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Author Topic: U47 schematic, component-by-component  (Read 27368 times)

soapfoot

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U47 schematic, component-by-component
« on: January 15, 2011, 04:15:58 pm »

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.

http://img.photobucket.com/albums/v283/jazz347/47-inspired%20mic%20build/u47.gif

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)

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brad williams

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Re: U47 schematic, component-by-component
« Reply #1 on: January 15, 2011, 10:20:15 pm »

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.  
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soapfoot

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Re: U47 schematic, component-by-component
« Reply #2 on: January 16, 2011, 12:41:04 am »

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!
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brad williams

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Re: U47 schematic, component-by-component
« Reply #3 on: January 16, 2011, 02:24:08 am »

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.  
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They say the heart of Rock & Roll is still beating, which is amazing if you consider all the blow it's done over the years.

"The Internet enables pompous blowhards to interact with other pompous blowhards in a big circle jerk of pomposity." - Bill Maher

"The negative aspects of this business, not only will continue to prevail, but will continue to accelerate in madness. Conditions aren't going to get better, because the economics of rock and roll are getting closer and closer to the economics of Big Business America." - Bill Graham

johnR

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Re: U47 schematic, component-by-component
« Reply #4 on: January 16, 2011, 08:13:27 am »

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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J.J. Blair

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Re: U47 schematic, component-by-component
« Reply #5 on: January 16, 2011, 09:42:59 am »

Thanks, thanks for the clarification.  I've even heard it called a "grid shunt," and when using that term, David Bock corrected me, saying it wasn't shunting anything.  

I didn't say "grid stopper," but I have noticed an extension in the high end on the U47 when increasing the value of R2.  The information I had found said "very high frequency LPF," in explaining the function, which corresponded with my experience.  The only thing I could find describing the circuit called it the control grid resistor.  

I appreciate you're chiming in.  I actually didn't want to be the first to offer anything up, but nobody was saying anything, so I figured I'd get the ball rolling, and you guys could make corrections as we go along.  

Also, I'd love to know about the function of C1 and C3, if anybody can elaborate on that.
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They say the heart of Rock & Roll is still beating, which is amazing if you consider all the blow it's done over the years.

"The Internet enables pompous blowhards to interact with other pompous blowhards in a big circle jerk of pomposity." - Bill Maher

"The negative aspects of this business, not only will continue to prevail, but will continue to accelerate in madness. Conditions aren't going to get better, because the economics of rock and roll are getting closer and closer to the economics of Big Business America." - Bill Graham

soapfoot

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Re: U47 schematic, component-by-component
« Reply #6 on: January 16, 2011, 09:47:39 am »

thanks both.  I've made the correction(s) above.

Would love to learn more!

Here's a question I have.  It might be elementary, but like others I'm a "tinkerer" with no formal training--

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?
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brad williams

KaiS

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Re: U47 schematic, component-by-component
« Reply #7 on: January 16, 2011, 12:15:58 pm »

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?

No, the load is purely capacitive, this means no DC current does flow through the capsule or C1, except some parasitic due to imperfect isolation.
Therefore the voltage on both sides of R1 is the same.

R1 + C1 do form a LPF to remove hum and noise from the capsule bias voltage.
It is tuned  to 0.17Hz , suppressing hum about 50dB.
C1 2nd function is to create a low impedance AC ground coupling for the capsule backplate.

R6 + C3 do the same for the tube's plate voltage, the filter x-over is about 6Hz, filtering hum by about 20dB.
The filter even reduces DC fluctuations, caused by Line AC change, reaching the signal output.

R5+R6 form the plate resistor, where only R5 is active for signal's AC.
Together they set the DC working point for the tube in the plate branch.

R2 is the negative biasing resistor, R3 the positive biasing resistor (by placing the tube into positive voltage related to ground).
Together they are responsible for the grid/anode working point of the tube.
R3, in theory, acts as a negative feedback resistor too.
In fact this is neglectible, to my measurements there is no gain difference if it is bypassed with a cap.
The resistance of 29 Ohms is simply to small to build up a significant feedback voltage.
It's inductance (it's wire-wound) can, of course, build a RF filter the reduces the possibility to catch up radio interferences.
The capsule capacity together with R2 form a HPF, tuned to 21Hz.
Increasing R2's value would widen the LF response but make the mic more sensitive to sub-sonics like wind and plosives.
The same applies to C2, which does form a 12dB/Okt. HPF together with the output x-former's inductance.
I can't tell the x-over frequency of this one.
I guess (only guess) it's around 30Hz.

