I am wrong here--I was trying and couldn't find information about these FETs used in Panasonics. I read somewhere that these are "in house" ones. I was under impression that most of the noise comes from leaky gate junction, and attributed it to lack of gate-leak resistor. These guys usually high Mohms--U-47-100M, U67-400M, M49-150M, etc. Smaller capsule mics use up to 2Gohm.
    Another question is about difference between WM60 and WM61. Once again, because of lack of information, I was under impression that the difference in their output and noise specs are because of different FETs, and mostly their different stray capacitance, which
    loads the input. The 'conventional' connection makes it worse. The 'source follower' connection, where the gain is essentially unity, but the output is still different, also suggesteded that. However, now I start thinking that it might be a result of different back chamber construction...

    I would appreciate your input or any info on that.

  10. #140
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    I had always thought that Vgs was held at zero by integrated diode leakage (many microphone FET's explicitly show them in the data sheets) but yesterday the following appeared in the micbuilders Yahoo group (formerly MicDIYers). I'd like to know what you make of it:

    http://groups.yahoo.com/group/micbuilders/message/469

    Frankly, this topic is starting to drift over my head and I would appreciate whatever you think the truth is.


    Bob

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