R/E/P Community

R/E/P => Klaus Heyne's Mic Lab => Topic started by: LLL on July 19, 2019, 10:55:40 AM

Title: U87 Head amp - Proper test setup and calibration input..
Post by: LLL on July 19, 2019, 10:55:40 AM

Hi All,

Had a conversation with a collegue earlier, regarding testing the head amp of his U87.
The method he was going to use, was to connect a sig gen direct to the gate of the FET. ( with head removed)

After looking at the circuit, I would go for possibly injecting across R6 ( with head removed) ?

But, would the correct way be to remove the capsule head, and inject signal through a small cap ( say 22-47pF ??) into pin 4 on the head socket ?

Could we then do a frequency sweep to plot response of the amp, as well as test the gain of the head amp ?

Or, would it better off using the calibration input for this ?

Title: Re: U87 Head amp - Proper test setup and calibration input..
Post by: RuudNL on July 20, 2019, 05:00:36 AM

According to Neumann:

"This (the 'Messeingang') socket enable an audio frequency test signal to be applied to the amplifier input via the capsule capacitance in the same way as a corresponding change in sound pressure.
Most of the electrical data of the microphone amplifier, such as gain, frequency response, self noise,
and modulability can thus be directly tested."

This is the only way you can measure the frequency response of the amplifier including the frequency dependent feedback network. At the moment you inject a signal directly to the gate of the FET, you will measure a 'flat' frequency response, without LF and HF correction.
So during the measurement you will have to keep the capsule head in place.

Title: Re: U87 Head amp - Proper test setup and calibration input..
Post by: uwe ret on July 21, 2019, 03:47:52 PM

The U87 is equipped with a test input across R10 (560 Ω). It is best to use with a test adapter (KA87) containing two 47 pF capacitors in place of the dual capsule. In a reasonably quiet environment the K87 capsule head can be used as well, but may not yield good results for self noise measurements. Due to the very high input impedance of the FET stage with corresponding susceptibility for interference pick-up it is furthermore important to carry out any measurements in a well shielded environment.

Here are the nominal test targets for the U87:
Phantom power:  48V ±4V
Current consumption:  0.4 mA ±0.05mA
Maximum AF signal level at the test input: 320 mV
Self noise (CCIR-468-2 weighted): -109 dB (re. 0.775mV)
Self noise (A-weighted): 113 dB (re. 0.775mV)
Gain: -5 ±1 dB
Maximum output (<1% THD): 200 mV
response @ 40Hz (re. 1 kHz): -3 ±1 dB
response @ 16 kHz (re. 1 kHz): -3 ±1 dB