Modding Neumann U87Ai to U87i voicing and headroom

[RuudNL]

the lower resistance shunts more of the overall feedback to ground, increasing the overall output.

Your comment triggered my curiosity!
So, as a test, I have put a 1.2 K.ohm resistor in parallel with the existing 6.8 K.ohm resistor. (R9 in the U87i)
Because I also assumed that this would increase the gain of the ciruit, but it did not!
Anyway no audible difference. Surprising...

 

[MagnetoSound]

Your comment triggered my curiosity!
So, as a test, I have put a 1.2 K.ohm resistor in parallel with the existing 6.8 K.ohm resistor. (R9 in the U87i)
Because I also assumed that this would increase the gain of the ciruit, but it did not!
Anyway no audible difference. Surprising...

Yes, that is surprising. Did it have any effect on the low end curve? And did you reduce C3 to 3.3pF as well?

 

[RuudNL]

I left the small feedback capacitor as it was. (I won't do too much soldering in a microphone.)
I even went one step further, and shorted R9 and R10 completely.
The influence at 1 KHz was about 0.5 dB (more), but of course this also disabled the de-emphasis.

 

[MagnetoSound]

Very interesting, Ruud. I wonder if C3 at 10pF is the dominant feedback path here. It looks as if the R8/C6/C5/R9+10 network is doing a more subtle tone shaping than I thought.

That would explain the fact that when I tried reducing C6 in my own mic from 220pF to 160pF some years ago the effect was actually quite marginal.

The early U87i used a 3.3pF feedback capacitor, in fact between gate and drain.
Later this value was increased to 10 pF.
I suppose the value Andreas Grosser used in his modification will be in this range. (3...10 pF)
Of course this can also be done in the U87ai. And if you don't like it, it is easy to undo...

Edit: I just checked the effect of an added drain-gate capacitor in my own U87ai.
A 10 pF capacitor reduced the output in my case a little more than 9 dB. (So: 9 dB more headroom!)
Since we are dealing here with a very high impedance, a good leak free capacitor should be used!

9dB is a lot! I bet that with C3 at 3.3pF the tone network would then be doing more of the donkey work, and if you tried your experiment in this condition perhaps the changes might become more effective.

Of course the Ai has no C3, so the tinkering of the network will be all the more crucial. I might try and find time to experiment more with my 87s, it’s something I haven’t done for a long time now - I just love them the way they are!

 

[oscarhuang]

I have always liked the design of the u87, I think it is a genius design. I recently rebuilt a u87 with arienne's capsule, the 16th version, the output transformer is sowter, I like the sound so much.

 

[mean_street]

I have always liked the design of the u87, I think it is a genius design. I recently rebuilt a u87 with arienne's capsule, the 16th version, the output transformer is sowter, I like the sound so much.

Might go that way too, read many good things about the capsules.

After recording some vocals yesterday, I found my mic to be a touch too mid heavy and with the added bottom it sounded like a phase issue? Liked it on drums, but on vocals the stock configuration was superior. Might change it back to stock to compare.

 

[Gus]

That’s a fairly accurate description of what my vintage U87s sound like, and how they respond to internal padding.

Don’t forget that the bass roll off cap (C104 on your schematic, C5 in the non-A circuit) is different too - and in fact has evolved quite significantly through the years.

View attachment 111563

ran some sims the posted network has less gain

 

[Migs 31]

The early U87i used a 3.3pF feedback capacitor, in fact between gate and drain.
Later this value was increased to 10 pF.
I suppose the value Andreas Grosser used in his modification will be in this range. (3...10 pF)
Of course this can also be done in the U87ai. And if you don't like it, it is easy to undo...

Edit: I just checked the effect of an added drain-gate capacitor in my own U87ai.
A 10 pF capacitor reduced the output in my case a little more than 9 dB. (So: 9 dB more headroom!)
Since we are dealing here with a very high impedance, a good leak free capacitor should be used!

I'm tempted to "alter" my U87Ai... and reduce T1 saturation at high levels.
To see how it sounds.
When you say: " I just checked the effect of an added drain-gate capacitor in my own U87ai.A 10 pF capacitor reduced the output..."

A Gate-Drain capacitor, as in the early U87i. Have I understood this correctly?
Please see pic, noted in red on U87Ai schemo below. Second pic is U87i.

Regards
M

 

[GeorgeToledo]

What are you recording with it that leads to loss of headroom? Remember the specified max thd is just 1% for the named spl.

But the Neumann capsules are still the best around and highly regarded clones like Stam and Serrano both didn’t to much for me. And if I factor in the costs for high quality parts like mic body, pcbs, transformer, components and capsule and factor in the time that I need to research, build, troubleshoot and tune, the total wouldn’t be far away from a used 87Ai, at least in Germany or Europe.

I can see you've been following the wrong forums, i hope you will stick around here to gain some valuable info. Stam and such are nowhere near highly regarded here, and we have managed to find some more than good (maybe even better?) than Neumann's capsules. In any case more consistent than the originals.

What capsules are available that are more consistent than Neumann capsules? I don’t remember that one.

 

[GeorgeToledo]

The main thing that makes the 87A circuit sound nastier than the 87 is that the output transformer is under specced for the increased output level the head amp gives, and the distortion from it is subjectively bad sounding. It’s not necessarily popular opinion but I wouldn’t mind them keeping the 87A circuit and putting a better transformer in there.

