R/E/P > Klaus Heyne's Mic Lab

U47 schematic, component-by-component

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Here's something I thought might be a cool exercise.  I know there's a lot of knowledge here, and mine is limited, so I hope others can help to fill in the blanks.

I thought it might be educational to go through the U47 schematic component-by-component and explain the purpose of each component one at a time.  The idea would be to learn a little bit more about how tube microphones in general work.

If anyone thinks this is a worthwhile thing, add your insight and I can update this post, hopefully until it's complete.  If any of my initial suppositions are wrong, let me know and I'll correct.

Here's the schematic.

Obviously KK47 is the capsule head, and U1 is the BV8 transformer.  The VF14M is obvious as well.  But what are the functions of the other components?

Let's go one at a time.  There are 8 resistors and 3 capacitors-- shouldn't be too overwhelming.  Thanks to Kai, JJ, JohnR, and others for helping fill in the blanks and increase my (our?) understanding of this seemingly simple circuit.  If there are any additional corrections/clarifications, I'll keep updating this post.

R1 - With C1, forms a LPF to remove hum/noise from capsule bias voltage (Kai)

R2 -  negative biasing resistor, works with R3 to determine bias point of the tube. Together with capsule capacitance, forms a high pass filter at about 21 Hz. (Kai)

R3 - allows a positive voltage tapped off the heater supply to develop on the cathode. (Kai called this "positive biasing resistor")

R4 - Dropping resistor to obtain filament voltage from B+ voltage

R5 - 100k plate load resistor for the VF14M, active for AC signal

R6 - With C3, forms low pass filter to filter out noise from plate supply, also combines with R5 to form plate resistance to set the DC working point of the tube's plate. (Kai)

R7 - part of voltage divider to get capsule polarization (JJ)

R8 - part of voltage divider to get capsule polarization (JJ)

C1 -With R1, forms LPF to remove noise from capsule bias voltage & forms AC ground coupling for capsule backplate. (Kai)

C2 - Blocks DC before U1, shapes timbre (OT coupling cap)

C3 - With R6, forms a low pass filter to filter out noise from plate supply (Kai)

J.J. Blair:
Since nobody of the experts are chiming in, here's what I'll offer (corrections welcome):

R1 is the capsule bias resistor.

R2 is the control grid resistor.  A very high frequency low-pass filter.  Different values sill determine the top end of the mic, which is why you see many 1GΩ resistors in that position in modern mics.

R5 and R6 are part of the plate loading circuit.  I'd love more explanation on this from somebody, please.

R7 and R8 form the voltage divider to you your capsule voltage.  Resistor to ground (3) / the sum of both resistors (2+3=5) = 3/5 x 105 = 63

C2 is the output / transformer coupling cap. I think you have this confused with C3.  

nice!  Thanks J.J.  I understand a little more already.  You were right about my typo.

I fixed that and also updated the OP with your additions.

In case you know in greater detail-- what is the function of the capsule bias resistor?  What's its purpose in life and what happens if you move the value up or down?

(According to johnR, the following does not apply--thanks!):

The 60M grid stopper (R2) makes sense now that you mention it.  In guitar amps, these are used to filter out RF, prevent parasitic oscillations, and help prevent the grid from going positive with respect to the cathode.  I assume a similar function here?  Am I safe in assuming that as the resistor increases, the corner frequency of the LPF moves upward... 60M seems very high (high enough to get out of the way of the audible band?)  Interesting that people tweak it higher to get increased HF response.  I wonder what the tradeoff is.

I'm especially curious also about C3.

Thanks for your input!

J.J. Blair:
Brad, I'm not an electronic engineer or designer.  I'm simply a tinkerer.  I wish I could give you a more informed answer, and I'm hoping that some of the resident geniuses here will fill in the blanks, as well as correct what misinformation I might have.  

There seems to be some confusion here. R2 isn't a grid stopper. It's a grid bias resistor, and as such it just holds the tube's control grid near ground potential. It doesn't actually have anything to do with low pass filtering (although the resistor's parasitic capacitance may have some effect at high frequencies).

R2 does form a high pass filter in conjunction with the capsule capacitance, so increasing its resistance would extend low frequency response at that point in the circuit (whether that's desirable is another matter).


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