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[GThomp]

Hey Guys,

Had a few questions for the experts here. Recently worked with a U67 that the owner took in for a check up and it had a mod done to it. The mod was described as "opening up the top end." The mic was returned with 2 caps removed (I have him looking for them) and one cap (the output) replaced. He has since noticed a boost in the hi's and isn't too happy with it.

Would I be correct in assuming that this could be related to the C12, C13 filter cap removal I've seen in a few discussions here? I looked for any photo based discrepancies and couldn't see any except the output cap change. Where would these caps had been located?

I attached some photos, and any and all input is greatly appreciated!
Thanks for your time guys!

Greg

[klaus]

The original C9 capacitor in your mic was replaced with a metalized film cap (yellow). That will add a bit of metallic sound. (True film & foil caps, when chosen with the appropriate capacitance and voltage rating, will be a feasible sonic alternative, but need rearranging of the mic amp's layout and components to fit, unlike the cheaper and smaller metalized cap installed in your mic.)

I assume C17, which used to be where the yellow cap was placed, was also removed. That will make the mic sound more wispy in the highs by throwing the feed-forward and feed-back of the patented design out of whack.
Removal of C12 and C13 will not add highs, and seem to be still installed in your mic.

Not addressing the complicated eq in this model as a whole, but messing with the easy stuff (like removing a capacitor) does not add up: where this was done, these mics sound too bright and unmusical.

If you really want to undo the proximity suppression circuitry, you would have to do more work, including the redesign of some sections in the mic amp.

[GThomp]

Thanks Klaus,

Sorry for the delay on my end here. I appreciate the in-depth reply. It's nice to find a place that is open and friendly to questions.

My intent is actually to undo this mod, and return it to factory conditions, before those changes. What would you recommend part wise to get this back to the starting point?

Thanks Again!

Greg

[klaus]

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

Restoring C9 to its original type will be harder: This was a dry polarized electrolytic 1.0mfd. and a rather good sounding one. Hunt around for an old Siemens/AEG metal jacket type. But measure its capacitance before installation.
I would not use one of the tiny Japanese electrolytics like Nichicon.
The original cap was installed in the space underneath the top circuit board that contains C3, C5, R3, etc.

[GThomp]

Thanks again Klaus!

I'll let you know the results!

Greg

[Uwe]

Value of the original C 9 is 0.5 uF (not 1 uF), and C 17 was not installed in the early runs of the U67. C 12 and C 13 are ceramic feed-through types for mitigating RF-interference, and with their value of 2000 pF have practically no effect on audio frequencies (their combined total  impedance at 20 kHz is around 2 kOhm or 10 to 40 times the source impedance)

[klaus]

Hello Uwe,
To my knowledge, both, 0.5 mfd and 1.0 mfd, were used as U67 coupling capacitors (see Neumann U67 schematic 1020390101, for example), with the majority using 0.5mfd, as you mentioned (and I often replace those with 1.0, to augment the mic's body).

Regarding C17: I have not seen a mic (or schematic) without it. It was an essential broadcast Braunbook requirement to attenuate highs. I would love to add a schematic to my collection that shows omission of C17.

Best,
KH

Value of the original C 9 is 0.5 uF (not 1 uF), and C 17 was not installed in the early runs of the U67. C 12 and C 13 are ceramic feed-through types for mitigating RF-interference, and with their value of 2000 pF have practically no effect on audio frequencies (their combined total  impedance at 20 kHz is around 2 kOhm or 10 to 40 times the source impedance)

[Uwe]

For example the initial U67u 00-00-00S. (Considering that 17 is the highest number for a capacitor in this model, the designation  as C17 may be an indication that is may have been an afterthought...):

[klaus]

Absolutely fascinating! Your schematic is dated one day after the first series I have (21.6.1960) which sports the C17.

Your schematic is curiously labeled "U67u", so let me speculate: In addition to the NWDR-rejected model U67 (later re-submitted as M269), Neumann initially contemplated also offering an export/U.S.- version of the U67, without the HF broadcast attenuator ("u" is a Neumann suffix often used for export power supplies, specifically for the North-American market; also note that the U67u schematic is using English language and U.S. component measurements/symbols). 

[David Bock]

The low cut positive feedback network (feedforward) has no relation to the high cut feed back network.

[klaus]

Correct. The feed-forward was a Neumann invention, part of a circuit to reduce proximity effect, while the HF attenuator feedback was a German broadcast board requirement to shield the mic's signal against multiplex stereo interference (@19kHz).

Though both systems are not directly related in their implementation, removing one (high-end attenuation) without addressing the other (low-end feed-forward) will make the mic sound imbalanced and unpleasant. That was my point.

[GThomp]

Hello again gentlemen,

Yes, this was the 0.5 mfd version of C9. This did have C17 as well. I got the bag of parts from the owner and was able to replace everything back to stock. I've attached some photos again, because anytime I work on something that costs more than I make I like to triple check my work before trying anything. I did however notice a jumper in the bag, and was wondering what this may have come from and what it could affect?

Thanks again!
Greg

[klaus]

The jumper wire above is identified on the schematic as "S2". When installed, it affects frequencies below 40Hz, attenuating 8dB @ 20Hz. For a flat low end response of the mic, down to almost 20Hz, you remove or snip the jumper.

(I prefer the mic's sound with S2 open.)

[J.J. Blair]

Interestingly, as Klaus and some of you know, the SM69 does not have the equivalent of the C17 circuit.  Did German broadcast use the SM69?  I've seen it in many videos of classical recordings. 

[klaus]

Yes, German broadcasters were the main customers of the SM69. Lots of radio plays featured that mic, in addition to live orchestral broadcasts. That's why the SM69 also needed to be "high frequency free" at 19kHz.

The SM 69 achieves the same high-cut attenuation as the U67's C17 through capacitors C11 and C12 (down ca. 4dB @16kHz). It's done a bit simpler in the SM69 than the more complicated path of the U67, but both mics drain high frequencies to ground, via a capacitor.