U87 and negative feedback

[Silvas]

Hello,

I´m learning a bit about this microphone, as i´m finishing reviving a very mistreated one, from early 70´s, and i paid almost nothing for it. For money reasons, i chose to reskin the capsule over a new original capsule (too much $$$ right now). After many hours troubleshooting, finally it´s working as i´ts supposed to, and begging to be modded...i´m not keeping the mic for collecting reasons but i want to use it everyday as a tool and don´t mind modding it rather than trying to keep it completely original.

I´m putting new BC264 FET on it, and for now, i´m leaving a trimmer for biasing it, which i´ll replace with an adequate resistor later. I replaced the coupling cap, but i´m looking for the nicest film cap i can put in the mic. I also removed the 220pF cap in paralell with the 1m resistor, which, iirc, disconnects the HF attenuation of the mic amp, and i actually like the 87 without it. A couple of the studio´s 87´s are modded this way (i think this mod was done by someone on gotham audio many years ago) and i like how they sound, specially for distant miking.

What i´m trying to understand is how the negative feedback works on this circuit. I would love to hear the mic with all the NFB removed, as some people say NFB in this mic adds some honkiness to the sound.

Could some mic experts here help me understand NFB on the 87 ?

Also, i´m wondering if it´s a good idea to replace both 470pF caps between capsule and FET with nicer ones, what do you think? maybe a bit of experimentation with the values here ?

Thanks so much !

 

[Martin B. Kantola]

Never played with this one, but how about disconnecting/removing C9? Too late for more thoughts today...Zzzzzz

Martin

 

[gyraf]

C3 (10pF) looks like a good candidate...

Jakob E.

 

[Silvas]

In theory, if i remove C9, gain should go up and the amp should lose a bit of headroom right ?

What about c3 ? i don´t get it...

I´ll remove C9 and see what happens.

Thanks !

 

[Silvas]

Found this on Klaus Heyne´s forum, interesting, have to try that....

I must say, I am not any closer to an answer to the original question.
Lots of fine, intelligent words by the creme de la creme, but what about the original issue: why put in a (de-coupling?) capacitor, when you don't need one?

Case in point: C4 in the original U87.
Here, a 470pf cap is placed in series between the capsule's front diaphragm and the FET's gate. (I routinely remove it in the course of my U87 modifications- it does not contribute to anything audible or measurable; all it does is introduce phase shift at a crucial point in the circuitry. (Yes, David Satz: phase shift! Audible!)

If it's not decoupling anything, if it does not eq, if its removal does not cause the collapse of the Roman Empire, what the hell is it for?
I still think that Neumann's answer to me makes the most sense: low frequencies, those close to and including DC, are curtailed. Maybe it serves as a safety valve in case there is any kind of DC (electrostatic?) charge left on the diaphragm. But if that's the case, 25 years of living without it has not got me in trouble yet.

Let's bring this baby to a satisfying conclusion!

Klaus Heyne
German Masterworks

full discusion here :

http://recforums.prosoundweb.com/index.php/t/1756/0/

 

[Silvas]

Ah, and i think i should remove either one of the (now paralell)1gig R6 or R7 if removing C4. I´ll do all of this and report later...

 

[rrs]

Yes please keep us informed.

 

[Silvas]

Hello

Removed C9, C4 and one of the 1gig resistors....

Mic is still alive in all patterns and sounds nice, at least with my ugly voice and a borrowed 30 year-old capsule. My reskinned capsules are still in Germany waiting to be shipped. I still have to test the mic with the reskinned capsule and in a real high-SPL situation. I also rechecked the bias of the temporary FET i'm using. I didn't compare the gain of the amp with a stock 87, but i'll do that later.

I'm very anxious to compare the modded 87 with a couple nice mics i have here, a 414EB with brass CK12, M149, Blueberry and of course with an stock 87 and 87ai.

I'll report my findings when i do the test...

Mods done so far :

Replaced all electros and tants with new ones
New coupling cap (C7, film)
Removed C6
Removed C4
Removed C9
Removed R7

 

[strangeandbouncy]

Hi Guys,


I have found this thread just in time! I was just about to fergal around in my u87. I have got the Innertube valve jobby, and hence my u87 guts are redundant! I understand the removal of C4, C6, C9 and R7. Can anyone please tell me what components form the high pass network? C4 i understand will work only at approx 3Hz. What causes the 40 Hz roll-off? And what do C1 and C3 do?

questions questions . . .


Any help gratefully received!


ANdyP

 

[Silvas]

Jakob suggested removing c3 but i´m not sure of it´s function. I think i can use a bigger coupling cap to move the phase shift way below audible. My reskinned capsules (done by Mr. Thiersch) are almost here, so soon i´ll find out how the mic behaves.

 

[Silvas]

Hello

My capsules finally arrived (Thanks Mr. Thiersch!)

So i put the new capsules in 2 mics, my modded one and an unmodded one but with better coupling cap and without c6 (the HF filter?). Both fitted with custom-biased BC264 fet.

