797 hf schoeps de emphasis

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diegocowes90

Member
Joined
Sep 14, 2009
Messages
7
Hi I m Diego, and i am new at this forum. As i ve been reading the 797 capsule is a close replica of the k87 but it has a hf emphasis and is ussually conected to schoeps like circuits. I was wondering if someone could help me understand wich are the components in the u87 mic that reduce the high frequency emphasis http://recordist.com/ampex/schematics/neumann/u87sch.jpg
  And how could i put this filter in a schoeps circuit like this one
http://f1.grp.yahoofs.com/v1/YELxTZ_4CUz3FPFo_Vd4BqT9Ajmmk2BA-hwvGEvDPw5hJQ2mmOKAc4R5KIYordBLvMw04oFvAxyg_zW6oAt8o7VSoRvWheXDytLN/Zapnspark/GenChinaMic.pdf  borrowed from zapnspark from the micbuilders forum.
thanks
 
Is anybody else not seeing anything in the body of this post, unless you quote it?

EDIT: it's because of the links, Diego. use the
Code: 
[URL][/URL]
tags to post links.

Hi I m Diego, and i am new at this forum. As i ve been reading the 797 capsule is a close replica of the k87 but it has a hf emphasis and is ussually conected to schoeps like circuits. I was wondering if someone could help me understand wich are the components in the u87 mic that reduce the high frequency emphasis http://recordist.com/ampex/schematics/neumann/u87sch.jpg.
  And how could i put this filter in a schoeps circuit like this one
http://f1.grp.yahoofs.com/v1/YELxTZ_4CUz3FPFo_Vd4BqT9Ajmmk2BA-hwvGEvDPw5hJQ2mmOKAc4R5KIYordBLvMw04oFvAxyg_zW6oAt8o7VSoRvWheXDytLN/Zapnspark/GenChinaMic.pdf  borrowed from zapnspark from the micbuilders forum.
Click to expand...

anyway, welcome Diego!
 
Hi Diego! Welcome to the forum.

I tried both the links, but they don't seem to be working. I am glad you brought this up because I couldn't seem to find much information on it on the web. I just recently built an mk 012 circuit with HF roll off using the k67 capsule from an Apex 435. Worked great! However, the schoeps circuit is fully balanced, so it might be a bit trickier.



 
I wanted to do something like this to a studio projects C1 mic... After tracing the schematic I started to think it would be easier to make a sort of U87 circuit with a Cinemag CM2480 trafo I got from a forum member.  I think the ratio might be about right. 


 
cool Insomniaclown, so you used the circuit of the oktava mic and added the hf rollof? how did you do it, do you have the schem? i fixed the link off the u87, could you tell me which components do the roll off?

hi mitsos, do you know how much does the cinemag transformer costs?, i think it would be an option to clone the u87, but it would be more expensive and harder than implementing the hf roll off. luckly there is lot of info about the neumann mic so i don t think it would be extremely hard, but i ve read that it isn t easy to bias the fet.

 
 
I'm not sure how much the trafo costs, I bought it in the BM but don't remember, maybe $40?  It's quite small, but I know people have used mic preamp input trafos as output trafos on microphones, so that might be another option if you can find one cheaper.  Anyway, I don't think it sould be THAT hard to bias the FET, though I've not built the mic circuit yet. 

One more disclaimer on the C1 circuit... Some of the diodes marked 1N4148 are probably zeners. I did not remove them for testing, but looking at the schoeps schematics around the net, you can kind of guess at some of them.

Anyway, the following thread has some details on the HF.. looks like it's C9 in the feedback loop.

http://www.gearslutz.com/board/geekslutz-forum/350566-how-mod-u87.html
 
Yeah, it's about $40 or so. I just bought one from Cinemag a few weeks ago.

All the info about the Oktava circuit including adding the HF roll off is in this thread:
http://www.groupdiy.com/index.php?topic=41986.0

I have a couple of pics of the mic I built. Still have to finish the head basket and paint it, but it works! Sounds awesome too.

I'll check out that link in a sec. I have poured over the U87 and U87a circuit, but I am still not sure how the roll off is implemented.

Biasing the FET isn't so bad. I have some info somewhere about the voltages and procedures for that.
 
well its on then, here i ve uploaded some u87 board pics, two u87 and one u87ai schematics that will be useful to decide about the
components http://www.mediafire.com/?k7eph8cycuddrub
Wich model you prefer best? u87 or u87ai? here s acool link http://recordinghacks.com/2011/03/19/neumann-u87ai-vs-u87i/ , personally i like the old one better, and i think it should be easier to build.
Also i ve found this transfomer o ebay which is 11.5:1 do you think it should work fine? it does n t seems to be very good though.

http://cgi.ebay.com/KCM-TUBE-CONDENSER-MICROPHONE-OUTPUT-TRANSFORMER-11-5-1-/120687470730?_trksid=p5197.m7&_trkparms=algo%3DLVI%26itu%3DUCI%26otn%3D4%26po%3DLVI%26ps%3D63%26clkid%3D572400659732143314#ht_1692wt_1139
 
Thanks for all that info! Those pics are great, and the schematics are a much better resolution than I have found previously.

