R/E/P Community

Please login or register.

Login with username, password and session length
Advanced search  

Pages: [1] 2  All   Go Down

Author Topic: Different NU67 - different output levels?  (Read 5311 times)

Koen

  • Newbie
  • *
  • Offline Offline
  • Posts: 33
  • Real Full Name: Koen Gisenesd
Different NU67 - different output levels?
« on: September 26, 2015, 09:41:54 am »

A thing that bugs me regarding NU67's:
I employ 3 U67's and consequently use 3 original Neumann NU67 power supplies. Two of them are of the same (later?) type, with serial n°s in the 10000's. Now: when recording, using the very same microphone, but switching between the power supplies, the output differs by more the 3-4 db. I do have a pair that come quite close, but another one is really way quieter. How is this possible? And, is it at all possible to 'match' them a bit more?
Thanks for your input,

Koen.
Logged

klaus

  • Moderator
  • Hero Member
  • *****
  • Offline Offline
  • Posts: 1660
Re: Different NU67 - different output levels?
« Reply #1 on: September 26, 2015, 01:02:58 pm »

Start by measuring the supply voltages of all three mics with their respective power supplies (you should always keep mic and power supply as a permanent team, and not switch around: different tubes can have quite a bit differing current draw, and once the power supply is calibrated for the specific mic, it should stay with it).

Then substitute the mic/power supply combo that gave you lower gain, and measure that combo. You may find that the mic is voltage starved.
Let us know what you find!
Logged
Klaus Heyne
German Masterworks®
www.GermanMasterworks.com

Koen

  • Newbie
  • *
  • Offline Offline
  • Posts: 33
  • Real Full Name: Koen Gisenesd
Re: Different NU67 - different output levels?
« Reply #2 on: September 26, 2015, 01:30:00 pm »

Thanks,
Will do later this week and post back.
210V polarisation voltage and 6,15V filament, right?
Greetings,

Koen.
Logged

klaus

  • Moderator
  • Hero Member
  • *****
  • Offline Offline
  • Posts: 1660
Re: Different NU67 - different output levels?
« Reply #3 on: September 26, 2015, 01:31:26 pm »

Right! And measured at the mic!
Logged
Klaus Heyne
German Masterworks®
www.GermanMasterworks.com

Katerin Stone

  • Newbie
  • *
  • Offline Offline
  • Posts: 3
  • Real Full Name: Katerin Stone
Re: Different NU67 - different output levels?
« Reply #4 on: September 29, 2015, 02:28:37 am »

A thing that bugs me regarding NU67's:
I employ 3 U67's and consequently use 3 original Neumann NU67 power supplies. Two of them are of the same (later?) type, with serial n°s in the 10000's. Now: when recording, using the very same microphone, but switching between the power supplies, the output differs by more than 3-4 db. I do have a pair that comes quite close, but another one is really way quieter. How is this possible? And, is it at all possible to 'match' them a bit more?
I would just suggest you to start by measuring supply voltages from three mics, and then substitute combo and measure it as well.
____________
Katerin, professional consultant on  essay trust 
Logged

klaus

  • Moderator
  • Hero Member
  • *****
  • Offline Offline
  • Posts: 1660
Re: Different NU67 - different output levels?
« Reply #5 on: September 29, 2015, 06:32:15 am »

I cannot emphasize enough to :

1. calibrate the power supply voltage to nominal 210VDC/6.15VDC, which adjusts the supply to a specific mic's (actually its tube's) current draw.

2. not switch supplies after having done the calibration, in order to maximize tube life, minimize diaphragm stress, and optimize the mic's noise floor
Logged
Klaus Heyne
German Masterworks®
www.GermanMasterworks.com

Koen

  • Newbie
  • *
  • Offline Offline
  • Posts: 33
  • Real Full Name: Koen Gisenesd
Re: Different NU67 - different output levels?
« Reply #6 on: October 01, 2015, 03:36:56 pm »

Will do, thank you!
Logged

J.J. Blair

  • Full Member
  • ***
  • Offline Offline
  • Posts: 130
Re: Different NU67 - different output levels?
« Reply #7 on: October 02, 2015, 09:00:16 am »

I concur with Klaus.  All my PSUs are assigned and color coded to go with a respective mic, and calibrated to that mic.  You should never mix and match, unless you want to shorten the life of your tubes.
Logged

Koen

  • Newbie
  • *
  • Offline Offline
  • Posts: 33
  • Real Full Name: Koen Gisenesd
Re: Different NU67 - different output levels?
« Reply #8 on: October 12, 2015, 12:30:23 pm »

Mystery solved. Well, sort of.
The polarisation voltage of said NU67 supply measures 299V (!) (under load), the filament heater 6,11V.
B+ is way off in other words...
Also, it's a early serial number and the schematic and layout look somewhat different from the later ones (this is type N-084-03, my other ones are N-084-07). I can't find a schematic for this version unfortunately. Any ideas?
regards,