R2 does not have any direct influence on the HF response of the mic. Even the parasitic capacity of R2 only dampens the signal over the whole audio range very slightly (less then 0.1dB).
But changing the low end can make us believe the high end does sound different.

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.
100MOhms allow some more negative charging of the grid (by parasitic electrons), which adds to the bias caused from R3.
This would mean less current is flowing through the tube, the working point is shifted, distortion can increase.
I've never measured the amount of extra biasing, although it's possible using a high voltage probe with an Ri of e.g. 1GOhm.
One could even do a relative measurement by shorting R2 with a low value resistor and measure the dc change on the plate.
DC on the plate should go down if the high R2 value did change the biasing.

BTW: The U47 circuit is very nice for understanding how a tube condenser mic works, as it's the most basic one possible.
It uses the least number of parts for a working mic.
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.
If you have a perfectly regulated PSU you could even leave away R6 and C3, and replace R5 by 130kOhms, but that's all.

Regards
Kai
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MagnetoSound

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Re: U47 schematic, component-by-component
« Reply #8 on: January 16, 2011, 02:01:32 pm »

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.




Doesn't R1 need to be that big to provide the high DC impedance needed for the capsule, as without it the capsule capacitance would need to be impractically huge?

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.


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Music can make me get right up out of my chair and start dancing or it can get me so pumped up I have to walk around the block.
It can also knock me back and make me sit there and cry like a little baby. This shit is as powerful as any drug!!!
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KaiS

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Re: U47 schematic, component-by-component
« Reply #9 on: January 16, 2011, 02:12:23 pm »

MagnetoSound wrote on Sun, 16 January 2011 13:01

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.

Doesn't R1 need to be that big to provide the high DC impedance needed for the capsule, as without it the capsule capacitance would need to be impractically huge?

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.
Exactly!
Neumann didn't have another chance as to use 60MOhm on R2, as the capsule capacitance is fixed to 80pF by design and cannot be significantly magnified.

Don't underestimate the noise filtering by C1, without it all PSU noise would be coupled into the audio and amplified together with the music.
Therefore R1 is that big to provide sufficient filtering with a relatively small cap.

Regards
Kai
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MagnetoSound

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Re: U47 schematic, component-by-component
« Reply #10 on: January 16, 2011, 02:15:32 pm »

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.




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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Music can make me get right up out of my chair and start dancing or it can get me so pumped up I have to walk around the block.
It can also knock me back and make me sit there and cry like a little baby. This shit is as powerful as any drug!!!
- Larry DeVivo

johnR

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Re: U47 schematic, component-by-component
« Reply #11 on: January 16, 2011, 03:29:58 pm »

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.

The resulting change in bias may increase harmonic distortion, which could account for at least some of the perceived increase in high frequencies when R2 is increased.

Edit: I'm just speculating here. See Oliver's (better informed) comment below.
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soapfoot

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Re: U47 schematic, component-by-component
« Reply #12 on: January 16, 2011, 06:00:11 pm »

This is awesome.  I'm learning a lot.  I've been updating the first post to reflect the clarifications you all are bringing about.  Thanks to Kai, especially.
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brad williams

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Re: U47 schematic, component-by-component
« Reply #13 on: January 16, 2011, 08:59:01 pm »

Yes.  Thank you Kai and Dan for explaining some things that I've never had properly explained to me, or that I've been unable to find.  Great thread idea, soapfoot.  

BTW, through some trial and error, I'd arrived at using 200MΩ in the R2 position, as being what seemed most pleasant to my ears.  Interesting note about the harmonic distortion, John.  Thanks for that.
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They say the heart of Rock & Roll is still beating, which is amazing if you consider all the blow it's done over the years.

"The Internet enables pompous blowhards to interact with other pompous blowhards in a big circle jerk of pomposity." - Bill Maher

"The negative aspects of this business, not only will continue to prevail, but will continue to accelerate in madness. Conditions aren't going to get better, because the economics of rock and roll are getting closer and closer to the economics of Big Business America." - Bill Graham

mad.ax

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Re: U47 schematic, component-by-component
« Reply #14 on: January 17, 2011, 11:04:01 am »

It is worth noting that the current flowing through the filament is also flowing through R3. Given that this current is much higher (37mA) than the one flowing through the tube, the bias is stiffer, and cannot be affected as it would with other biasing scheme...

Axel
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