The U87 would be easier to build if you can find a dual back plate capsule. I honestly have no clue who makes one. The toughest part about the U87ai would be getting the oscillator working properly. Worse comes to worse, you can always build an external DC supply to get your +/- DC instead of using a DC converter board. Although I have looked at a couple of chinese designs, and they appear simpler than the Neumann one.

Found a thread on the hf roll off!
http://www.groupdiy.com/index.php?topic=20652.0
Haven't read through it yet, but C105 looks to be where it's happening.

I think that transformer would work fine for prototyping. Might even sound pretty good too. If you are doing the U87ai, you will need a transformer that has a centre tap for phantom such as the cinemag CMO 15/200CT.
 
mitsos said:
Here is the schematic I traced from the C1. It may not be 100% correct, and the switches are On-Off-On, which is not quite clear from looking at it.  The capsule is shown as a capacitor, the side toward the FET is the front, the other side is the backplate. 

http://www.twin-x.com/groupdiy/displayimage.php?album=random&cat=10439&pos=-4434
Click to expand...

Thanks for posting this schematic: it will help me to understand the LSD2 circuit, which looks similar at a first glance.  Are you SURE that S2 is connected correctly in it?  I am not sure what you mean by On-Off-On.  Do you mean break before make? According to your diagram, hichever position it is in, it will short out R15.

David
 
Hi David,

It's been a while since I traced that, I have no idea if it's correct, if you can confirm with your mic that'd be great as I don't have that exact mic anymore. I did take a couple of pics though, let's see if they attach. you can see both sides of the PCB (the bottom has a mess of wire and hot glue, but you should be able to make it out.  As for On-off-on, it's a SPDT switch with an off position in the middle (both sides of switch are open, and nothing is shorted.  BTW, the 20dB pad shorts both R19 and R15.  The 10dB pad shorts only the R15.  And for no pad, switch is in the middle position. I did check continuity of all this when I did the trace, but I'd appreciate it if you let me know of any mistakes you find.

cheers!
-d

EDIT: btw, the bottom picture is mirrored so you can see where the traces go. 
 

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A bunch of years ago, I posted the question of achieving a Neumann-ish low-pass / high-shelf response out of a Schoeps circuit over on the Micbuilders group, and one of the guys there (I forget who, but I have the document he put together, saved on my computer) concluded that it's easily achievable with a series RC in parallel with the JFET drain resistor.
 
Khron said:
A bunch of years ago, I posted the question of achieving a Neumann-ish low-pass / high-shelf response out of a Schoeps circuit over on the Micbuilders group, and one of the guys there (I forget who, but I have the document he put together, saved on my computer) concluded that it's easily achievable with a series RC in parallel with the JFET drain resistor.
Click to expand...

Do you have any circuit/schematic for De-emphasising a schoeps type circuit?
Can you please post it?

thank you so much
 
I believe I've already described the "cure" ;) But I'll attach that document I mentioned after I get back home, in a couple hours.

Here's to hoping that won't be terribly unethical or whatever...

[later edit] Well, since Richard got some of his information and inspiration from here (GroupDIY), i shouldn't be making any faux-pas by sharing the document here ::)
 

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Khron said:
I believe I've already described the "cure" ;)
Click to expand...

humm not for me at least , maybe I missed something in thread but, but I don't see any description on circuit or parts I could had to my mic to provide de-emphasis.

Thank you so much for the doc
 
I hadn't?  ???

Khron said:
A bunch of years ago, I posted the question of achieving a Neumann-ish low-pass / high-shelf response out of a Schoeps circuit over on the Micbuilders group, and one of the guys there (I forget who, but I have the document he put together, saved on my computer) concluded that it's easily achievable with a series RC in parallel with the JFET drain resistor.
Click to expand...

Whoops said:
humm not for me at least , maybe I missed something in thread but, but I don't see any description on circuit or parts I could had to my mic to provide de-emphasis.
Click to expand...

PS: Not 10000% sure how accurate it is, but i seem to have a schematic of the SP C1 in my collection. I'm a bit confused why there are two 1G resistors (where one would be enough) as well as a coupling capacitor, though ???
 

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Khron said:
A bunch of years ago, I posted the question of achieving a Neumann-ish low-pass / high-shelf response out of a Schoeps circuit over on the Micbuilders group, and one of the guys there (I forget who, but I have the document he put together, saved on my computer) concluded that it's easily achievable with a series RC in parallel with the JFET drain resistor.
Click to expand...

I read the article, thank you so much for that.
I'm not versed on RC implementation so I'm not sure how to implement it.

Can you please check if I understood correctly,is it implemented this way?
Thank you

index.php

 

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