soapfoot wrote on Sun, 16 January 2011 08:47 |
The voltage divider created by R7 and R8, given an input voltage of 105v, will output a voltage of 63v. However, between that point and the capsule backplate, there's a series resistance of 100M. I was trying my best to do the math (not my strong point) and I'm wondering how there can be +63V on the capsule backplate? Wouldn't there be some (rather large?) voltage drop across that 100m series resistance? |
KaiS wrote on Sun, 16 January 2011 17:15 |
In theory only the hum filters could be left away or build different. E.g. you could leave away R1 and increase C1 to 1uF to achieve the same result. |
MagnetoSound wrote on Sun, 16 January 2011 13:01 | ||
C1 is primarily there to provide the AC path to ground. Without this, there would be no signal (or only a very poor signal) from the capsule and the mic would not work. The filtering action is a nice bonus. |
KaiS wrote on Sun, 16 January 2011 19:12 |
R1 does not have any influence on the frequency response, it's not loading the capsules output signal against ground, but is connected to the backplate. |
KaiS wrote on Sun, 16 January 2011 17:15 |
Increasing R2 would bring the tube circuit out of the tubes spec's. In fact it's already far of (x200), the spec says no more then 0.5MOhms (for Pentode use, in this quasi-triode circuit it doesn't seem to be such a problem)! So changing it will change the working point and stability of the circuit. |
Oliver Archut wrote on Mon, 17 January 2011 08: |
The grid leak and polarization cap is a matter of compromise, the higher the grid leak the lower in frequency the capsule will be. But getting higher the noise of the tube will increase, depending on the state of function the VF14, this resistor can be everything from 500M to 1G and in nearly all cases a sound improvement. |
Oliver Archut wrote on Tue, 18 January 2011 15:07 |
Grid leak might not the proper name, it should be bias/ground reference resistor, but the same as the Buffalo is actually a Bison, it might be not correct but it sounds proper after all those years of mis-use. |
MagnetoSound wrote on Tue, 18 January 2011 08:08 |
I know this as the grid leak resistor, so named because the grid leak current flows up through this resistor and into the grid. This is old school amplifier terminology, but certainly still applicable today - and definitely proper. |
Oliver Archut wrote on Wed, 19 January 2011 14:34 |
but talking to old tube engineers, they point out if something is leaky it is broken. |
MagnetoSound wrote on Sun, 16 January 2011 13:15 | ||
No, correct me if I'm wrong, but I meant that since the capsule operates by varying a DC charge, that a high DC source impedance is necessary for such a small capacitance in the capsule to pull against. You're saying that the only reason for the high value of R1 is to allow such a tiny filter cap value for C1? |
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R1/C1 is connected to the backplate of the capsule, which is the low impedance side and being held on a steady 60V bias. It is therefore AC-connected to the ground, while DC-GND-connection is isolated by C1. This way the capsule can be polarised with +60V through R1. |
gk wrote on Fri, 21 January 2011 14:36 |
Such a straightforward circuit, one might think it would be noise free |
soapfoot wrote on Fri, 21 January 2011 18:10 |
One question I have: Plate voltage in the VF14 is listed as +34V. I see a voltage divider between R7+R8 an R5+R6. However, this wouldn't get you all the way down to 34V... more like 102V. So where does the rest of that voltage go? There must be something 'hidden' that I'm not seeing. |
David Bock wrote on Thu, 20 January 2011 15:38 | ||
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Oliver Archut wrote on Mon, 17 January 2011 17:58 |
The THD is given by the capsule and won't change to much with external components. |
Klaus Heyne wrote on Sun, 23 January 2011 14:17 |
I think distortion measurement protocols for capsules are hard to design. Suppose you can somehow extract and eliminate THD distortion from all electronic components in the chain, including the mic processor itself, how do you want to trigger (or simulate) and isolate the distortion emanating from a capsule? With a sine wave, fed into a speaker, fed into the capsule? |
volki wrote on Mon, 24 January 2011 14:44 |
(...) Thus, the speakers on their own only generate THD, their two signals combine acoustically in front of the mic, and you can be sure that the DFD you measure is actually caused by the mic itself. |
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(Oliver) The capsule of a 47, it does not matter M7 or K47, will start to distort earlier than the buffer amplifier itself, |
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(Klaus) There is also the electro-mechanical interaction of the mic amp's high impedance stage coupled to the capsule. |
volki wrote on Mon, 24 January 2011 18:20 |
PPS. Maybe I’m mistaken, but it seems to me that manufacturers used to get their published max. SPL data by simply measuring THD of the electronic part of the mic, assuming that this were the dominant figure. |
Klaus Heyne wrote on Tue, 25 January 2011 01:22 |
What would transient measurements tell you about how a microphone sounds, or how useful the mic may be for music recording? |
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Well, the same way we have transient measures for power amp to reproduce music. Music is the contrary or static and the way any audio device can "track" those micro changes in the music/sound can get a good idea of how much accurate a mic would be in transducing a pressure or gradient point in the sound field. |
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More accurate transient response is one of the reason people will love condenser for example. |
David Bock wrote on Tue, 25 January 2011 12:00 | ||||
... in those measurements, transient response doesn't come up so much. FWIW greater minds than mine have and are addressing this topic, with still, no overwhelming evidence that one could use measurements instead of listening.
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Oliver Archut wrote on Tue, 25 January 2011 05:59 |
in a 47 the capsule starts to distort earlier than the electronics (...) My point is that the static value of testing distortion does not take this into account. |
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For example, if sound waves hitting the membrane are accompanied by strong capsule exertion caused by exhaled breath, that might easily distort the fairly linear swinging process of a diaphragm* sooner than the amp distorting. |
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David, if you have some reference to those paper on new ways to measure microphones, I'd love to know about them. |
Oliver Archut wrote on Wed, 26 January 2011 02:44 |
Most of the statement are based on my own measurements (...) The membrane material is the 2nd problem, most capsules are not 100% tensioned with reference to dead center of the capsule. There are only a few capsule out of any given series that collapse uniformly when the polarization voltage is pushed up (...) the less uniform the capsule collapses the more the distortion. |
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Additional the material itself is one factor itself for distortion, as an example a multiple layer membrane will produce less distortion (...) AKG rolled the mylar with a special pattern aside several other production advantages it did reduced the capsule distortion. |