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

[chunger]

No way this will fit into a taper body setup unless we integrate a mic pcb. . . The syt5s are nice because they have so much internal volume.  Prototypes for the gen 2s are done btw. . . Should have good news in the next 2 days.

 

[Matador]

This also is too wide to fit a C12 type body.

I took a chance on this first design:  I will typically have 5 PCB's made on a first prototype before I even attempt to mass produce, however it appears to work properly on the first try.   I still need to get it integrated into a mike however to be sure.  It was severely over-designed since this was my first polarization design using SMPS (I've done many other SMPS supplies however), just never at these high impedances.

I am currently stuffing a second PCB that eliminates the last RC filter.  If this works, I can cut the board area by half.  If I eliminate the adjustable resistors as well I think I can shave this down to the bare essentials, and have it fit the other body styles.  I have the opportunity to do another large-ish quantity panel so I might try to have a smaller version compatible with the narrow taper U87 bodies soon.

 

[sr1200]

Correct me if im wrong, but the issue of the low output was solved after we figured out that the AMI transformers weren't all color coded correctly.  Mine was one of the backwards colored ones, re-wired it, and it's all good now. 

Not that im trying to thwart your project, (in fact, ill probably pick one of these up after its all sorted out) but someone that might have missed the bit about the wonky transformer leads might be here looking for a solution.

 

[Pip]

Correct me if im wrong, but the issue of the low output was solved after we figured out that the AMI transformers weren't all color coded correctly.  Mine was one of the backwards colored ones, re-wired it, and it's all good now. 

Not that im trying to thwart your project, (in fact, ill probably pick one of these up after its all sorted out) but someone that might have missed the bit about the wonky transformer leads might be here looking for a solution.

One thing is not the other. The AMI transformer issue you are correct about. The color coding of the leads was reversed which if you missed would lead to a lower ouput.  This speaks to the difference between the U87i and the U87Ai. Th "A" coding speaking to the microphone having a DC to DC conversion circuit in it that brings the phantom voltage input up to 60 VDC for a higher capsule biasing scheme. This results in a thoroughly different mic that is more sensitive with higher output and different transient response characteristics.

IMHO it is not as musical a mic as the U87i. Though I at some time might try this circuit out for other mic schemes. It is for sure way cool!

 

[sr1200]

Ahh thanks.  I thought it was just a level thing.  Good to know

 

[poctop]

Ahh thanks.  I thought it was just a level thing.  Good to know

Here is the thread related to this erratum,

http://groupdiy.com/index.php?topic=50533.msg640599#msg640599

Best,
Dan.

 

[chunger]

Sorry for the delay, I've had production copies of this PCB in hand for a while, but have not had time to do build documentation and testing.  If anyone is interested in doing BETA testing, here they are!

http://store.studio939.com/product/u87-ai-booster-conversion-pcb

 

[Adky]

May I ask , how do I hook this up to a first generation U87 ? thanks!!

 

[Matador]

May I ask , how do I hook this up to a first generation U87 ? thanks!!

Please check post #3.

 

[barbaroja]

Interesting. Hope they can fit in donor bodies.
Would this require either using a K67 capsule or joining both backplates on a K87?
Could then be done a mod to switch patterns on a K67 if using Dan's Pcbs?

 

[Arkenberg]

I was unable to order the low ESR capacitors from mouser, will low impedance caps work?

Thanks
Jon

 

[Matador]

I was unable to order the low ESR capacitors from mouser, will low impedance caps work?

Thanks
Jon

In general, yes.  The caps should be rated at 5 ohms or less at about 250kHz, and general "low impedance" is meant to signify this.

 

[Arkenberg]

Thanks for the info.
Also I am looking for an O-scope for my build. Which one do you prefer to use? Also would I need to re bias the mic when I install this board?

Thanks
Jon

 

[Matador]

Thanks for the info.
Also I am looking for an O-scope for my build. Which one do you prefer to use? Also would I need to re bias the mic when I install this board?

Thanks
Jon

The Rigol DS1000Z series is an amazing value.

This board supplies polarization voltage to the capsule, and leaves the JFET bias unchanged.  No re-bias should be needed.

 

[Arkenberg]

Thanks for the info!

 

[ChuckD]

Hello

I have tried this chip

LM5002 ( In Current) = 3.1mA
6.81 K * 3.1mA = 21.11 Volts dropped

The rest of the circuit will suffer and may no longer operate.

 

[Arkenberg]

So you are saying that this chip will not work in a microphone?

 

[Matador]

Hello

I have tried this chip

LM5002 ( In Current) = 3.1mA
6.81 K * 3.1mA = 21.11 Volts dropped

The rest of the circuit will suffer and may no longer operate.

What do the 6.81K resistors have to do with anything?  It's part of the phantom standard.

The preamp's 6.81K phantom supply resistors are essentially in-series with the 2.21K phantom sourcing resistors:  the total supply impedance seen by the chip (and the JFET) looks like 4.5K.  Even 4mA (worst case current for the boost chip) would drop 18V, which leaves over 30V going in to this chip and the rest of the circuit:  it's even 7V of margin for the Zener to operate against.

In short:  it works fine.

 

[kante1603]

So what went wrong in your build chuck?


Curious now,


best regards,


Udo.

 

[Matador]

So what went wrong in your build chuck?


Curious now,


best regards,


Udo.

Oh wait - Chuck may be referring to a retrofit of the original U87 - I designed this to retrofit Dany's U87 large format design, which uses a 24V Zener voltage to regulate the JFET operating point.  The original Neumann uses a 33V Zener after a large dropping resistor.  If you tried to draw this 4mA current through a stock Neumann design, the Zener voltage would collapse and the operating point of the JFET may no longer be valid.

However with Dany's stock design it's no problem, as essentially the draw from the LM5002 is diverted away from the Zener diode which is sinking a lot of current to ground to hold the internal rail at 24V anyways (probably even more than chip does in any case).

If anyone wants to try and retrofit an original Neumann, it requires a few more circuit changes (which I wouldn't recommend doing to preserve the value).  But if anyone is curious I can explain it in concept.