Adding de emphasis to tube mic circuits

[gary o]

I have good sounding cheap K67 capsule I wanna try in some mics I have made, I made a Neumann U87 i type circuit with a switch to switch on or off de emphasis Im using a real neumann transformer by the way,  I have a real neumann K87 to compare with, I should mention Im only wiring these capsules in cardiod mode... comparing the capsules in the same mic I actually like the china cheap K67 without de emphasis its less bright than the neumann & maybe a little too dull with the de emphasis switched in so i plan to change capacitor  value in the de emphasis circuit to fine tune the de emphasis.......Im also now thinking of trying the china capsule in Elam M49 and other tube mic circuits I have made.....

My question is instead of getting a transformer like in a U67 with its tertary feedback winding can add a similar set of components like in the the U87 ??? to a tube mic design..... I just want to tame the china capsules top alittle as its not as bright as the neumann capsule...

Big thanks Gary O.

 

[ln76d]

Put capacitor (styroflex, MKP, MKT etc.) from plate to ground. 100pF - 330pF for example.
You can use switch (rotary for example) put few values on it (or 3-4pc. of 100pF - for switching in parallel connection), two wires and your ears

 

[micaddict]

Suggested capacitor values are very useful and yes, do use your ears.

But somehow the actual frequencies to shoot for are less often discussed (although searching should provide some results).
A Swedish member on the Gearslutz forum wrote this last week:

12dB LPF at around 10k for chinese SDCs and 6.5k for chinese LDCs. This basically simulates the HF deemphasis stage that the manufacturers left out to keep the retail price low.

The thread concerned equalizing after the fact (recording) BTW.

 

[Khron]

Is that supposed to be a 12dB/octave low-pass? Seems a bit "drastic to me"; might be a bit much even if it were a -12dB shelf, but... I guess "the proof is in the pudding"

 

[micaddict]

Yeah, might be a little steep.

But I hope we get more reactions regarding frequency and filter values, if the OP permits.

 

[micaddict]

Hey, I did a search and this thread came up first. Guess who started it. 

http://groupdiy.com/index.php?topic=54388.0

I have a feeling Ruud will chime in this time, too.
If not, I will kindly ask him to.


Henk

 

[Khron]

"Unfortunately", the OP was asking about implementing such a de-emphasis circuit into a tube mic

Although, to be fair, i don't see why you couldn't use that chain of components in a tube mic - just swap the JFET with a triode in the schematic, and it's done

 

[micaddict]

Yes, I realize tube microphones make this a new question. But like you say, it shouldn't be too different.

Anyway, not all K67-type capsules are the same. So ideally (!) we would need a frequency plot of given capsule.

I'll hush now, because I don't want to hijack Gary's thread.

 

[Khron]

A while ago, i decided to redraw the U87 circuit in LTspice, precisely to see how the frequency plot changes, when altering the values of some of the feedback circuit components. It's no big deal, but if anyone has use for it, i can attach it here

 

[gary o]

Yes, I realize tube microphones make this a new question. But like you say, it shouldn't be too different.

Anyway, not all K67-type capsules are the same. So ideally (!) we would need a frequency plot of given capsule.

I'll hush now, because I don't want to hijack Gary's thread.

Please no need to hush please expand , everybody I want to learn...... its all useful & thanks.......Yes my original question is can I add say the U87 de emph chain of components in a similar way in a tube mic ? Khron seem to think will be ok Im bit worried about the voltage in the tube mics ( I should just add that Im not the best at understanding basic electronics) but I will make stuff and experiment , I already have switchable caps in my DIY Ela mic, Im guessing its not the best way to do it because its cheap & simple not saying doesnt work but neumann went to great lengths with the U67s transformer & network of feedback components must have cost a bit.... also the 87s components.

I agree the capsules sound different for my experiments I have these capsules to play with & compare a real neumann K87 a Tim cambel a violet vin67 a terrible RK47 and this cheap K67 ( I cant believe how good it sounds) getting back to the de emphasis part comparing the real K87 to the cheap K67 in my U87i circuit with real neumann tran & switchable de emphasis, the K87 is brighter to my surprise unusable for my vocals without de emphasis in my opinion no surprise really, surprise is I think china K67 is usable with no de emph...... however would like to tame the top a tad..... so it goes with de emph engaged the mic sounds a little duller more vintage I guess some would say and still very usable.....

Experiments to try

1. Change de emph cap values for the china capsule in the U87i mic
2. Try china K67 in Ela mic, maybe try more plate to ground cap values, compare to tim cambel capsule in that mic                                                3.  Maybe work out safe way to add U87 de emph circuit to Ela mic ......
4. Put china K67 in MKU47 I have add plate to grid cap to that add U87 feed back to that too, Im intersted to hear the effect of the headbasket (U47 clone) I have noticed the bat crossing the capsules acts as de esser....
                                           

 

[Khron]

One small hint: punctuation - it makes sentences SO much easier to read & understand...

 

[ln76d]

Mentioned by me, plate to ground capacitor (100pF - 330pF) will do the best job for you.
Adaptation electronic u87 deemphasis circuit to tube mike, probably is possible, but it can change too much.
In my opinion - not worth!!!
Please remember, that less parts in the citcuit (especially capacitors) usual better sound.
Try this one cap and you will see what's the difference. Switching testing method is the best, you can hear difference in A/B comparison without spending time to slodering/desoldering parts!

