U67 de-emphasis network

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marcus4audio

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Apr 23, 2013
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Hi, I started this same question under the Drawing Board, but nobody reacted. Maybe mic gurus dont read there, so I repeat the question here ;) . I would like to understand how it works.  I read that its somehow related  with the transformer feedback windings, but cant figure out all the components involved :(
U67-schematic.jpg
 
Tertiary windings (White/Brown+White/Brown) are connected in series and are terminated with R18 (470 ohm).
This voltage is fed back to the backplate of the capsule.
Later C17 (80..160pF) was added. (This capacitor was not present in the first issues of the U67!)
This capacitor introduced HF feedback, also to the backplate of the capsule.

EDIT: added schematic of the U67u (Without C17!) Also a different value for R19
 

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C17, R18, R19, R20, C3 and connected transformer windings to it.
This is feedback network which provide main deemphasis eq to the backplate.
It's little bit more complicated, but here you have good point to start ;)
 
ln76d said:
C17, R18, R19, R20, C3 and connected transformer windings to it.
This is feedback network which provide main deemphasis eq to the backplate.
It's little bit more complicated, but here you have good point to start ;)
Aaa, weird :) So,  feedback AC signal is mixed with DC polarisation voltage and "reflected" to the membrane? If i understand correctly.
 
the transformer has a special winding for feedback
R19 R20 are almost negligible
first feedback C3/R18
there's another feedback through c17
they are mixed in  R3,R5,R6,R7
As it's an intricate network, i'd tweak with C17 and eventually R5 at first,  and further i'd simulate with LTspice  to fine tune it...
 
"R19 R20 are almost negligible"
I'm not so sure about that.  10K is a low value compared to R3, R4 and R5.
If the influence would have been negligible, the tertiarly winding would have been 'floating'.
(Because this is the only ground reference for this winding.)
 
As Rudolf wrote.

R19 / R20 (+R18) noott only provide testt signal injection. It works same way as R9/R10 in U87 or R105/R107 in U77.
For diy builds, where there's no need to inject signal to this node, can be used only grounded R19 = 10k. 60ohm is neglible.
Changing also resistor values you can also get different curve and use different capacitors values.
It's definately much more simple in U87 or U77 but managing few parts values you can get completely different  response.
 
granger.frederic said:
the transformer has a special winding for feedback
R19 R20 are almost negligible
first feedback C3/R18
there's another feedback through c17
they are mixed in  R3,R5,R6,R7
As it's an intricate network, i'd tweak with C17 and eventually R5 at first,  and further i'd simulate with LTspice  to fine tune it...
Thanks again. You mentioned LTspice simulation.  Did you simulated whole network or just the C17 or compete de emphasis? How you simulate capsule backplate influence to the membrane?
 
I swept the amp freq response using the calibration input back when I built a u67
https://groupdiy.com/index.php?topic=50021.msg646101#msg646101

The filter is just negative feedback as the others have posted. It is a gentle rolloff down 10dB at 15kHz
There is also a low cut with Switch 2 and Switch 1, which is also negative feedback to the capsule backplate.
 
dmp said:
I swept the amp freq response using the calibration input back when I built a u67
https://groupdiy.com/index.php?topic=50021.msg646101#msg646101

The filter is just negative feedback as the others have posted. It is a gentle rolloff down 10dB at 15kHz
There is also a low cut with Switch 2 and Switch 1, which is also negative feedback to the capsule backplate.
Got it. U talk about the instruction from the 1963 manual. Right?
 
RuudNL said:
"R19 R20 are almost negligible"
I'm not so sure about that.  10K is a low value compared to R3, R4 and R5.
If the influence would have been negligible, the tertiarly winding would have been 'floating'.
(Because this is the only ground reference for this winding.)

you right , i meaned ; not so important in  the feedback even if R19 is somewhat an actor ...

Marcus, i haven't simulated the whole circuit, i have a M367 which has nearly the same diagram and i'm happy with the mic's stock response...
you could reproduce it with LtSpice and simulate the capsule with a simple condensor, it should work enough to understand what component is doing what... but you have to know the specs of the transformer's windings...
 
[quote author=granger.frederic
Marcus, you could reproduce it with LtSpice and simulate the capsule with a simple condensor, it should work enough to understand what component is doing what...
[/quote]
Yes, I understand that 50pf will act as the capacitive source but have no clue where to tie the de emphasis network since it has no galvanic connection with the circuit. Did I misunderstood this approach :(
 
For simulation you just need connect deemphasis network to one end of capacitor and other to grid.
If you want multipattern (there's no any sense) the connecting two capcitors and deemphasis in the middle connection.
Simulation can give you little bit different results than true circuit, not always show everything exactly as it should be, but it is good point to start ;)

 
ln76d said:
For simulation you just need connect deemphasis network to one end of capacitor and other to grid.
If you want multipattern (there's no any sense) the connecting two capcitors and deemphasis in the middle connection.
Simulation can give you little bit different results than true circuit, not always show everything exactly as it should be, but it is good point to start ;)
Thanks ln76d, thanks guys. This makes sense and thanks to you I have some knowledge to play with this circuit. Cheers!
 
In this review of the U67 reissue by Klaus Heinz, this paragraph stood out:
"The 9dB high-frequency emphasis of the dual-backplate K67 capsule with its phase-inducing offset ports, and its low-frequency proximity effect have been elegantly tamed through a combination of transformer feedback, feed- forward and shunt capacitors in the amp; the 4dB drop@16kHz prevents an overly thinned-out top; the U67’s mid range ranks among the most authoritative of any mics ever made, and the low end, while more choked than in the velvety M49 or the reedy U47, still has enough heft to convince as a mic of choice even for a bass-baritone."

My understanding from the 1963 U67 manual is the de-emphasis should be -10dB at 15kHz, based on the calibration instructions. Why "4dB@16kHz"?

Does anyone know what emphasis the k67 capsule had and what the correct de-emphasis should be (via the tertiary winding and C17)? A source would be appreciated.
My understanding is Neumann created the k67 capsule with a high frequency emphasis that is compensated by a equal and opposite de-emphasis in the amplifier in order to reduce the microphone noise.
 
Velvety M49 and the reedy U47? I'd say ignore anything Claus related and you'll save yourself lots of time and energy. You got the rest right.
 
To start with, I never measured a microphone capsule that had a high frequency boost of 9 dB...
A measurement on a U67 microphone through the 'messeingang' shows -5 dB @ 15 KHz.
 
I have been experimenting with this lovely circuit,I have built the circuit from parts laying around no boads just point to point.I have been trying to understand how it works, Im useing a cheap K67 someone gave me ( I may upgrade to a neumann capsule next ) Im using a Max K Bv12 some one gave me & I have been reading there maybe something off about this transformer, at first I thought the tertary winding was dead... Question can anyone confirm or enlighten ,my tertary winding measures 12 ohms on my multimeter does tha sound right ?

I have put the mic together with a switch to switch in C17 100pf, to my humble ears its seems to make no difference, would it be that subtle ? I have since tried several values 80 pf ,300pf , i was expecting a change of sound, I did read that max BV12 users were leaving no C17 to get a decent sound, all in all even with this cheap capsule the mic sounds good Im sure it doesnt sound like a reall U67 tho.

before I threw this together I originally the Max BV12 was kput and was going to order a Moby Im just a pit confused now
 

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