U67 de-emphasis network
« on: March 13, 2018, 05:08:48 AM »
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 :(


RuudNL

Re: U67 de-emphasis network
« Reply #1 on: March 13, 2018, 08:12:28 AM »
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
« Last Edit: March 13, 2018, 01:00:58 PM by RuudNL »
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ln76d

Re: U67 de-emphasis network
« Reply #2 on: March 13, 2018, 08:14:38 AM »
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 ;)

Re: U67 de-emphasis network
« Reply #3 on: March 13, 2018, 08:26:28 AM »
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.

RuudNL

Re: U67 de-emphasis network
« Reply #4 on: March 13, 2018, 08:28:42 AM »
That's right! The AC is 'superimposed' on the DC polarisation voltage.
There is a solution for every problem!

http://www.vansteenisaudio.nl

Re: U67 de-emphasis network
« Reply #5 on: March 13, 2018, 08:33:32 AM »
That's right! The AC is 'superimposed' on the DC polarisation voltage.
Thanks guys. Very interesting approach. I learned something new :) thanks a ton!

Re: U67 de-emphasis network
« Reply #6 on: March 13, 2018, 10:18:11 AM »
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...
« Last Edit: March 13, 2018, 10:26:08 AM by granger.frederic »

RuudNL

Re: U67 de-emphasis network
« Reply #7 on: March 13, 2018, 10:42:46 AM »
"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.)
There is a solution for every problem!

http://www.vansteenisaudio.nl

ln76d

Re: U67 de-emphasis network
« Reply #8 on: March 13, 2018, 12:40:45 PM »
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.

Re: U67 de-emphasis network
« Reply #9 on: March 13, 2018, 12:58:11 PM »
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?


dmp

Re: U67 de-emphasis network
« Reply #10 on: March 13, 2018, 02:24:09 PM »
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.

Re: U67 de-emphasis network
« Reply #11 on: March 13, 2018, 03:15:14 PM »
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?

Re: U67 de-emphasis network
« Reply #12 on: March 13, 2018, 06:26:26 PM »
"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...
« Last Edit: March 13, 2018, 06:31:39 PM by granger.frederic »

Re: U67 de-emphasis network
« Reply #13 on: March 13, 2018, 06:32:48 PM »
[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 :(

ln76d

Re: U67 de-emphasis network
« Reply #14 on: March 13, 2018, 07:11:58 PM »
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 ;)


Re: U67 de-emphasis network
« Reply #15 on: March 13, 2018, 08:47:05 PM »
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!