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

[JMPGuitars]

Today I was bored, so I figured out that if I mount C5 horizontally on the back of the PCB, the PCB would fit in the budget body with the blue PCB version of the U87.

Obviously I decided to test it before connecting to a functional mic. I seem to have similar problems to others in this thread that have gone unanswered. Is this project abandoned?

I didn't use R8 or R9, so I have R3, R5, and R6 on the PCB. R8 is bridged across all 3 vias.

With a source voltage (bench PSU) of 12V, I had only 10V on the Vout. I saw somebody else with a similar issue, and higher voltage was suggested. I raised the bench PSU to 24V, and then (I believe it was) the diodes started smoking. They're very shiny now, and look a little melted maybe.

Assuming these boards aren't completely abandoned, maybe somebody should post a functional layout/schematic of this board in use with the U87 project?

Thanks,
Josh

 

[Matador]

Apologies I missed this.  It isn't defunct, it's just that not many people seemed interested in the PCB.

I still have my 4 prototypes, and I can measure them.  There isn't much to debug here, as there are only a handful of components and 99% of the circuit is handled within the chip itself.  However if you need some test voltages I can measure them for you.

 

[JMPGuitars]

Apologies I missed this.  It isn't defunct, it's just that not many people seemed interested in the PCB.

I still have my 4 prototypes, and I can measure them.  There isn't much to debug here, as there are only a handful of components and 99% of the circuit is handled within the chip itself.  However if you need some test voltages I can measure them for you.

Test voltages would be great, thanks.

Question- how much of an output increase does this give the U87?

Thanks,
Josh

 

[RuudNL]

Output level is linear with the polarisation voltage.
Incresing the voltage from 46 to 60 volts will mean 2,3 dB gain.

 

[JMPGuitars]

Output level is linear with the polarisation voltage.
Incresing the voltage from 46 to 60 volts will mean 2,3 dB gain.

How does that 2.3dB gain translate to real world use? If a 3dB increase is double the power, and 10dB is double the volume, how does that gain in this case affect the real world perceivable output level? Will I need only 2.3dB less increase on my preamp?, or would I need 2/3rd less preamp boost based on power?, Or 25% less based on volume?

 

[SparkleBear]

Hey friends,

I am wanting to integrate this circuit into a custom pcb I'm designing right now.

Where are you deriving VCC to operate the IC? Is it just tied to the cap and to ground or are we giving it voltage?

Thanks!

 

[rivermAn]

That circuit seems a bit complex for such a simple function? ..... I've had success using the often copied multiplier based on Rory Holmes 1983 ETI article... (see here: www.jp137.com/lts/Voltage_multipliers_with_CMOS_gates.pdf )

A version of that circuit is used by Rode in their low noise NT1 model, for example.

I used a simple stripboard version for my LDCX2 project .....(details in the lower half of these notes: http://www.jp137.com/lts/LDCX2.notes.pdf )

125KHz oscillator - ripple of around 5mV. Works well -- and it should be simple to modify the stripboard layout to fit your requirements, if necessary...

And it's simple to vary the voltage too .... with a 9V supply and all capacitors fitted, you'll get c.60V.
Remove (or switch out) C8 and that drops to around 50V.

There are lots of other permutations -- the 40106 will work with a supply rail of anything between 3 and 18V, and with a higher supply rail it may be desirable to remove or switch our more than one multiplying capacitor.
Lots of variable options from a simple circuit..

this looks awesome. how feasible would it be to modify this to convert from phantom power to 200v? given max input of 18V, if we drop the 48v to 18v for input, and then introduce more schmit triggers to bump the total output V higher, could we reach 200v? i imagine this could be accomplished by using multiple CD40106B chips. my understanding is that each schmitt provides an additional 18v, is that right? so we need roughly 10 schmitt triggers total, since 18 + 10*18 ~ 200. the goal is to power gefell mv102 microphone which requires a very nice healthy 200v

 

[Khron]

this looks awesome. how feasible would it be to modify this to convert from phantom power to 200v? given max input of 18V, if we drop the 48v to 18v for input, and then introduce more schmit triggers to bump the total output V higher, could we reach 200v? i imagine this could be accomplished by using multiple CD40106B chips. my understanding is that each schmitt provides an additional 18v, is that right? so we need roughly 10 schmitt triggers total, since 18 + 10*18 ~ 200. the goal is to power gefell mv102 microphone which requires a very nice healthy 200v

So what would you want to power the heater with, then?

 

[rogs]

this looks awesome. how feasible would it be to modify this to convert from phantom power to 200v? given max input of 18V, if we drop the 48v to 18v for input, and then introduce more schmit triggers to bump the total output V higher, could we reach 200v? i imagine this could be accomplished by using multiple CD40106B chips. my understanding is that each schmitt provides an additional 18v, is that right? so we need roughly 10 schmitt triggers total, since 18 + 10*18 ~ 200. the goal is to power gefell mv102 microphone which requires a very nice healthy 200v

The last schematic - fig 11 - on this page: https://gyraf.dk/schematics/Voltage_multipliers_with_CMOS_gates.pdf shows how adding further stages can provide higher voltages........ Don't forget to allow for fitting higher voltage capacitors for the later stages!
That article from Rory Holmes published in 1983 seems to have been the inspiration for many of the voltage multipliers found in todays' hobby mic schematics. .... I certainly used it for my OPIC mics. (See from page 7 of this pdf: http://www.jp137.com/lts/OPIC.41.pdf ).
I would think an extra 6 stages from a second 40106 should get you somewhere near 200v, using a 15v supply.. (not all 4000 CMOS types list the full 18v, as I recall?)..

 

[RuudNL]

If you need 6.3 V and 200 V, this may be an easy and reliable option.
(Not to be powered from phantom power though...)
https://nl.aliexpress.com/item/1005002998520867.html

 

[rivermAn]

yeah i know i cant use this to power a tube mic lol - what i have isn't a tube mic. might have quoted wrong mic number. will double check.

 

[Khron]

Well then, what sort of current do you need, from that 200v?

 

[RuudNL]

If it is not a tube microphone, I suppose the 200 V. is needed for the polarisation voltage?
(In that case the current would be 0 mA.)

 

[vinylwall]

When you say bridge across R8 do you mean to short all three leads together?

And do both ground points need to be connected to the mic ground?

 

[elskardio]

When you say bridge across R8 do you mean to short all three leads together?

And do both ground points need to be connected to the mic ground?
View attachment 104542

If you're using R5 & R6 to create the voltage divider, yes you should jumper all R8's pads together.

All GND points should be interconnected (double check with a multimeter to confirm), so you only need 1 connection to the mic's PCB GND.