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R/E/P => Klaus Heyne's Mic Lab => Topic started by: GThomp on July 10, 2013, 10:10:06 pm

Title: U67: Add More Top End?
Post by: GThomp on July 10, 2013, 10:10:06 pm
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
Title: Re: U67 filter caps...
Post by: klaus on July 10, 2013, 10:15:48 pm
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.
Title: Re: U67: Add More Top End?
Post by: GThomp on July 16, 2013, 01:40:34 pm
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
Title: Re: U67: Add More Top End?
Post by: klaus on July 16, 2013, 02:40:29 pm
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.
Title: Re: U67: Add More Top End?
Post by: GThomp on July 16, 2013, 03:39:12 pm
Thanks again Klaus!

I'll let you know the results!

Greg
Title: Re: U67: Add More Top End?
Post by: Uwe on July 22, 2013, 03:38:39 pm
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)
Title: Re: U67: Add More Top End?
Post by: klaus on July 25, 2013, 06:17:11 pm
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)
Title: Re: U67: Add More Top End?
Post by: Uwe on July 26, 2013, 10:00:01 am
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...):
Title: Re: U67: Add More Top End?
Post by: klaus on July 26, 2013, 03:16:30 pm
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). 
Title: Re: U67: Add More Top End?
Post by: David Bock on July 26, 2013, 04:17:41 pm
The low cut positive feedback network (feedforward) has no relation to the high cut feed back network.
Title: Re: U67: Add More Top End?
Post by: klaus on July 26, 2013, 05:33:41 pm
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.
Title: Re: U67: Add More Top End?
Post by: GThomp on July 29, 2013, 06:18:09 pm
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
Title: Re: U67: Add More Top End?
Post by: klaus on July 29, 2013, 07:11:53 pm
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.)
Title: Re: U67: Add More Top End?
Post by: J.J. Blair on July 30, 2013, 01:09:44 am
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. 
Title: Re: U67: Add More Top End?
Post by: klaus on July 30, 2013, 01:26:51 am
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.
Title: Re: U67: Add More Top End?
Post by: David Bock on August 02, 2013, 06:24:19 pm
Are you saying the U67 High cut is achieved by a HF shunt to ground?
Title: Re: U67: Add More Top End?
Post by: klaus on August 03, 2013, 12:29:27 am
C17 taps off plate, goes via 530 Ohms to ground. That's how I read it.
Title: Re: U67: Add More Top End?
Post by: David Bock on August 03, 2013, 06:39:27 pm
C17 connects to  resistor string *R18 (470r) R19 (10K)  R20 (60r)* Rt=10530r then to ground, so, an RF shunt to ground not an audio band shunt (80-160pF + 10.5k series R= almost invisible to audio). C17 also AC couples through the capsule backplate to form the  HF cut feedback network with the active element (the tube). Like the positive feedback network that forms the LF cut, this HF cut network has it's values calculated from the gain of the tube.
Title: Re: U67: Add More Top End?
Post by: klaus on August 04, 2013, 03:11:11 pm
You are right. I overlooked the 10KΩ, which, without the feedback, would shift the HF cut way to the limits of hearing. Feedback therefore trumps ground shunt.
Thanks.
Title: Re: U67: Add More Top End?
Post by: Uwe on August 05, 2013, 09:02:14 am
The U67 circuit would be easier to understand, if the HF-feedback were redrawn with C17 as it effectively functions, namely connected between the tube's anode to the junction of R2/C2 and the capsule back electrode.
Title: Re: U67: Add More Top End?
Post by: David Bock on August 06, 2013, 12:12:45 pm
Agreed it can be a confusing schematic. I had to re-draw it to simplify and see the multiple feedback operations.
Along those same likes, there was a mod by SPA which inverted the HF cut feedback circuit to create a HF boost on that mic.
Title: Re: U67: Add More Top End?
Post by: J.J. Blair on August 06, 2013, 07:13:03 pm
I bet that boost sounded great, when combined with a 1Ķ capsule.   :o
Title: Re: U67: Add More Top End?
Post by: klaus on August 06, 2013, 07:18:28 pm
You are kidding, right?
Title: Re: U67: Add More Top End?
Post by: David Bock on August 06, 2013, 07:19:43 pm
not me.
Title: Re: U67: Add More Top End?
Post by: J.J. Blair on August 08, 2013, 10:03:20 am
I was certainly kidding.
Title: Re: U67: Add More Top End?
Post by: Jim Williams on August 08, 2013, 11:02:24 am
Quite a few forgetable pop recordings have been done with those SPA 1 um capsule modified mics. Those solved some of the "please add more top end" problems of 1990 era recordings. I suspect they were selected to overcome Dolby SR losses, but maybe not.
Title: Re: U67: Add More Top End?
Post by: GThomp on August 10, 2013, 02:14:38 pm
Hey Klaus and gang,

Thanks for the overwhelming amount of help and information provided on this thread. I am happy to report that the mic has been restored to it's original condition (minus the S-2 jumper as per your recommendation Klaus), and I'm happy to report it is sounding as good as ever!

