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Old June 22nd, 2006, 07:55 PM   #31
Fred Retread
 
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Quote:
Originally Posted by Seth Bloombaum
Well, if both mic inputs are feeding phantom into the Y cable that could be as much as 96 volts to the mic. Or, it could be a lot less, depending on Marantz & Oade...
Two 48 volt sources in parallel supply 48 volts, so the smart money says the mic will see 48V.

And in terms of signal voltage, we're trying to distribute voltage these days, not power, so impedance mismatches are less significant. For transferring voltage, you just want the load to have a lot higher impedance than than the source. Besides, the 4073 has voltage to spare.

I don't disagree that this is not best practice, but I think the odds are in Kevin's favor.
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Old June 22nd, 2006, 08:54 PM   #32
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Phantom POWER is POWER, not just voltage. Power = Current x Voltage. We already have existing examples of distortion due to low milliamp capability of some Phantom Power supplies.

I'm bemused by the "how to get by with less" attitude exhibited here. I once had a client suggest that he reduce his ad budget by 10% to see if his sales went down 10% or hopefully less (there's at least one in every crowd.) I suggested he shoot one toe off and see if it had any effect on his abilty to walk properly. If not, shoot another!

If you're doing this for yourself, it really doesn't matter. If you are getting paid as a professional, it does.

Thanks,

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Old June 23rd, 2006, 07:04 AM   #33
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Quote:
Originally Posted by A. J. deLange
If the sensitivity of the microphone is 71 mV/Pa it will drop to about 32% (square root of 10) of this or about 21 mV/Pa with a 10 dB pad installed.
Thanks A.J. And am I guessing correctly that the
drop with a 20dB pad would be the square root of 20
i.e. 4.5% of the mV/Pa?
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Old June 23rd, 2006, 09:05 AM   #34
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No, -20 dB is a drop to 10% of the reference level. dB is 20 log (new level / reference level). Going the other way, the ratio of new level to reference level is 10 raised to the power (dB / 20).

So for the earlier example of -10dB that was 10 raised to the power of -10/20 or 10 raised to the power of -1/2, which is 0.316 (not actually the square root of 10. +10dB is the ratiothat happens to be equal to the square root of 10).

For the case of -20dB it's 10 raised to the power of -20/20, or 10 to the -1 power, or 1/10
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Old June 23rd, 2006, 11:11 AM   #35
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Quote:
Originally Posted by Fred Retread
Two 48 volt sources in parallel supply 48 volts...
Right. Parallel, not series. Thanks for the correction.
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Old June 23rd, 2006, 11:11 AM   #36
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***edit*** why did the forum server make 2 of these?
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Old June 23rd, 2006, 12:02 PM   #37
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Quote:
Originally Posted by Fred Retread
Well, I'm glad Doug was there to straighten that out--if you had listened to Dave in post #4 you might have thought you had brickwalled the preamp and needed -20 dB of attenuation ... 8>)
I knew it was possible I just misunderstood the mod and there is no addendum added to the package to clarify things... with a stock Marantz the -20db pad is useless as it just makes things so noisy... I thought they got rid of the pad with the mod so that's why I wasn't sure what the problem was.

But yes Doug cleared it up, they are a great company making some great products with very good support.
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Old June 23rd, 2006, 12:05 PM   #38
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Quote:
Originally Posted by Fred Retread
Two 48 volt sources in parallel supply 48 volts, so the smart money says the mic will see 48V.

And in terms of signal voltage, we're trying to distribute voltage these days, not power, so impedance mismatches are less significant. For transferring voltage, you just want the load to have a lot higher impedance than than the source. Besides, the 4073 has voltage to spare.

I don't disagree that this is not best practice, but I think the odds are in Kevin's favor.
This is correct which is why it's not a problem for supplying voltage... as far as current is concerned, I am combining it yes but the mic will only draw what it needs at least in this case. That is why when you add batteries in parellel to supply something it just powers it for longer as it still draws same amount of current and you are just giving it a bigger current source to draw from.
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Old June 23rd, 2006, 01:30 PM   #39
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The voltage factor for 10 dB is sqrt(10). In this case as you are padding (reducing) level you divide by sqrt(10) = 3.1622... Thats equivalent to multiplying by 1/sqrt(10) = sqrt(10)/10 = 0.31622.... I wasn't very clear on that.

There is no code requirement for any of the commercial radiotelephone licences. The commercial radiotelegrah licences did have a requirement. I think it was 25 wpm random groups of 5 for the first class.

Impedance matching isn't too critical at audio frequencies as is clear by the fact that the sources (microphones) tend to have low output impedances and sinks (mixers, cameras, recorders) tend to have high input impedances. Thus it's clear the mic is intended to serve as a voltage source. So if, for example, the mic has an impedance of 100 ohms and a mixer 5900 (to make the math easy) 5900/6000 = 98.3% of the microphone's open circuit output voltage would appear across the mixer's input. If two mixer channels are paralleled (with a Y cable) the mic is loaded by 2950 Ohms so that 2950/3050 = 96.7% of the mic voltage appears across the input. This is a very small loss (0.14 dB) and all should be fine assuming that the Y cable is properly wired.
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Old June 23rd, 2006, 02:01 PM   #40
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Quote:
Originally Posted by A. J. deLange
...So if, for example, the mic has an impedance of 100 ohms and a mixer 5900 (to make the math easy) 5900/6000 = 98.3% of the microphone's open circuit output voltage would appear across the mixer's input. If two mixer channels are paralleled (with a Y cable) the mic is loaded by 2950 Ohms so that 2950/3050 = 96.7% of the mic voltage appears across the input. This is a very small loss (0.14 dB) and all should be fine assuming that the Y cable is properly wired.
True - I was more concerned that it might have some adverse effect on the mic's own internal preamplifier and impedance transforming circuitry that might make it more susceptible to clipping in the mic.
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Old June 23rd, 2006, 04:06 PM   #41
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Quote:
Originally Posted by A. J. deLange
...There is no code requirement for any of the commercial radiotelephone licences...
I must be misremembering. I guess it was the Southeastern Signal School at Ft. Gordon, GA in 1970 that I had to pass that code test. It was in 1973 that I got my 1st Phone license, and that was a long day as I took all three of the written elements the same day.
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