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[ilcaccillo]

Hi,
I find my U67 too dark sometimes would like to try different values in the place of C17.
What is the voltage rating you recommend for this cap?

Also for the C10 Styrene cap I can see there's different values depending n the schematics,
I have a schematic where C10 is 270pf and other it is 500pf.
Whats the difference between C10 being 270pf or 500pf?
Also whats the voltage rating you recommend for C10 also?

Thank you so much
Best regards

[klaus]

You can play with C17 values, all the way down to nothing, but it may unbalance the high end.
C10 differed in value through the years, because initially the mic came with a -14dB cut in the attenuated position, that was later reduced to a more user-friendly -10dB.

I would not play with the attenuation values, or even use the attenuation switch, if you can help it, as it reduces the s/n by the number of the gain reduction achieved: 10dB or 14dB.
It's a primitive way to attenuate a mic.

All caps should be rated 400 VDC.

[ilcaccillo]

Thank you so much for your reply Klaus.

[ilcaccillo]

All caps should be rated 400 VDC.

Hi Klaus,
When you mentioned 400V were you talking about the voltage rating for C10 and C17 or the other caps in the circuit?

Is that at least for C17 you advised in another thread a 160V capacitor,
here is what you suggested:

“ Missing C17 replacement is easy: get a 80-100pf polystyrene with a 160VDC rating and install it between its original solder points (see schematic). ”


https://repforums.prosoundweb.com/index.php/topic,36297.0.html


So can C17 be rated at only 160V instead of 400?
How about C10?

Thank you so much

[klaus]

C10 = 160V is certainly fine, as a max of 60VDC flows through it. 
C17 Nominally should be nominally 250V rated, but I have seen plenty of original C17 with a 160V rating.

But if you want to sleep well at night, use 250V.

[ilcaccillo]

Thank you Klaus and thanks for sharing your experience

[Kai]

... or even use the attenuation switch, if you can help it, as it reduces the s/n by the number of the gain reduction achieved: 10dB or 14dB.

It’s not quite as bad.
Due to the lower shunt impedance at the amplifier’s input, it’s self-noise is reduced by several dB.

In praxi the attenuator is used with very loud sources to avoid mic’s internal amp’s overload (NOT overloading the following mic-pre), the maximum possible S/N ratio may still be reached.

To avoid overloading the following mic-pre’s damping pad should be used.

[klaus]

I respectfully disagree: reducing gain at/before the FET gate does not remove any amplifier self-noise past the FET stage. At equal gain settings between padded and unpadded U87, the mic's signal-to-noise ratio is audibly reduced, around 6dB if not more.

Perform the test yourself and listen.

[RadarDoug2]

So are you saying that with the pad inserted, the signal to noise is worse, with the following preamplifier set for the same gain?
So the noise level increases with pad in? That makes no sense. Since the pad introduces a capacitor in parallel with the capsule, the input impedance to the fet amp drops. This should definitely result in slightly lower noise.

[klaus]

While the s/n up to the FET stage remains the same with the -10dB pad in, the noise of the amp POST FET stage remains unchanged, which, due to the now reduced s results in an overall worse s/n for the whole mic.

[gtoledo3]

The original poster was writing about U67. I keep looking through the thread and trying to figure out why the topic changed to U87. Was there a deleted post before your comment about U87, Klaus?

[klaus]

Thanks for your alert observation.
I modified my responses to include the U87, as both models use the same method of gain reduction at the mic amp's input via capacitor between diaphragm and backplate.

[RadarDoug2]

The whole reason for the pad is that you are putting more signal in to the mike. So assuming your signal goes up 10 dB, the output level of the mike will be the same, and the signal to noise will be the same. If you put the same level of signal in to the mike, then you leave the pad out.

[gtoledo3]

Thanks for your alert observation.
I modified my responses to include the U87, as both models use the same method of gain reduction at the mic amp's input via capacitor between diaphragm and backplate.

Ahh, ok, nice.

One thoughtful reason that people might choose to use the pad in U87 regardless, is that the small output transformer under performs and exhibits non linear overload at high output level. This leads to the results being a bit nasty sounding in some contexts.

This may be the case with U67 as well but I haven’t delved into the issue.

[David Satz]

RadarDoug, the standard definition of signal-to-noise ratio for a studio microphone is based on the voltage that it puts out at an SPL of 1 Pa (about 94 dB SPL). It's not like in consumer audio, where the reference level is the highest voltage that the device can put out. Because of this special definition, signal-to-noise ratios for studio microphones may seem rather low--e.g. only 68 dB for the U 87 A in the cardioid setting, as shown in the attached, abbreviated specification list. But if you add the equivalent noise level to the s/n ratio you get 94 dB, the reference level used for this specification. It's clearly nowhere near the highest SPL that the mike can handle--and that's completely intentional.

If your reference is a fixed SPL, then using the pad on a microphone will detract from its s/n ratio by around the same amount as the attenuation of the pad. Klaus is presumably speaking from that standpoint. You, on the other hand, seem to assume that the reference sound pressure level will be higher when the pad is on--so to your mind, the s/n ratio isn't being reduced by the pad. But according to the standard, the reference level for measuring the s/n ratio is always the same--it's whatever the microphone puts out when a 1 kHz, 94 dB SPL tone is played into it. Throw the pad switch and that signal level goes down 10 dB while the noise floor of the mike stays approximately constant.

Thus the pad is really costing you 10 dB, and shouldn't be used unless it's really needed to prevent the microphone's electronics from being overloaded by the output from its own capsule. If there's no risk of such overload but a "downstream" component such as a mixer input or preamp is threatened with overload from the microphone's output, then a resistive pad should be used at the input of the downstream device; using the pad switch on the microphone in that situation will add 10 dB of unnecessary noise to the channel, whereas a resistive pad will not.