R/E/P Community
R/E/P => Klaus Heyne's Mic Lab => Topic started by: afterlifestudios on February 14, 2017, 12:13:11 AM
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When I purchased my u67, I measured it against my 87's for noise etc. Good noise floor results, and the output levels between them were quite matched.
But on a suspicious nagging feeling I measured them again today (about a year later) and the u67 is about 6db HOTTER output than the two 87's (both 87's have the same output level.)
I swapped tubes in the 67 with no change in output level.
I measured PSU voltages and got 6.25v heater and 208v B+.
I swapped to my spare PSU (slightly less ideal PSU voltages, 5.8v
and 220v) with no change in output level.
Any thoughts?
I don't have other u67 cables or capsules to test with.
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Let's make sure to compare these mics properly before suspecting something may be wrong:
Do you have the original U87 (pre-1986) or the newer U87Ai?
If these are pre-"A" models, what transformer output strapping is set? Are the pads engaged?
If these are "A" models, their output should be roughly equal to a U67 strapped for 200 Ohms.
If they are pre- "A" a 50Ω strapping and/or pads engaged can easily explain the output difference.
Let's start there and then we talk.
But one thing is for sure: your U67 did not secretly go to the refrigerator late at night, gaining 6dB! Mics never increase output over time. If they would, we would have a new form of cheap energy production.
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Thanks Klaus. Indeed. These are 87 (non A) versions. They are both strapped for 200ohm output impedance. But I neglected to check the pad resistors. I just looked now, and the resistors (R20 and R21) are not bridged. So that explains my 6db, correct?
Thanks again.
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That explains a good portion of it, correct. However, even after bridging the pad resistors, the U67 may still be ca. 1-2 dB hotter than the U87, due to other design differences.
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Assuming the transformers in both models are wired for 200Ω output impedance, then the nominal output levels for a SPL of 1 Pa (94 dBspl) are 8 mV for the U 87 and 20 mV for the U 67 (cardioid), 11 mV (omni) and 14 mV (∞ or figure eight). With other words, the U 67 will have 3 dB higher output level in omni, 8 dB more in cardioid, and 5 dB more in figure eight than the U 87!