Adding de emphasis to tube mic circuits

[Matador]

A cap on the plate to ground is not quite the same circuit action as is employed in the U87:  the U87 is done through negative feedback, not simply filtering certain frequencies to ground.

U87 injects an out-of-phase sampling of the output back through the polarization network.  The feedback increases as frequency increases, which means the closed loop gain has a pole in the response and gain is reduced at about the same point that response is rising through the capsule.

I don't see why a similar technique can't be done with a tube circuit:  you could tap from the plate (which will be opposite in polarity to the incoming signal), and then figure out a way of coupling that signal back into the polarization string.

 

[micaddict]

Matador explains the principle better than I ever could.

I will add that the U87 of course is or was meant to be a fet version of the U67, so it's similar in many ways.
(Not counting the short lived U77.)

 

[ln76d]

A cap on the plate to ground is not quite the same circuit action as is employed in the U87:  the U87 is done through negative feedback, not simply filtering certain frequencies to ground.

Of course is not the same, but plate to ground cap have it's own properties
I really like use it in some designs, even with a small amount of hf damping -  it add  some "vintage" (i really don't like this word )
mojo. Probably by some phase swift.

I don't see why a similar technique can't be done with a tube circuit:  you could tap from the plate (which will be opposite in polarity to the incoming signal), and then figure out a way of coupling that signal back into the polarization string.

It's possible but is it needed?
Of course all is matter of taste

Thanks for great explanations!

 

[gary o]

What I mean by SN ratio is mic has higher output than with de emph I engaged so you dont have to have your mic pre/ compressor whatever is in your chain up as high, if Im misunderstood then Im sorry , Im here to learn & do DIY 

As most of us, that's why debate like this is usefull for everyone i think.
I hope, that you don't think it is about who have the right

But, about the deemphasis, did you mean whole feedback or capacitor only?

Up to now I had switch to disconnect C7 from the FETs drain I thought this was turning off the de emphasis the audio result is disconnecting C7 the output level of the mic goes up & there are more highs in the SSSSS region  .... with the de emph engaged it acts like a de esser brilliantly, way better than the plate to ground thing in some tube mic & thats why Im asking can we do this style de emph in a tube mic so we can use capsules with the bump.

Thanks Matador for your input brilliant as ever explanation of what going on in the circuit 

Sadly I dont understand electronics as well as most here thats why I experiment & ask lots of questions ??? so here are some more questions Im afraid.

Is my disconnecting of C7 from the drain turning off the de emph fully ?

Are the de emphasis  components these..... C5 C6 R8 R9/10. Is C3 part of the de emphasis circuit?

My reason for this thread was to ask if I can make a circuit from these components similar values & add to a tube mic

If we wanted NO de emphasis in the circuit say we like our capsules flat sound we could lose more components right ? BUT we would need to change circuit a little for it to work ? I guess would be like the KM84 circuit.

 

[Khron]

All these part numbers are referenced to... what schematic? ;D

 

[ln76d]

with the de emph engaged it acts like a de esser brilliantly, way better than the plate to ground thing in some tube mic

All depends how you make it, what capacitor type, what value etc. 

Is my disconnecting of C7 from the drain turning off the de emph fully ?

Meaning as the filter - YES, but still you have whole feedback network, which also affects low frequency response.
There's no better self noise or higher output -  in overall meaning are the same. Only you aare not attenuating high frequency - that's it.

Is C3 part of the de emphasis circuit?

No

My reason for this thread was to ask if I can make a circuit from these components similar values & add to a tube mic

Make simulation.

If we wanted NO de emphasis in the circuit say we like our capsules flat sound we could lose more components right ? BUT we would need to change circuit a little for it to work ? I guess would be like the KM84 circuit.

KM84 isn't flat circuit, there's C2 - 4pF capacitor which is also feedback path.

Schematic - marked all components in feedback path

 

[Khron]

A discussion / explanation of the KM84 circuit (scroll down a bit) - http://audioimprov.com/AudioImprov/Mics/Entries/2015/4/23_Basic_FET_Microphone_Circuits.html

Measurements of the "charge amplifier" circuit - http://audioimprov.com/AudioImprov/Mics/Entries/2015/12/20_Modding_a_BM-800_Mic.html (you'll notice it IS virtually flat so there's no real tone-shaping going on in the electronics of the KM84)

 

[ln76d]

Glad, did you use word "virtualy". Show me the results of KM84 measurements with large diaphragm capsule -  with cap and without - not audioimprov mambo-jumbo-combo circuit

Gary

Here's "flat" circuit for cardioid use - simplified, without pad and hpf - for cardioid only. Higher input impedance and no input cap.
Unfortunately senisitivity will be lower, because there's no feedback. Even with feedback sensitivity in U87 is low.

 

[Khron]

So you're saying that a Schoeps output stage, or the plain transformer output, have (such) drastic effects on the frequency response?

For what it's worth: http://taperssection.com/index.php?PHPSESSID=12d2bbabd0b95ed9e35f41cd516a192b&topic=168837.msg2098976#msg2098976

And a couple posts blow that: http://taperssection.com/index.php?PHPSESSID=12d2bbabd0b95ed9e35f41cd516a192b&topic=168837.msg2106182#msg2106182

Glad, did you use word "virtualy". Show me the results of KM84 measurements with large diaphragm capsule -  with cap and without - not audioimprov mambo-jumbo-combo circuit

 

[ln76d]

One blah... blah... vs. other blah... blah...

Did you ever tried this?

One thing!
I truly like Dave Thomas, because his really nice Guy and he have rally good knowledge but sometimes, yes - he post some  statements which are not so real.

Anyway, try, measure and we can talk

 

[Matador]

For the U87, C6 (220pF) sets the frequency/magnitude of the de-emphasis.

Check this post:  http://groupdiy.com/index.php?topic=51761.msg657996#msg657996

 

[ln76d]

A, yeah it's C6 not C7

 

[gary o]

Thanks for schematics by the way

Im still trying to understand the parts of the 87 circuit & what they do, I have de constructed my point to point verison of the U87 circuit and listened to the results

I have few more questions if anyone would care to answer...

What is the role of C3 .....

In cardiod mode are C1 C3 R3 R5 doing anything

Doesnt  C9 play a part in de emphasis..

Big thanks.

 

[ln76d]

I have few more questions if anyone would care to answer...

What is the role of C3 .....

In cardiod mode are C1 C3 R3 R5 doing anything

Doesnt  C9 play a part in de emphasis..

I forget about C9
Without deemphasis/feedback, you can remove it.
Now schematic is correct.

C1/C3/R2 are part of HPF.

R3/R5 does nothing in cardioid.

 

[gary o]

I have few more questions if anyone would care to answer...

What is the role of C3 .....

In cardiod mode are C1 C3 R3 R5 doing anything

Doesnt  C9 play a part in de emphasis..

I forget about C9
Without deemphasis/feedback, you can remove it.
Now schematic is correct.

C1/C3/R2 are part of HPF.

R3/R5 does nothing in cardioid.

This is helpful , im getting there  C 1 & C3  still in circuit when HPF not engaged tho ...

Big thanks