Author Topic: question about schoeps M221  (Read 1277 times)

adrian

  • Member
  • Posts: 126
  • Lyon France
question about schoeps M221
« on: August 08, 2012, 03:39:48 PM »
Hello!
I've got a question about my schoeps M221,
one of theme sound AMAZING!! and the other sound good but not as good as the first (less bottom end, less rich)..

so i decided to open them to check the electronic… the first (the better one) seem to be untouched but on the other the yellow cap (on the photo) had been changed,
the original one is a 0,47uF 100V,  i can't see the value on the second but when i compare the value on my multimetter i find 0,760 uF on the first and 1,9 uF on the second…
on the schoeps schematics they write 0,5 - 1 uF …



so i think i gonna change the one that had ever been replaced..
can you confirm it's an electrolytic cap?

do you have any advice before i change it? do i need to find a very special model or any caps will be ok?
« Last Edit: August 08, 2012, 03:43:33 PM by adrian »


Walrus

  • Member
  • Posts: 506
  • London, UK
Re: question about schoeps M221
« Reply #1 on: August 08, 2012, 07:23:08 PM »
THe one in the top pic is a foil cap. The bottom one appears to be an electrolytic.
I'd replace it with a foil type.
Kevin.

mad.ax

  • Member
  • Posts: 605
  • france
Re: question about schoeps M221
« Reply #2 on: August 09, 2012, 05:10:50 AM »
What Walrus said.
You definitely want a foil cap in both. (Maybe even match them if you plan to use those mics as a pair)

Also, your mics seems to be from different rev. the resistor below the transformer for instance, is 270KΩ in the top mic, and 200KΩ in the one with the electro...
You may want to check for other differences... And have a look a the transformer, which may be another version, or just be wired for 50Ω or 200Ω operation.

Axel

adrian

  • Member
  • Posts: 126
  • Lyon France
Re: question about schoeps M221
« Reply #3 on: August 09, 2012, 10:56:22 AM »
thank you for these answers!
so i found a foil cap, I gonna change and report what happens!

there also is a strange things about the PSU i bought with these mics... I have 2 voltage : 125v and 12,2v  ,
i'm confused because on the schematics the ac701 should be 4v on Vf ...
the precedent owner used these mics a lot with this PSU without problem,  today i modified it to deliver 4v on Vf.... but the mics doesn't work anymore...
i'm scared because what i read on the internet is that if Vf is different from 4v the Filament will burn...

any idea?

pasarski

  • Member
  • Posts: 280
Re: question about schoeps M221
« Reply #4 on: August 09, 2012, 11:21:17 AM »
You measured the voltages with the mic connected?

What ever your next step is, be very careful. You don't want to fry those expensive and rare tubes.

mad.ax

  • Member
  • Posts: 605
  • france
Re: question about schoeps M221
« Reply #5 on: August 09, 2012, 11:34:14 AM »
I have heard about a Schoeps PSU that was able to work either with tube or transistor mics (hence the 12V) but have yet to see one in the flesh...
Do you have a schematic? Or pics to share?

As Pasarki hinted, it is possible with a passive design, that you measure 12V unloaded. That would also explain the 125V that seems a bit on the high side...

Take care!

Axel

adrian

  • Member
  • Posts: 126
  • Lyon France
Re: question about schoeps M221
« Reply #6 on: August 09, 2012, 11:56:56 AM »
here are some picture of the PSU,
what i know is that it was build for one mic but then modified to feed 2 mics,
i don't have any schematic but we can read "4V adj" near the LM317...  but i had to add a resistor in parallel to the green one (see picture)  to be able to down the voltage to 4v  because the trimmer wasn't enought!




adrian

  • Member
  • Posts: 126
  • Lyon France
Re: question about schoeps M221
« Reply #7 on: August 09, 2012, 01:34:38 PM »
hey! i found the problem!
it was because I checked the voltage on the channel where the mic wasn't plugged... when a mic is plugged the voltage drop from 12v to 4v...
so it's ok i will now try to change the capacitor...
i will let you know what's happen!


Walrus

  • Member
  • Posts: 506
  • London, UK
Re: question about schoeps M221
« Reply #8 on: August 09, 2012, 01:48:58 PM »
You need to mow the lawn too!   ;D
Kevin.

adrian

  • Member
  • Posts: 126
  • Lyon France
Re: question about schoeps M221
« Reply #9 on: August 09, 2012, 03:41:39 PM »
Shure!

some news about the condenser....
the sound totally changed! It's much better! there still is a little difference between the 2 mics but it's ok for me, I gonna keep it as is...

thanks a lot for your help!


mad.ax

  • Member
  • Posts: 605
  • france
Re: question about schoeps M221
« Reply #10 on: August 09, 2012, 05:29:58 PM »
That isn't an original Schoeps power supply. Looks like an Alcatel or LIE-Belin, custom made for the ORTF (French Broadcast Organization) to power the M221.
I'm not devaluating your PSU at all! Those are perfectly fine, and even more robust than the Schoeps ones.
Original schematics should be silkscreened on a plate riveted on the bottom cover...

Yours have obviously been heavily modified. LM317 is not stock, nor the 3 lytics, nor the XLRs...

