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Got a GR 1559-B Microphone Calibrator
Got a GR 1559-B Microphone Calibrator
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Old 20th February 2019, 03:33 PM   #51
stijena1973 is offline stijena1973
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Join Date: Nov 2012
Quote:
Originally Posted by Conrad Hoffman View Post
Those are probably the same scans as on the jptronics site. If your B version is a later one, I'd love to see it.

I'm sure I can fix you up with some adapters. Email or PM me with what you need.

I'm getting stable results now and part of the secret is having a high impedance detector with low capacitance- the cable is important. That might be why they supplied a cable with the instrument. Use low C cable and keep it short. I even pondered putting a little FET buffer inside the XLR connector, but that's probably overkill.

I'm still not positive about the best way to calibrate the internal pots and position of the wiper/small knob. It's close, but I think better is possible. My present understanding is that achieving an accurate SPL requires that the attenuator produce an exact voltage. That seems to depend on the meter calibration, which depends on the PZT tube sensitivity. OTOH, that may be why they have three pots when two would have been enough for just the meter. Still thinking about it. Remember that all GR equipment was capable of better performance than the published specs. It wouldn't surprise me that with careful setup, this might be good to .1 or .2 dB.
PZT tube sensitivity is not a critical factor here. It is just one of the three microphones. Math takes care of any variation of its sensitivity.
Potenciometer also has some finite resolution, and also beginning of the scale , but that is all sub dB...
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Old Yesterday, 06:57 PM   #52
1audio is offline 1audio  United States
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Join Date: Mar 2004
Location: SF Bay Area
Got a GR 1559-B Microphone Calibrator
I figured out the reason for difference adapters for the GR (WE, HP, Gefell) 1" (.946') microphones and the B&K mikes. The core comes from the different grids on them. The original WE640AA has a retaining ring on the front and the diaphragm is recessed. it was not designed as a measurement microphone but worked well for that application. The WE, GR, HP and Gefell microphones copied the WE grid, with a flat face with a hole pattern on it. B&K redesigned the structure with the diaphragm closer to the front surface and no retaining ring. The "famous" B&K grid was to allow off axis response to better match the grid off and has the openings down the side. The adapters for the GR type grid won't seal properly on a B&K microphone. The B&K adapters will have an o-ring to seal behind the grid. There is also a difference in the front volume between the WE and the B&K that would require spacing the microphone back a little to correct or correction values (the Omnical manual has a lot of correction values and matching confusion. . .). I have not seen this specifically documented at all except in an old writeup in the B&K journal.

Now I can move forward with some of what I want to do. I'll still need some adapters. I'll PM Conrad with what I need. Details on the 1559-6080 would be interesting since I can't find references to it.
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