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Author Topic: U67 high output  (Read 453 times)

afterlifestudios

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U67 high output
« on: February 14, 2017, 12:13:11 am »

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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klaus

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Re: U67 high output
« Reply #1 on: February 14, 2017, 04:00:43 am »

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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Klaus Heyne
German Masterworks
www.GermanMasterworks.com

afterlifestudios

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Re: U67 high output
« Reply #2 on: February 14, 2017, 12:11:52 pm »

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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klaus

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Re: U67 high output
« Reply #3 on: February 14, 2017, 02:09:46 pm »

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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Klaus Heyne
German Masterworks
www.GermanMasterworks.com

uwe ret

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Re: U67 high output
« Reply #4 on: February 14, 2017, 07:22:08 pm »

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!
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