 

[soliloqueen]

The main thing that makes the 87A circuit sound nastier than the 87 is that the output transformer is under specced for the increased output level the head amp gives, and the distortion from it is subjectively bad sounding. It’s not necessarily popular opinion but I wouldn’t mind them keeping the 87A circuit and putting a better transformer in there.

the trafo in the ai is comically dinky. agree that it is a big portion of the problem. at least it was IME when I swept the head amp of the Ai w/o a capsule.

i agree most of all with the opinion from a page ago. anatomical differences in the capsule are responsible for a lot of the difference in sound.

 

[kingkorg]

What capsules are available that are more consistent than Neumann capsules? I don’t remember that one.

Maybe newer Neumann ones, all the vintage ones I've measured were all over the place. I have to admit that could have been due to aging, or intentional variations in the design.

There are many modern manufacturers that make mics where any two can be used as matched pair. 797 audio being one of least expensive where every capsule comes with anechoic measurement, detailed specs. All the capsules I received from them were identical, and my measurements matched their graphs. Which is why 797 capsules are used in modern modeling mics where consistency is the key.

Drawback with their capsules is they are not Neumann clones, they have more high end and need more de-emphasis, which in the end results in less noise, and some even consider that improvement on the original design. Others complain about excessive HF boost.

 

[Recording Engineer]

the trafo in the ai is comically dinky. agree that it is a big portion of the problem. at least it was IME when I swept the head amp of the Ai w/o a capsule.

mmm… Love me Marik’s Samar CT-10! Completely solves the dinky problem. Ha!

 

[LevinGuitar]

Does reducing the gain stage by lowering the polarization or with that small C3 cap helps the transformer job lowering the THD?

 

[Migs 31]

Does reducing the gain stage by lowering the polarization or with that small C3 cap helps the transformer job lowering the THD?

That is what I am trying to find out. And specially: how does it sound to the ear?!?

Let's try again: " I just checked the effect of an added drain-gate capacitor in my own U87ai.
A 10 pF capacitor reduced the output..."

Putting a Gate-Drain capacitor on the U87Ai, as in the early U87i. Have I understood this correctly?

Regards
M

 

[LevinGuitar]

Call me crazy but I do hear quite difference in the response, in a KM84 build with 6.5 trafo, using a small cap made the mic sound nicer, less bright and better low end. The same mic with a 9.5 trafo (similar size and quality) and without that cap did not sounded as good. The compromise is the gain, I would not use 10pf, but smaller value.

 

[Gus]

a very good link

https://repforums.prosoundweb.com/index.php/topic,36194.msg531542.html#msg531542
IMO the 87a in the first post in this thread has a better gain stage circuit so lowering the capsule voltage makes the most sense to me. There are different ways of lowering the capsule voltage.
The SN is better in the a and if you need to pad the microphone the SPL should be high enough not to matter
It has a better circuit before the gain stage voltage regulator Zener.

 

[living sounds]

Call me crazy but I do hear quite difference in the response, in a KM84 build with 6.5 trafo, using a small cap made the mic sound nicer, less bright and better low end. The same mic with a 9.5 trafo (similar size and quality) and without that cap did not sounded as good. The compromise is the gain, I would not use 10pf, but smaller value.

I think Andreas Grosser used a 3.3pf cap, that's what I used when I modded clones to resemble my U87 last year, too.

 

[SparkleBear]

the trafo in the ai is comically dinky. agree that it is a big portion of the problem. at least it was IME when I swept the head amp of the Ai w/o a capsule.

What would you recommend to mitigate this? Is there a drop in trafo swap you could recommend. I used my u87a body tube, bell and head basket for a u67 build with custom PCB 1:1 with neumann circuit and layout... But im thinking of sprucing up my AI headamp. I noticed when looking at some of klaus' mods that he has a custom AMI transformer in there. Also looks like he is using a phatty highV output decoupling capacitor too. Noticed the 1uf in the AI is a tiny little electrolytic. Perhaps swapping this could make an improvement? There seem to be a few stray PS caps and a removed impedance converter board.

Guts of a KH u87ai mod on left / my stock u87ai on the right

Top side observations: Mod removes the impedance converter board. Maybe fet is beneath hidden by the big caps or on the back side of the pcb. Big film cap i assume to be the decoupling cap. Extra added styro caps. Looks like r108 removed and looks like there is a 60pf cap in that position. R101 is removed. The blank c107 pads look like populated with a styro. Styro cap added at the rt test point thing (not clear).

.

DC-DC side observations: Looks like C26 was swapped out for a film cap. R1 position bridged and both high z resistors connected to those 200k (i think) resistors. Not sure of the scheme though. Big MP cap coming from the top r1 pad


Not sure if this comparison is fruitful for any mod progress but i found it to be facinating.

I do want to do something with these AI guts that is useful.

Maybe as simple as following some of the swaps in this thread including the Gate-Drain capacitor and swapping the value on the feedback caps like @RuudNL pointed out.

Ive never had the chance to listen to a KH mod. But im excited to try out some of the stuff yall wise wizards are suggesting. Thanks for sharing such wonderful wisdom.

Cheers friends

PS '80s u87 guts just for fun.