K.A. Gisen.
Logged

Koen

  • Newbie
  • *
  • Offline Offline
  • Posts: 33
  • Real Full Name: Koen Gisenesd
Re: Different NU67 - different output levels?
« Reply #9 on: October 12, 2015, 12:38:29 pm »

Another question:
at this voltage, wouldn't it be wise to make some sort of 'dummy' load instead of attaching the microphone itself?
I mean, couldn't these voltages damage the capsule?
If at all possible, anyone any clues for me on how to device such a load?
Thanks,

K.A.G.
Logged

Uwe

  • Jr. Member
  • **
  • Offline Offline
  • Posts: 67
Re: Different NU67 - different output levels?
« Reply #10 on: October 12, 2015, 07:15:34 pm »

Please check R7 (160 kΩ / ¼ W), which is typically located across the final filter capacitor C4 (25 µF). I suspect it may be open. With this resistor and a load resistor of 240 kΩ / ¼ W (equivalent to a normal U 67) the voltages against ground should be within 5 % as follows:

at the junction of the rectifier Gr1, first electrolytic filter capacitor C1 and R1 = +243 V;
at the junction of R1, second electrolytic filter capacitor C2 and R3 = +225 V;
and at the junction of R2, third and final filter capacitor C3 and R7 = +210 V (= across R7 and equivalent load or U 67).
Logged

Koen

  • Newbie
  • *
  • Offline Offline
  • Posts: 33
  • Real Full Name: Koen Gisenesd
Re: Different NU67 - different output levels?
« Reply #11 on: October 13, 2015, 07:26:34 am »

Many thanks Uwe,
will try asap!

Regards,

Koen.
Logged

Koen

  • Newbie
  • *
  • Offline Offline
  • Posts: 33
  • Real Full Name: Koen Gisenesd
Re: Different NU67 - different output levels?
« Reply #12 on: October 13, 2015, 07:41:30 am »

Uwe, if you're reading this:
I'm afraid this PSU has a different schematic and layout than the later ones (as I pointed out, this is a N-084-03) the codes of the large caps for example already differ from your schematic, eg. C5, C6 and C7 are 500uF here and C1, C2 & C3 are 25 uF cans. C4 is, as far as I can see, a Wima 0,1 axial cap. Confusing without a good schematic...
Logged

Kai

  • Sr. Member
  • ****
  • Offline Offline
  • Posts: 314
Re: Different NU67 - different output levels?
« Reply #13 on: October 13, 2015, 09:16:15 am »

Uwe, if you're reading this:
I'm afraid this PSU has a different schematic and layout than the later ones ...Confusing without a good schematic...
HEADLINE:
NEVER MANIPULATE OR TOUCH ANYTHING INSIDE OF SUCH A PSU WHILE THE POWER PLUG IS CONNECTED.
EVEN AFTER DISCONNECT LETHAL VOLTAGES ARE STILL PRESENT IN THE CHARGED CAPACITORS FOR A LONGER TIME.
SPECIALLY IF THE THING IS BROKEN THESE VOLTAGES MAY NOT BE DISCHARGED FOR HOURS.
IF YOU ARE NOT SKILLED GET SOMEONE DOING IT FOR YOU!!!

First check if the line voltage selector is set to the correct value.
Even check the secondary voltages at the transformer to confirm.

The 25uF caps are the high voltage (B+) filter stages.

POSSIBLE REPAIR:
Check all resistors in-circuit (without desoldering) for if the value is in a reasonable range (about what their color coding shows).
Usually they are either working or fully open, so the faulty one should be be locatable.
If this doesn't help, check the voltage of each filter step with no load condition.
Voltage should significantly drop from stage to stage. Where it doesn't you've found the faulty stage.
If the voltages are all the same the output shunt R is open (like Uwe mentioned).
If this doesn't help, unsolder the resitors at one end, one by one, and check their value.

POSSIBLE MOD:
It might even be that someone or Neumann has modified the PSU for whatever reason.
The different model No. point's to Neumann doing it, maybe for a different mic version.
You would need to replace some parts to get the correct output voltage then.
The basic circuit is quite simple, shouldn't be hard to (re-)modify it to N-084-07 circuit version.
Do you see something unusual like a transistor that might have been inserted for regulation?
Throw out everything (in the high voltage path only) except the caps and put in the 6K8, 8K2 and 100K resistors that are standard.
Play with the 1st stage's R's value (6K8) if the voltage is still a bit off under load (240K Ohm refering to Uwe) condition.
Not a big deal.

Regards
Kai
Logged

Koen

  • Newbie
  • *
  • Offline Offline
  • Posts: 33
  • Real Full Name: Koen Gisenesd
Re: Different NU67 - different output levels?
« Reply #14 on: October 13, 2015, 10:14:09 am »

Thanks Kai,
this is very helpful!
Logged
Pages: [1] 2  All   Go Up