Thanks again for all the help, and mind blowing knowledge!

Greg
Title: Re: U67: Add More Top End?
Post by: klaus on August 10, 2013, 04:12:51 pm
You are welcome!
Title: Re: U67: Add More Top End?
Post by: David Bock on August 10, 2013, 08:21:42 pm
Quite a few forgetable pop recordings have been done with those SPA 1 um capsule modified mics. Those solved some of the "please add more top end" problems of 1990 era recordings. I suspect they were selected to overcome Dolby SR losses, but maybe not.
To be fair there were a lot more 3 micron mods than one micron mods. My theorum on the selection of "enhanced brightness" mics, which started in the mid 1980's, is that the combination of thousands of passes of the slave tape to record and comp the vocal (loss of high end for those of you who came up in the post-tape era)  combined with the omnipresent cocaine and the infinite record budget led to selection of said mics, especially if it was "going to be a hit" (as well perhaps then you would be willing to give up a few points on the record for the privilege of renting an aphex aural exciter- remember that!?) . Possibly a good dose of browbeating salesmanship as well.  I mean, who wouldn't want the newest advancements in technology in their old mics? IIR tube gear, U47's (let alone all the broken ribbons left unused in the mic locker) etc had fallen out of favor during that time.
Title: Re: U67: Add More Top End?
Post by: Je55 on February 12, 2015, 05:03:15 am
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.

Are you sure that you can use a 160v Styroflex for c17? The Neumann data sheet calls for 500v
Title: Re: U67: Add More Top End?
Post by: Jim Williams on February 12, 2015, 10:54:38 am
Polystyrene film caps are harder to get these days since all but one of the polystyrene film makers closed shop. The only Amercan brand still making them is Reliable Capacitors in Santa Fe Springs, CA. Their limit is 1000 pf. Some asian caps are still available.

In a pinch a good sub would be a quality polypropylene film and foil cap like the WIMA FKP-2 series. Those come in 1000 volt ratings, 100, 68, 47 and 33 pf are available values.
Title: Re: U67: Add More Top End?
Post by: klaus on February 12, 2015, 01:08:10 pm
Are you sure that you can use a 160v Styroflex for c17? The Neumann data sheet calls for 500v
Yes, I am sure.
Title: Re: U67: Add More Top End?
Post by: soapfoot on February 12, 2015, 01:09:51 pm
Polystyrene film caps are harder to get these days since all but one of the polystyrene film makers closed shop. The only Amercan brand still making them is Reliable Capacitors in Santa Fe Springs, CA. Their limit is 1000 pf. Some asian caps are still available.

In a pinch a good sub would be a quality polypropylene film and foil cap like the WIMA FKP-2 series. Those come in 1000 volt ratings, 100, 68, 47 and 33 pf are available values.

Do you have an opinion on the performance of teflon-dielectric capacitors as a substitute for polystyrene? The obvious downside being cost.
Title: Re: U67: Add More Top End?
Post by: klaus on February 12, 2015, 03:19:18 pm
Bass Lim's Rel-Cap made me some Teflon film & foil capacitors in the 1mfd range, and, hoping that higher cost would tranlate to higher utility, I did not find them particularly musical or better in resolution or other relevant parameters than tin film & foil, or most intriguing, but very hard to make and find: copper film & foil.

It is also worth repeating what I wrote a while ago: different capacitances demand different materials in the audio chain. For example, I've found that in the capacitance range of 10.000pf or lower nothing beats styrene. But styrene's advantages and sonic attraction diminishes with values above that.
Title: Re: U67: Add More Top End?
Post by: Je55 on February 12, 2015, 09:32:20 pm
Cheers Jim, I think i'm going to stick with styroflex though. Its just good to know that I can use a lower voltage cap, they are much more readily available in lower voltages than 500v and 630v

Jess
Title: Re: U67: Add More Top End?
Post by: Je55 on February 12, 2015, 09:34:27 pm
Thanks Klaus,

While this is a u67 thread, is there a reason that Neumann didn't use a choke in their psu? I'm leaning to believe a massive part of the u47 ultra smooth sound is due to the chokes in the psu.

