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Author Topic: U47 schematic, component-by-component  (Read 33463 times)

Oliver Archut

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Re: U47 schematic, component-by-component
« Reply #15 on: January 17, 2011, 11:58:33 am »

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.

Sorry guys, there is a lot of speculation on your side, the THD won't increase and the higher the grid leak the better for the low end. The THD is given by the capsule and won't change to much with external components.

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.

The same is valid for the polarization resistor.
The ground reference cap will change the sound most drastically depending on type. Neumann used everything from ceramic to film caps in this position, one of the reasons why U47 can sound so different.

The 29 Ohm with the 1750 Ohm is a voltage divider to keep the bias always at 1.1V, increasing in this voltage will increase the noise of the tube.

Coupling cap can be 05uF for flat response or 1.0uF for 3dB boost at 30Hz.(with BV8s, GN8 have a build in low cut).

Best regards,
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Oliver Archut
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J.J. Blair

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Re: U47 schematic, component-by-component
« Reply #16 on: January 17, 2011, 03:31:50 pm »

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.



Oli, thanks for this info, but can you please clarify: when you say "getting higher," do you mean the grid leak itself, or the value of the resistor in that position?  And should we refer to this resistor then as a "grid leak resistor"?  
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gk

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Re: U47 schematic, component-by-component
« Reply #17 on: January 18, 2011, 09:37:26 am »

Hope this lends to the discussion- if anything, my 47 (orig m7)
exhibits a bit of hiss when used for things like bare dialog,  and prob one of the biggest contributors to this artifact would be the noise value of R5.  Anyone know if a high quality metal film might gain a hedge on this?  
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soapfoot

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Re: U47 schematic, component-by-component
« Reply #18 on: January 18, 2011, 09:54:58 am »

Have you tried replacing R5 with a new carbon composition resistor before going the metal film route?

From my experience in other types of high voltage tube circuits, replacing plate load resistors with resistors of a different type can sometimes affect subtle changes in the sound of an audio circuit.

Carbon composition resistors can often absorb moisture if improperly stored.  If this happens, they will hiss like crazy.

Apologies if you already knew this.
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Oliver Archut

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Re: U47 schematic, component-by-component
« Reply #19 on: January 18, 2011, 10:07:19 am »

Oli, thanks for this info, but can you please clarify: when you say "getting higher," do you mean the grid leak itself, or the value of the resistor in that position? And should we refer to this resistor then as a "grid leak resistor"?

Hello J.J,

getting higher refers to a higher value of R2, during the time the U47 was made there was nothing higher available that 200M and then super expensive.

R2 was chosen at 60M because during the 1950s 40Hz was the norm for recording mics. Changing this resistor to somewere from 500M to 1G will open up the lowest frequency of the mic, but also depending on the state of the tube might increase the tube noise.

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.

All resistor should be updated to metal film or carbon composition for better noise specs.

Best regards,
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Oliver Archut
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MagnetoSound

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Re: U47 schematic, component-by-component
« Reply #20 on: January 18, 2011, 11:08:51 am »

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.




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.


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J.J. Blair

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Re: U47 schematic, component-by-component
« Reply #21 on: January 18, 2011, 01:10:36 pm »

I prefer the sound of the carbon comps resistors.  I certainly don't care for those blue Xicon metal films.  PRP resistors are nice, as far as metal film goes.

For the R1 and R2, I have some TRW carbon spiral resistors that have been fantastic.  They might not pass muster with Klaus, but the mics I installed them in, as well as used carbon comps for the rest, sound really fantastic.
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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

J.J. Blair

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Re: U47 schematic, component-by-component
« Reply #22 on: January 18, 2011, 01:12:55 pm »

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.


Thanks.  That's what I'll call it then, from now on, just so people know what I'm talking about.  LOL.
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studio info

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

Oliver Archut

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Re: U47 schematic, component-by-component
« Reply #23 on: January 19, 2011, 09:34:19 am »

It might be right that old school techs used grid leak but talking to old tube engineers, they point out if something is leaky it is broken. I am with you grid leak is fine with me, but the proper description is ground reference or bias grid 1 resistor.

We also had a discussion about that about a year ago were other techs point out that grid leak is not the right word.

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Oliver Archut
www.tab-funkenwerk.com

We are so advanced, that we can develop technology that can determine how much damage the earth has taken from the development of that technology.

J.J. Blair

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Re: U47 schematic, component-by-component
« Reply #24 on: January 19, 2011, 11:59:19 am »

There needs to be an agreed upon nomenclature.  Let's put it up to a vote!  
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studio info

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

MagnetoSound

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Re: U47 schematic, component-by-component
« Reply #25 on: January 19, 2011, 12:33:19 pm »

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.



What are they, tube engineers or plumbers?  Very Happy


Wanna go with grid bias, then?

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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

KaiS

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Re: U47 schematic, component-by-component
« Reply #26 on: January 20, 2011, 03:40:54 pm »

MagnetoSound wrote on Sun, 16 January 2011 13:15

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?
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.

The diaphragm side is the high impedance signal output.
This is the reason for R2 being a high 60M.

The capsule as capacitor keeps the bias from reaching the tube grid, saving Neumann one extra cap (+one resistor) that they would need if they would bias the diaphragm instead of the backplate.

One more advantage of this biasing scheme:
the diaphragm is on GND potential and therefore doesn't attract dirt as much as a if it was charged by 60V.

Regards
Kai
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David Bock

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Re: U47 schematic, component-by-component
« Reply #27 on: January 20, 2011, 04:38:52 pm »

Quote:

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.
So what happens if you 1)increase 2)decrease 3)remove C1?

MagnetoSound

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Re: U47 schematic, component-by-component
« Reply #28 on: January 20, 2011, 06:13:08 pm »

Oliver, I would be grateful if you would clarify what you meant here with regard to the polarization resistor.


Thanks!

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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

gk

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Re: U47 schematic, component-by-component
« Reply #29 on: January 21, 2011, 09:36:58 am »

Great Post-   grid leak works for me,  isn't that what it does?
Such a straightforward circuit, one might think it would be noise free, but hiss can still be an issue with the 47. Is there a best choice for R5? Oliver, might you or someone have experience at a solution for this, thanks.
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