ELA M250/251 had this capacitor, 100pf - from plate to ground, to damp HF

Fro U87 deemphasis, marked red, four capacitors are important.
C4 - input cap.
C5 - HPF deemphasis cap.
C6 - LPF deemphasis cap.
C7 - output cap.

All depends in used capsule.

C4 - the best no cap at all.
C5 - try 100nF (AKG use this value), then you can try to lower value of C7 - for example 680nF.
C5 - also 100p-300p range. For extremely boosted hf response use 270p-300p. For less booste 100p-150p.
In chinese microphones also do another (let call it) "mod", put 1G resistor to ground as in neumann circuit.

 

[micaddict]

OK then. 

Apart from the U67 obviously (for which the K67 was designed) there are more (high end) tube mics that have a K67-style capsule. But without the de-emphasis. These are very (!) bright indeed, but don't sound as aggressive as the cheap Chinese mics. No doubt the quality of the capsule and electronics has to do with this.
One classic example is the Sony C800G. Another is the Manley Reference Cardioid. Again, those are BRIGHT. But on some voices/sources they really shine.

Then there's Brauner. Those (mostly tube) microphones all have a K67-style capsule, too. Dirk wants no negative feedback, let alone tertiairy windings on trannies and such. And AFAIK there's no de-emphasis to speak of.
The capsules used to be custom made by Haun and are now made in house, from what I understand. Both the custom Haun and the newer,  in house capsules seem to be tamed from the get go. Still, Brauner mics are on the bright side. But not nearly as much as the Sony and the Manley.

As a matter of fact, there are many other tube mics with a K67-style capsule. But I'll limit myself to those modern classics.

My point being, even without de-emphasis, you can build an interesting K67/tube microphone.
It need not be aggressive, harsh or abrasive.
But bright it will be.

 

[ln76d]

"But bright it will be" young Jedi

 

[granger.frederic]

my advice on chinese capsules  is to lower the voltage capsule polarisation near 45V, it wil tam the top end without negative feedback and extend the low end as well
the sensitivity will be lower thus the electronic must be as quiet as possible
regards
Fred

 

[ln76d]

My advice is, don't doooooo that
In my opinion, most of chinese capsules (especially k67)  which i tested worked much worse on polarisation voltages below 50V (in fet circuits especially). There's a lot of other options to tweak response, than lowering polarisation voltage which also increases noise in the circuit. In many tube microphones there's option to put potentiometer instead resistor divider and find by ear or measurements best fitting polarisation voltage.

 

[gary o]

my advice on chinese capsules  is to lower the voltage capsule polarisation near 45V, it wil tam the top end without negative feedback and extend the low end as well
the sensitivity will be lower thus the electronic must be as quiet as possible
regards
Fred

My advice is, don't doooooo that
In my opinion, most of chinese capsules (especially k67)  which i tested worked much worse on polarisation voltages below 50V (in fet circuits especially). There's a lot of other options to tweak response, than lowering polarisation voltage which also increases noise in the circuit. In many tube microphones there's option to put potentiometer instead resistor divider and find by ear or measurements best fitting polarisation voltage.

Interesting Isnt the vintage U87 i polarisation voltage around 46V ish, lower than the 60V in the more modern Ai U87.
I read several peoples opinions that that is the reason its sounds better, better low end more musical sound due to more relaxed diaphram, Im aware or the noise penalty.I was thinking maybe thats why this crappier capsule sounds good in the 87i circuit.

A while back when I got the cheap K67 I tried it in a valve mic with 60V polarisation voltage & plate to ground caps installed & it didnt sound good, to me all my DIY tube mics have more detail in the high end than any of my FET mics I thought this well contributing to making it sound shrill.

Im going to re try the cheap K67 in a Ela mic I have as I said I already have the plate ground caps installed I have a 3 way switch cant remember the values I those, think one value same as K49 other same as original Ela s cap value & maybe something inbetween, i will check.

I have put the head basket and capsule back on my U87Ai for now to use as my control mic others to reference to.

Im testing the cheap K67 in an old AKG solid tube basket for now. K67  has less high end without de emphasis than the 87Ai it sounds surprisingly good so maybe I dont need de emphasis anyway. more output & better signal to noise..... all good.

Im going to transplant cheap k67 onto Ela tube mic now

 

[micaddict]

Do keep us posted. 

What Chinese mic did you rescue the capsule from?

 

[afdobbert]

The Sony's sound comes from a (mostly) untammed K67 capsule, I beleive
there is actually a 150 pf filter cap in the circuit but I haven't looked at the schematic in a bit.

The compression of the tube in that circuit is a big part of why the brighness still sounds good.
The high end really hits that tube and takes on a nice articulate but squished sound.

A good cheap and fun way to do something similar to this, which I've been meaning to try
would be to build a copy of the Lomo 19a9 curcuit (its a pretty damn simple circuit)
and let the compression of the EF95 tube smooth out the high end of the capsule. I bet the
35 mm K67 Chugger sells in his store (which has a slightly less aggressive high end bump) would
sound really awesome in that setup.

 

[Khron]

I'm not so sure about that 150p cap in the C-800G

http://www.freeinfosociety.com/electronics/schematics/audio/pictures/sonyc800g.gif