Dan Dugan wrote:
> Walt, you wrote,
>
>
>>I'm not using the phantom power on the MP-2. But did find a considerable
>>current drain into the MP-2 from the mic with direct connect.
>
>
> The MP-2's transformer would be conducting the 5V to ground through
> the coil, also magnetizing it in the process! This would reduce the
> saturation level for loud sounds. Who knows what pulling the voltage
> down at the output does to the performance of the mike's circuitry,
> can't be good.
I certainly did not keep it connected once I discovered it, though it
had run for maybe a total of 10 minutes by then. The supply voltage took
a large drop, which is how I discovered it. Very upsetting to make a
basic error on them. The MP2 runs MKH mics the same as before with no
apparent problems or change in sound. Is the magnetizing something I
need to worry about?
They don't show much of the internal circuit in their diagram. I did
take one apart and look at it today. signal is feeding direct out a
transistor through a coil to pin 1. With small caps going to mic case
all along the way. This appears to be the RF filter. That looks very
similar to the circuit for a MKH 104, which I do have that diagram,
except the 104 has a cap in series. It was seeing that cap on that
diagram that lead me to believe that I was ok. The 104 also uses 8
volts, but draws 5ma. They do not rate it's power supply for running the
110, even though it appears to be the same powering.
>>The
>>Portadisc must have AC coupling inside it as it does not have that
>>problem.
>
>
> Mebbie it's a transformerless preamp? All such have input caps.
my guess too.
>>I'm now at the point trying to sort out a value for the cap as
>>Sennheiser did not specify. And that's all they show, just a cap symbol.
>
>
> Big, big, big cap in series for low-frequency response. What's the
> output Z of the mike?
I have a impedance of 90 ohms. It's all in german, just to make it more
fun. I don't have a lot of specs for the mic, sounds like you have about
as much as I do.
The biggest non polarized cap I have right now is 30mf, I'll give that a
try, though it sounds like bigger from what you are saying.
>>They did spec the cap on the 8volt line at 500mf.
>
>
> a filter cap to ground?
Across the 8 volt supply (between pins 2 & 3), the zenar is shown the
same. I assumed this was filtering/voltage regulation. The rest of their
powering is a feed resistor, size unspecified. I calculated that from
the 8ma spec so as to drop the voltage the right amount from the
battery. And then used trimpots so I could adjust it with a voltmeter.
When I first powered the mics up it was right on voltage, but I trimmed
it up a bit to give longer before the voltage fell too low. I set it for
a fully charged battery to give it 8.8volts, and then drop as the
battery discharged. At 8 ma that takes a long time.
Drawing through that feed resistor, the voltage was halved when hooked
to the MP2, Surprising that the mics were still putting out.
>>Also speced a 8 volt
>>zenar in there, but I've not been able to locate those, so just have the
>>voltage adjusted with trimpots.
>>
>>I'll be happy for any suggestions on that part if you have worked with
>>the mic.
>
>
> I haven't, just looked it up in old files here.
That's been part of the problem, I've not found anyone who knows what
the actual power supply for these was like. I do know that they are used
in infrasound studies. Every one of those studies I've read seemed to
use the 110 to record. I got into them because they were available
relatively cheaply for the specs they have. Needed something to move on
with the SASS.
Walt
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