I may add one or two in my 67 psu between C4 and R7.

Cheers

Jess
Title: Re: U67: Add More Top End?
Post by: klaus on February 13, 2015, 02:35:58 am
Neumann's NG 1 power supplies for U47 did feature chokes (through 1955), then dropped them, and within a couple of years, dropped chokes from all its tube mic power supplies.
I guess, they thought it overkill, and not worth the extra expenses?
Title: Re: U67: Add More Top End?
Post by: Brian Campbell on March 03, 2018, 12:28:45 pm
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...):
I'm curious about the omission of C-17 as shown in the schematic posted by Uwe (U67u).
My understanding is that removing C-17 was a common modification and seems rather controversial.
What effect does this have on the sound?
The other differences in these two schematics are R4, R5  and R19.
Title: Re: U67: Add More Top End?
Post by: Brian Campbell on March 03, 2018, 12:33:26 pm
here is schematic posted by Uwe
Title: Re: U67: Add More Top End?
Post by: klaus on March 03, 2018, 01:16:52 pm
Reply #8 goes a bit deeper into the history of the (short lived) omission of C17.

The mic is too bright without C17. Removing C17 does not "restore" full frequency response of capsule or mic amp to a balanced output. The capacitor removes the 9dB boost at 9KHz the capsule would otherwise produce.



Title: Re: U67: Add More Top End?
Post by: Brian Campbell on March 04, 2018, 01:39:09 pm
Given that schematic exits, I wonder if any mics were delivered without C17?
Title: Re: U67: Add More Top End?
Post by: klaus on March 04, 2018, 04:24:13 pm
I have a client's U67 prototype here- the U60, of which ca. 30 were made before the name was changed to U67. I will check out whether C17 is present in the prototype and report back.
Title: Re: U67: Add More Top End?
Post by: AusTex64 on March 05, 2018, 11:38:35 pm
Polystyrene film caps are harder to get these days since all but one of the polystyrene film makers closed shop. The only Amercan brand still making them is Reliable Capacitors in Santa Fe Springs, CA. Their limit is 1000 pf. Some asian caps are still available.

In a pinch a good sub would be a quality polypropylene film and foil cap like the WIMA FKP-2 series. Those come in 1000 volt ratings, 100, 68, 47 and 33 pf are available values.

RelCap will still make polystyrere caps up to 3uf. http://www.reliablecapacitors.com/audiocap_5.html
I have used these caps in mic preamps with excellent results.
Title: Re: U67: Add More Top End?
Post by: klaus on March 06, 2018, 12:40:42 am
... not according to the page you cited.

I commissioned Bass Lim, in charge at Relcap, to roll me a few 1.0 mfd styrenes, so I speak from direct experience:
Not only do styrene caps sound lousy as coupling caps in values above 10.000mfd, they also would be so large at 1.0mdf (the standard in condenser mics) that they would not fit inside a U67.

Tin film & foil is the way to go, and even there, the specialty under-sized cap Relcap makes for me would not fit a U67 unless I reposition the tube and its socket (PITA).
Title: Re: U67: Add More Top End?
Post by: Kai on March 06, 2018, 06:31:09 am
Wima FKP3 are available in much lower voltages and therefore smaller sizes.
Depending on the purpose these may be a good option, even paralleled for higher capacitance values.
They have the lowest distortion of all available (I built my rig now and can measure down to -154 dB or 0.000002 %).
Selected specimen come down to this figure, even unselected they're not too much worse.
The downside: they are really hard to find if you don't want to buy really large amounts.

Of course, if you want a cap to deliver a special "sound" others might be the choice.
Title: Re: U67: Add More Top End?
Post by: soapfoot on March 06, 2018, 07:29:51 am
Wima FKP3 are available in much lower voltages and therefore smaller sizes.
Depending on the purpose these may be a good option, even paralleled for higher capacitance values.
They have the lowest distortion of all available (I built my rig now and can measure down to -154 dB or 0.000002 %).
Selected specimen come down to this figure, even unselected they're not too much worse.
The downside: they are really hard to find if you don't want to buy really large amounts.

Of course, if you want a cap to deliver a special "sound" others might be the choice.