The LM317 need about 10mA load in order to regulate properly, that's probably why you read 12V with the mic unplugged...

As for the little difference that still remains, look at that 270K...

Axel

mjk

  • Member
  • Posts: 143
  • Eureka, CA
    • Redwood Coast Music and Studio Blade Electronics
Re: question about schoeps M221
« Reply #11 on: August 12, 2012, 12:02:22 PM »
Just to reinforce some comments.

 The electrolytic is totally wrong for the output cap. The leakage may even be responsible for your problem. You can check the plate voltage but I'd just replace the cap with a .47/100V film cap.

The 270K vs. 200K plate resistor will also make the 2 mics sound different.

Make sure the cathode bias components are the same values also (hard to see in the photos)

Make sure the transformers are both strapped for 200 ohms.

Any or all of these will change the sound of the mic.

You have already figured out that it's only relevant to measure the supply voltages with the mic(s) connected

adrian

  • Member
  • Posts: 126
  • Lyon France
Re: question about schoeps M221
« Reply #12 on: August 14, 2012, 04:08:08 AM »
thank you for these advice, :D
i will check the cathode bias components and change the plate resistor if needed, (transformers are both 200 ohm)
what is the better value for this plate resistor?

mjk

  • Member
  • Posts: 143
  • Eureka, CA
    • Redwood Coast Music and Studio Blade Electronics
Re: question about schoeps M221
« Reply #13 on: August 14, 2012, 12:56:43 PM »
thank you for these advice, :D
i will check the cathode bias components and change the plate resistor if needed, (transformers are both 200 ohm)
what is the better value for this plate resistor?

I think it will depend on whether the bias voltage is also different, hence the need to look at the bias components and circuit connections. I'm not aware of different plate resistors in these mics, but there must have been a design revision at some point which may also have had changes in the bias circuit. I'm not aware of any spec changes such as transformer ratio, etc.

Another thing to look for I remember from other AC701 mics is sometimes a feedback cap from the plate to grid.

I don't have one on hand at the moment to look at, but I'll look through my documents tomorrow at my studio and see if I can find out a little more. If the only difference is the plate resistor, I'd pick the one that sounds better.

PS as for HF loss, another potential cause is contamination around the grid circuit where the wire from the necked-down end part of the AC701 comes out and connects to the capsule post. That area is super sensitive to even fingerprints. Maybe solder flux from a previous repair, etc...
« Last Edit: August 14, 2012, 01:03:08 PM by mjk »

Wagtbtoobz

  • Member
  • Posts: 39
  • France
Re: question about schoeps M221
« Reply #14 on: August 26, 2012, 02:23:03 PM »
That isn't an original Schoeps power supply. Looks like an Alcatel or LIE-Belin, custom made for the ORTF (French Broadcast Organization) to power the M221. 


Mmmh, seems to me it's an LGT 48Vcc PSU from tape recorder  ;) The 12v /4v trimming was done to adjust length of cable /loss in heating . Avoid the LM317, a better result can me reached using LCLC filtering  :)
Tons of toobz to try, but the day is 24 hours long only ...

adrian

  • Member
  • Posts: 126
  • Lyon France
Re: question about schoeps M221
« Reply #15 on: August 26, 2012, 07:22:16 PM »
thank you for these advice,
i'm on tour now so i can't work on these mics for the moment, i will let you know what i can see when i come back,
what are you meaning about better results? the LM317 seem to works great... what could it improve?

thanks a lot! ;)

rmburrow

  • Member
  • Posts: 8
Re: question about schoeps M221
« Reply #16 on: October 23, 2013, 08:51:49 AM »
Adrian...You are correct that overvoltage or undervoltage on the AC701 heater can ruin the tube.  If you are unsure about the heater voltage from a unknown supply, you can "dummy load" the heater supply with a 40 ohm 1 watt resistor.  If you have over 4 volts across the resistor, don't connect the microphone.  3.8 to 3.9 volts is OK.

Get rid of the electrolytic coupling capacitor in the one mic.  The mylar ones don't have leakage.  Also, the value of the electrolytic could be higher or lower than the marked value.

rmburrow

  • Member
  • Posts: 8
Re: question about schoeps M221
« Reply #17 on: March 27, 2014, 10:55:47 AM »
Anyone out here have a schematic for a ELAM 931 power supply?  I can trace it out, but a genuine schematic and list of material would be great.  Neumann's NKM supply works fine on a M221....

dmp

  • Member
  • Posts: 1429
  • Madison, WI
Re: question about schoeps M221
« Reply #18 on: March 27, 2014, 02:18:47 PM »
Post schematic for this mic?


 

Related Topics

  Subject / Started by Replies Last post
1 Replies
733 Views
Last post April 08, 2008, 01:23:02 PM
by skipwave
0 Replies
667 Views
Last post May 13, 2008, 05:45:51 PM
by jensenmann
2 Replies
807 Views
Last post July 29, 2008, 03:03:12 PM
by moneypit
12 Replies
1852 Views
Last post October 12, 2012, 08:12:52 AM
by Gus
18 Replies
1878 Views
Last post June 07, 2012, 02:18:08 AM
by zephyrmic