[Peter Simonsen]

Hi Klaus,

Do you happen to know if these 2 mics (Schoeps 221B/Neumann KM56) can be powered from the same type of powersupply ???

Meaning if I only have the Neumann KM56 powersupply can I plug the Schoeps 221B to that supply without any harm done?

Thanks

Kind regards

Peter

[Jakob Erland]

Both mics are AC701-based small-diagphragme types, so this would be logical (if you are a german industrial designer)

From the schematics, they seem to be compatible. Only difference I can spot is the measurement-input on pin6 on the KM56, that is grounded on the 221B..

But you'd better doublecheck to be sure..





Jakob E.

[Peter Simonsen]

Hi Jakob,

First, thanks for the schematics..*S* very nice of you..

I knew that both power supplies are based on the the AC701 tube So I somehow figured, that they might be in the same "ballpark". I just don't know how important the "measurement input" is for the sound on the KM56. I mean, what would happen if it was shorted to GND..*S*

Kind regards

Peter

[Klaus Heyne]

Peter,
Principally both mics can be used with the same power supply, as both use the same tube. However, and this is big:[ current draw may be so different between the tubes that you may under- or worse, over-supply one of the tubes.

AC 701s are known to be finnicky and get easily damaged when hit too hard by heater juice.

Therefore, before you plug any mic into a power supply desigend for an AC701, hook up a DC voltmeter to the power supply's mic cable connectors to measure heater voltage: Turn the mic on, and don't let the heater voltage go past 4.00 Volts.

Ideally I want to calibrate to ca. 3.90VDC- 3.95VDC at the tube itself (by measuring directly at the tube filament wire, with the mic's housing off, then adjusting with the various forms of trimmers found in power supplies.)
Adjusting to ca. 4.00VDC at the power supply will roughly get you in the ball park without having to worry that the tube will be fried.

Regarding the measurement input (pin #6) on the KM56:
You can conveniently leave it open, i.e. unterminated.
I tend to shorten the calibration feed in most LD Neumann mics like the M49 to ground, but that's a different circuity than on your KM.

Kind regards,

[Peter Simonsen]

Klaus,

Thank you for your very kind, and informative reply. I will indeed tjeck the powersupply unloaded, and adjust to a maximum of 3.95 volts.

Regarding the Calibration input (#pin6). What kind of calibration is done here..??? If I may ask..*S* It is meant for injecting a test audio signal into the preamplifier, to check the mic electronics?

Thanks again Klaus.

Kind regards

Peter

[Peter Simonsen]

Jakob Erland wrote on Wed, 28 April 2004 10:21

It is ment for injecting a test audio signal into the preamplifier, to check the mic electronics. Jakob E.

Ahhh yes..thanks again Jakob Much appriciated..

Kind regards

Peter

[echorec]

Peter Simonsen wrote on Wed, 28 April 2004 09:18

Klaus, Thank you for your very kind, and informative reply. I will indeed check the powersupply unloaded, and adjust to a maximum of 3.95 volts.

Hello Peter,

You will have to check it loaded . I have a power supply for my Schoeps M221b microphones that delivers almost 10V unloaded and almost exactly 4.0V when loaded!

Gunnar Nord

[Klaus Heyne]

Point well taken. That's why I wrote:

Quote:

Turn the mic on, and don't let the heater voltage go past 4.00 Volts.

Measurements without a load (i.e. a microphone) connected to a power supply are always useless for calibrating.

[Peter Simonsen]

Gunnar, Klaus,

yes..sorry if I was a not clear enough..ofcause I meant to do the messurements loaded by the mic

Kind regards

Peter

[David Satz]

Hello, Klaus--

You wrote, "Current draw may be so different that you may under- or worse, over-power one of the tubes. AC 701 are known to be finnicky and get easily damaged when hit to hard by heater juice."

The current draw for the Schoeps M 221 B and the Neumann KM 56 are specified identically as far as the filament voltage is concerned:  4 V +/- 5% at 100 mA.  (Schoeps writes it as "4 V +/- 0.2 V" but that's the same thing, of course.) The anode current is specified by Schoeps to be "120 V +/- 5.0 V ca. 0.4 mA" while Neumann gives it as "120 V +/- 5% (0.5 mA)"--again the voltage specification is equivalent, but there is a difference in current, depending on the radius of that "ca."

Sources:  Cata0036.PDF in the Neumann InfoPool; Schoeps catalog for the M 221 B series with pages bearing both July, 1962 and March, 1967 date codes (identical data).

[Klaus Heyne]

Dear David,
In an ideal world you would be right. If both tubes in those two mics were brand new, of the same AC701sub-type, and the power supply (ies) correctly adjusted, you could indeed interchange mics without fear of negative consequences.

But this condition is rarely encountered today:
Just like an old light bulb, an old AC701 will draw more current compared to when new because of decreased resistance of the heater filament wire- about 10% or more.  

In addition, AC701 tubes came,in several versions with widely varying current draw specifications. (I have asked Oliver Archut to elaborate on the specs and variations of  the AC701 in the AC 701 thread)

If you combine a variation of these scenarios, you may end up with a tube which can be so severely stressed because its dedicated power supply was not properly calibrated, that its life span will be cut in half or worse.

Bottom line: The idealized operating conditions for an AC701 you describe are unfortunately not encountered in the real world, and tube damage can result if you don't carefully calibrate each power supply for each specific AC 701 tube

Kind regards,

[Oliver Archut]

Hello David,

in the times when you could order a AC701(k) from your closest Telefunken service center for about DM12.50 the hair splitting about the voltage wasn't needed.
Today only a hand full of the approx. 200000 AC701 are still in useable condition and replacement is hard to find, so it is easier to make sure your working AC701 will last for a long time.

An AC701 is a specialty tube, like no other ever made, as mentioned in the AC701 noise tread. Telefunken made four different versions of AC701/701k, the first one with a purple dot was the 00 or prototype series, tubes with the last two digits of the production code "01" have a 120mA filament, the "02" 104mA and the last "03" have just 100 mA.

Most historic power sups just have a simple regulator or even a crude adjust pot, so if you hook up a mic with a 03 to a 01 type tube you slowly overheat. Also the current over lifetime will change and if you replace the tube with a NOS, the current will be lower as the old burned out and we are back to toasting again.

An AC701 based mic should be checked adjusted every 3 to 4 month, if proper adjusted the tube lifetime is nearly infinity, I have AC701 with more than 100000h plus that still work nearly as good as the first day....

Best regards,

[Klaus Heyne]

You adjust the power supply's heater voltage output for the specific tube the power supply will be connected to, as I described earlier in this thread.