Many values seem to be available from Mouser with no minimum order:

https://www.mouser.com/WIMA/Film-Capacitors/FKP3-Series/_/N-1z0zldhZ9x371Z1z0ye2a
Title: Re: U67: Add More Top End?
Post by: Kai on March 06, 2018, 08:21:50 am
The lower voltage/higher capacity versions are hard to find, unfortunately.
I had to oder from Indonesia, via Ebay lately.
They are worth the hassle, from a technical point of view if you're heading for minimum signal deformation.
I can't comment on the sound, I never A/B 'd them to something else.

But:
The use of technically superior parts does not necessarily lead into better sound or even better technical results.
Sometimes compensatoric effects are built into something, and constructions must be seen as a whole.
Advanced mic's like the ones from Neumann were not just put together with what was lying around, they're well thought out and thoroughly tested, and every of the few parts carefully selected I guess.
Title: Re: U67: Add More Top End?
Post by: klaus on March 06, 2018, 02:12:43 pm
Wima FKP3 are available (...)
Depending on the purpose these may be a good option, even paralleled for higher capacitance values.

My experience has not been good with paralleled coupling caps. I first was THINKING (rather than listening!) that this made perfect sense, but abandoned this method years before WonderCap was launched (which was later bought out by Rel-Cap). I tested Rel-Cap's WonderCaps as well, and found them to be an overpriced naked emperor. Resonances MORE smear and no clarity.

When I noticed (late 1990s) that other modifiers copied my paralleling (1mfd film and foil with 10.000pfd styrene) I went back and listened and have since ripped out that combo from all of my earlier mods.

Here is a good explanation why paralleling caps is not working so well:
http://conradhoffman.com/hoffman_effect.htm
Title: Re: U67: Add More Top End?
Post by: Kai on March 06, 2018, 08:21:27 pm
The mentioned theory is based on combining very different caps with high and low losses.
I could observe effects that can be explained by this when building my ultra low distortions measurement rig (besides finding out that a perfercly clean audio path does not exist).

If you parallel same sized very low loss caps, e.g. 2 x 0.22 uF or 4 x 0.15 uF to replace a 0.5 uF cap, this does not apply, and I could not observe any negative effects in that configuration.
When you choose 0.15 or 0.22 || 0.33 uF in that situation (to hit the nominal value of 0.5 uF more exact) it needs some practical "Golden Ear" evaluation if the different resonant frequencies of the 2 caps (far outside the audio range) does come into the way of perfect sound.

BTW: one has to be careful if a theory is based on models only, not practical evaluation.
Even the author states that his model does not work with caps of same parameters (division by 0 problem).
Title: Re: U67: Add More Top End?
Post by: AusTex64 on March 06, 2018, 10:51:48 pm
... not according to the page you cited.

I commissioned Bass Lim, in charge at Relcap, to roll me a few 1.0 mfd styrenes, so I speak from direct experience:
Not only do styrene caps sound lousy as coupling caps in values above 10.000mfd, they also would be so large at 1.0mdf (the standard in condenser mics) that they would not fit inside a U67.

Tin film & foil is the way to go, and even there, the specialty under-sized cap Relcap makes for me would not fit a U67 unless I reposition the tube and its socket (PITA).

Perhaps I misread this: Capacitance Values:   .01 to 3.0 mfd

I should have been more clear - I was referring to mic preamps, not buffers/impedance converters inside mics.

Also agree about bypassing caps - I think itís BS.

Agreed that they would be too large for mics, and take your word on the sound. Youíve certainly done more critical listening of coupling caps in mics than I. Appreciate your observations!
Title: Re: U67: Add More Top End?
Post by: Jim Williams on March 07, 2018, 11:48:57 am
As much as I like my Audio Precision and the other test gear here, it does not tell the full story. In fact, it's very incomplete.

Sure, we can test down to the nano volt but it does not measure sonic colors nor qualify them. Similar measuring stuff doesn't sound similar in many cases. Noise gain tricks and FFT's will not show those colors. THD+noise and other distortion tests like CCIF IMD will not show the colors either. It will show related errors if measurable.

Dynamic response is also very hard to test. Violent musical waveforms would be the best test stymuli but that technology isn't developed. Higher output current devices also deliver a greater depth of low end response, something that also doesn't show up on audio tests until you get into power dissapation under load tests.

I like WIMA's scoopage design, it does overcome some capacitor errors but I find the brand to have sonic issues as well. The same values in Rel Cap RT caps have better resolution here. Exotic silver foil caps have even better resolution. I have also had Bass Lim make me their copper foil polyprop caps, I also found those wanting. They are available from Rel Cap if you want to try them out.

