Official U87 "AI" Booster Conversion Board - Build and Support Thread

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Matador

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I had an idea a while back based on all of the reports of people complaining about decreased gain on original U87 clones as compared to other large-diaphragm condenser designs (especially the U87AI).

One of the reasons for this is is the difference in polarization voltages between the two designs:  the U87 uses phantom power (albeit heavily filtered), and the U87AI uses a switch-mode power supply (SMPS) design to boost the 48V phantom voltage up to +60V.  This accounts for about 6dB of sensitivity difference between the two designs.  There are many other varients of this design in many Schoeps microphone designs as well.

I set out to design a small SMPS adder board that could be tacked on to existing phantom-powered mike designs to provide a full +60V polarization supply.  The intention is for this to be usable in U87 clone designs to convert them to near "U87AI" specifications, but there's nothing preventing them from being used in other designs that lack a full +60V supply as well.

Here is the schematic..  The design is based around an LM5002 SMPS chip with an integrated boost transistor.  This chip is good to over an amp, however given polarization supply really have no DC current requirements, the specs of the external parts can be greatly simplified.  This design features an input RC filter and an output LC filter to keep it both quiet as well as prevent disturbances of the circuit providing the power to the board.

Here is the layout:  my goal was to keep it to the same size as a regular 1.1" square DOA footprint, but it ended up being 1.4" square in the end.  I used all SMD components except for the low ESR caps, and I may switch the next iteration over to all SMD.

u87ai_booster_layout.jpg


This thread will hold build pics and BOM's to support this project.
 

Matador

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U87 modification instructions:
[list type=decimal]
[*]Run a ground wire to the booster board.  Any available grounding point on the U87 can be used.
[*]The booster power supply comes from the +24V Zener reference.  This is simply a wire from any of the following points:
  • The Zener side of R17
  • The positive side of the Zener itself
  • The positive side of C12
  • The Zener side of R14

[*]Lift the side of R16 (150K) that comes from the phantom tapping resistors R18 and R19.  The output of the booster is connected to this point.
[/list]
 

HellfireStudios

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Has it been tested yet? If so, what, if any, are the changes in sound?

On a side note, if a switch could allow operation at both voltages, it could replace the 10dB pad that is on most of the donor mics for the U-87 clone. It would allow for quickly A/B'ing both the vintage and more modern sounds.

-James-
 

kante1603

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HellfireStudios said:
On a side note, if a switch could allow operation at both voltages, it could replace the 10dB pad that is on most of the donor mics for the U-87 clone. It would allow for quickly A/B'ing both the vintage and more modern sounds.

-James-
Hi James,

a sudden boost of around 20vdc as polarization voltage?I doubt this will be.......hmmmmmm...."not audible",no?

Best,

Udo.
 

HellfireStudios

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kante1603 said:
HellfireStudios said:
On a side note, if a switch could allow operation at both voltages, it could replace the 10dB pad that is on most of the donor mics for the U-87 clone. It would allow for quickly A/B'ing both the vintage and more modern sounds.

-James-
Hi James,

a sudden boost of around 20vdc as polarization voltage?I doubt this will be.......hmmmmmm...."not audible",no?

Best,

Udo.

I would imagine the same protocol used for polar pattern switching would be used in this instance to prevent overloads. This method is still faster for vintage/modern comparisons than the other possible options.

-James-
 

chunger

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p355818965-4.jpg


I'm thinking the T13 transformer is quite small. . . likely there is space opposite the transformer at the bottom of the mic body to fit this.

Let's also not forget the sonic awesomeness that any yellow PCB adds to a microphone design!  ::)
 

Spencerleehorton

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Hi Guys,

this sounds excellent, im in for at least 4 boards if this pans out, i use the P-T49 peluso transformers they are nice and small and have higher output compared to others ive tried so the board could go at the bottom as already stated.
Whats the availability of these?

regards

Spence.
 

Matador

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Ok, some updates.

A photographer I am not, and I'm glad Chunger has build slots reserved.

Here is a shot of the stuffed board:

u87ai_stuffed1.jpg


The two pots I added at the last minute for experimentation.  The "top" pot (in the shot) lets you adjust the switch mode frequency.  At 52K the frequency works out to about 250kHz which seems like a good starting point.  The "bottom" pot lets one adjust the output voltage.

u87ai_stuffed2.jpg


The pots dominate the height, and I think I can shave down the height of the input RC filter caps because I used 100V parts (just to make my Mouser order easy), but 36V parts would work fine.

Here is a scope shot of the output voltage:

u87ai_scope_60V.jpg


Zoomed in and set to AC coupling:

u87ai_scope_ripple.jpg


The tiny 1mV blips are spaced at 4.28uS apart, which equates to 234kHz switch frequency.  Pretty spot on!  The "pass-band" ripple is in the 10's of uV range, and will also be subject the input filters on the U87 polarization supply.  This is conjuntion with the LC filter should make this dead quiet!

Here is a shot of the SW output of the chip:  this is the drain of the switch MOSFET.  It crashed towards ground as the inductor is charged, then "let's go" to charge the output cap network via the Schottky diode:

u87ai_scope_switch.jpg


Looks good to go!  Now I have to stuff it inside a mike!
 

Toure14

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Please don't leave us guys with the tapered body and pcb build out of this upgrade  ;D
 

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