My modded one is, well, amazing. I have never used a 87 that sounded so clear. It´s bright and open. And i like bright mics on vocals most of the time. Probably comparable with the blueberry or a C800 in that sense. I need to use it more and get a feel for this mic. Compared with a stock 87, the stock 87 sounds muffled, like listening thru a blanket. But this is probably due to the age of the capsule and the tantalum cap. The new capsule in the unmodded 87, the one with the better coupling cap, sounds nice, quite open, not as nice as the unmodded one, but really nice. I´m very happy with the work done by Mr Thiersch.

Now, in my modded mic, i lost the HPF (i don´t care anyway, i prefer not to use it in the 87), most probably due to the removal of c9. Otherwise, mic is working perfectly. Output is higher than a stock 87. Still don´t know about c3 (10pf). Headroom ? still have to test the mic on really high-spl vocals but for now, headroom is OK.

As my mic had a big hole on the body (what kind of mother##cker drills a hole in a U87 ????) , i covered it with a red pilot and put a ultrabright led running with less than 1ma, so i have phantom power indication, as the moving coil on the back is missing.

I´m very happy with my new mic. And i say new because it sounds totally different, almost like a new mic. Now, the transformer is another story....

 

[strangeandbouncy]

SilvaS,


HI! Thank you for sharing your modding experience. I am all the more determined to do something similar to mine, when I get some free time . . .


All the best,


ANdyP

 

[Gus]

When you remove C9 you remove the EQ so the removal of C6 does nothing. Also now the "end" of R13 that set the source resistance that the drain modulates via C9 is left hanging do you have more noise?

I would not remove C4 and R7 yes I know someone web famous posted about it. I would increase C4 to 1000pf or maybe more . I am guessing Neumann did that to protect the gate of the Jfet also there is a voltage offset that is removed (different for different fets source to ground voltage)because of the AC coupling of C4 remember when companies build stuff they often try to reduce costs so maybe there was a thought about reason for it.

I am surprised you like the sound without the EQ. A 87 capsule sounds way to harsh to me with a flat amp.

Don't believe what you read on the web and don't even believe me.

 

[Silvas]

Hello gus, very informative post, thanks !

As i said before, i like bright mics but i´m aware that sibilance will be a problem with some voices, and i know the mic will sound terrible on some people and wonderful on some other people...good for me i have many mic choices. I think that for distant pickup my mic will work wonders, have to try it.

I left R13...but the mic isn´t noisier than a stock one, should i remove R13 and hear what happen?

What about C3 (10pF) ? what´s its function?

Looking at the tube mic schemo from Analog, probably i can stick a cap in a similar way between drain and coupling cap, to tame the high end a bit? it would be a nice idea to make this switchable...

 

[Gus]

You want a switch?

use a SPDT center to the end of the .47uf that went to the drain, one outside to the drain other outside to ground. Draw it and I think it make sense

Leave the 330K in

 

[Silvas]

Got the point. Of course the switch is just an idea, not something i want...but the way you describe it is a nice idea. Thanks.

 

[Branko]

Hi all,
I'm no expert in electronics, but I find these forums an excellent learning tool.
I've read somewhere that the bypass capacitor C8 can increase the noise level of the circuit. It is there to assure the max gain that the FET can deliver, I suppose. What could happen if this one is removed?
Sorry if my question makes no sense....

Branko Neskov

 

[Gus]

I would keep C8.

The older u87s(non DC to DC converter) I looked at had source resistor values from 2.2K to 11K.  Neumann selects the source resistor so the drain is about 10VDC and the Id (drain current is the same  (22VDC - 10VDC) / 47K about .25ma)in the older battery holder models.   The gain of the stage with no source resistor bypass cap will be reduced by the higher source resistor value, so to make the gain closer with different fets the source resistor is AC bypassed.  Now the same drain current with different source resistor values will cause different offset with the charge voltage at the capsule if the 470pf is replaced with a jumper.  2.2K x.25 = about .55volts  11KL X .25ma = about 2.75Volt this affects the sensitivity by reducing it a little, it subtracts this voltage from the charge voltage on the capsule if direct coupled and it also affects the patterns.  Now maybe it does not matter that much if you are careful and select the fet for the gain and operating points you want.

  Note I posted CLOSER things change even with the same process number fets at different IDSS.  And if you read things about changing it to this or that fet number you also need to think about the different miller cap drain to gate and...  Look at the curves for more things to think about.

 

[Silvas]

When you remove C9 you remove the EQ so the removal of C6 does nothing.  Also now the "end" of R13 that set the source resistance that the drain modulates via C9 is left hanging do you have more noise?

a couple months ago, after reading this i grounded the end of the 0,47  the way you described on the switch idea...you were right, the mic was indeed noisier. All i can say is, i'm very happy with the result of the modding. Only thing pending is new (really high quality, don't want to use the term "audiophile grade") coupling cap. As i said before, the mic is BRIGHT! but i like it that way, and as i also said before, sibilance is a problem on some voices but after a good de-esser it's just the exact amount of brightness and airiness i like on a mic. I remember comparing it side by side with a blueberry, which is a really bright mic, and the 87 matched it in terms of brightness, but the blueberry sounded almost anemic compared to the modded 87...more balls and body, despite it's brightness. I'm also using this mic a lot for instrument and distant pickup and it's doing really well. Thanks for all the advice !

 

[Gus]

Try a cheaper PET Wima Panasonic etc. I often like a PET over a polypro as a coupling cap to the transformer