I do these tests here against my current ultra high quality capacitor base standard, a 1/2" piece of Kimber Black Pearls solid silver wire.

Yes, I am a bit anti-capacitor here as I spend much of my bench time on recording consoles and as such removing them by the thousands. The superior sonics of a well designed direct coupled audio system cannot be overstated. Most of my mics here are down to one capacitor in the signal path.

I also add selected bypass caps to larger el caps and some film caps. Not only myself but my customers hear an improved transient reponse and they always tell me the general sonics improve more with them than without. In this case, the customer's are right.
Title: Re: U67: Add More Top End?
Post by: Kai on March 08, 2018, 02:34:24 am
As much as I like my Audio Precision .... Noise gain tricks and FFT's will not show those colors..
Audio Precision isn't capable measuring the low distortions from high quality caps without the mentioned trick.
My system can, just straight analysis of an extremly clean generator signal.

This does not mean it can replace listening tests, but it's a good starting point.

...I do these tests here against my current ultra high quality capacitor base standard, a 1/2" piece of Kimber Black Pearls solid silver wire.
Can you explain that further, how can this extremly small cap (few pF) resulting from a 13 mm piece of cable serve as a reference for bigger caps?

Yes, I am a bit anti-capacitor here as I spend much of my bench time on recording consoles and as such removing them by the thousands.
In the aera of large scale analog consoles DC blocking caps were used all over in an inefficient attempt to avoid clicks of the mechanical switches and DC noise of controls.
Very often polarized electrolytics were used out of specs (for AC), making the situation even worse. Most of those are obsolete.
Title: Re: U67: Add More Top End?
Post by: Jim Williams on March 08, 2018, 11:21:04 am
The Kimber Black Pearls solid core silver wire is 26 awg, .0009% pure and coated with a 10,000V rated teflon dialectric. It's around $50 per foot.

It is my quality standard for audio electron transfer and easily beats any capacitor ever made for transparency. It is also used inside my mics to connect capsules to jfet gates or tube grids. I also use their 19 awg silver stranded wire here. That is used to connect mic pcb's to XLR connectors. I don't wear jewelry, I listen through it.

Millions of electrolytic capacitors were used in analog recording equipment, usually tossed in without checking the DC offset polarity. That was a cheap way out from designers that used them to stop the DC offsets from 072 and 5534 opamps. One will pay a sonic price for using them.

Modern opamps don't have those DC offsest in many circuits. Then those caps can be removed. Alternative techniques are DC servos and offset trimpots, very rare in audio gear.
Title: Re: U67: Add More Top End?
Post by: Brian Campbell on June 04, 2018, 08:04:21 pm
In the U67 schematics,  C17 is shown as variable 80-160pf. Is this to allow for variables in capsule, transformer or both?
Also what would be the range of frequency change with these values?
Title: Re: U67: Add More Top End?
Post by: klaus on June 04, 2018, 09:19:07 pm
As the test input only measures amplifier performance, without taking the variables of a hand-made capsule into consideration, it was probably the sum of all amplifier components-passive, tube, transformer- affecting the high frequencies. Hence a simple way was found to keep the highs within specs during final testing and adjusting.

You asked the same question on another forum, where at least one person speculated that the U67's transformer was so complicated that an adjustment for its variations in HF transmission alone would be necessary. I have my doubts as all C17 installed in U67 of a given period were always of the same value: either 80pf or 100pf (nothing above 100pf was ever installed in U67 I have seen).

Both values are so close, that I rather believe C17 was another way to show the German broadcast brown-book folks that the mic can be quickly adapted to their strict HF cut requirements. As the feature was retained in the broadcast version of the U67, it might have helped in the end to pass the mic. (All of this is just more speculation, I am afraid)
Title: Re: U67: Add More Top End?
Post by: Brian Campbell on June 05, 2018, 10:27:57 am
thank you
Title: Re: U67: Add More Top End?
Post by: Brian Campbell on June 13, 2018, 11:05:05 am
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...):

Interesting to note in that schematic that the heater supply is +6.3V on pin 4 not -6.3V.


(Pardon me for bringing this thread up again but I find this mic fascinating)
Title: Re: U67: Add More Top End?
Post by: klaus on June 13, 2018, 01:38:21 pm
A clerical mistake, not a design change: the second value on the schematic, a bit closer to the tube filament, shows the correct negative voltage.