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

[Wavesponge]

Can you make the 8.9V FB voltage change by turning the adjust pot R8?  FB pin in pin #6.  Can you also check pin #3?  There should be about 6V sitting there if the regulator is enabled and running.

FB at 8.9V doesn't change when adjusting R8.  pin #3 is at 6.9V.

 

[Matador]

Ok, 6.9V says the the regulator is enabled, as you have >1.26V on the EN pin with 27V on the input.

The fact that FB cannot be made to change by turning R8 says there's something suspect in the feedback path.  Even when disabled, the output will sit at about 1 diode drop below the input, so you should measure 25-28V on the output.  With R5 and R6 shorted then you should be able to adjust FB between 0V and 28V by twiddling R8.

Can you confirm you read 0V on the side of R6 that faces R8 (the "left" side of R6 in your picture?  If R6 is shorted, then you should read ground here.  You should also read the output voltage on the "right" side of R5 (if R5 is shorted).  Other than that, the pot is suspect.

 

[Wavesponge]

R5 and R6 are shorted.

Here's what I'm reading:
V in = 28.1
Left side of R6 = 0V
Right side of R5 = 27.4
V out = 27.4

Changing R8 makes no difference

 

[Arkenberg]

Let's start with the easy one:  leaving out R9 and stuffing R3 only is fine:  this sets the switch frequency, and the default frequency with 52K should be fine.

So R5/R6/R8:  the design is meant to be either adjustable or fixed.  If you want a fixed output voltage, you omit R8, and stuff R5 and R6.  You then bridge across R8.

However if you want adjustable output, you stuff R8, and you short across R5 and R6:  a short piece of solder wick does this job well, or you can buy SMD 0 ohm resistors if you want it to look clean.

If the BOM says "omit" R5 and R6, that is my fault, it should read "short out R5 and R6".

When you say bridge across r8 do you mean to short all three leads together?

 

[Arkenberg]

Not sure what I did wrong but I built 2 boards and they both only have 6.79VDC coming from them. I omitted the r5 and r6 and shorted them. I thought I might have had the chip installed backwards so I try reversing that but no luck still about the same. I then stripped the board of L2 and shorted it and removed D3 but still the same. I seem to have 6.7VDC at r5 and 0VDC at r6. Also the same before and after L1 and L2. I am not sure what went wrong. I do have a constant 23.6VDC in though.

 

[baloubass]

Hello, i want to know if with this i can make switchable, for have u87i and u87ai by the switch. sorry for my bad english.

 

[Khron]

Highly doubtful. If you compare the two schematics, apart from needing a "special" capsule for the non-A (with isolated backplates), you'll notice the biasing network and pattern-switching are totally different.

 

[baloubass]

Highly doubtful. If you compare the two schematics, apart from needing a "special" capsule for the non-A (with isolated backplates), you'll notice the biasing network and pattern-switching are totally different.

ok thus, it's not possible.

 

[RuudNL]

AFAIK most people think the 'old' U87 sounds different because the polarisation voltage on the capsule was lower (~ 46 V instead of 60 V.)
A higher polarisation voltage influences the membrane tension a bit.
I don't think it would be a problem to make the polarisation voltage switchable.
You will have to decide for yourself if it makes a real difference...

https://www.youtube.com/watch?time_continue=1&v=-tKG49pY9c4

 

[baloubass]

AFAIK most people think the 'old' U87 sounds different because the polarisation voltage on the capsule was lower (~ 46 V instead of 60 V.)
A higher polarisation voltage influences the membrane tension a bit.
I don't think it would be a problem to make the polarisation voltage switchable.
You will have to decide for yourself if it makes a real difference...

https://www.youtube.com/watch?time_continue=1&v=-tKG49pY9c4

Thank for your answer. So i need to add a dpdt switch or can i use the -10db for this mod. In fact i really not have any idea what i can wiring this  mod (newbbie in diy).
Thank if anyone can help me.

 

[RuudNL]

If you use the Matador voltage converter, I suppose a single resistor in parallel with R5 is sufficient to reduce the output voltage.
In this case you could use a simple on/off switch, to switch the parallel resistor in- or out of the circuit.
If you are following the original U87ai converter circuit, I suppose that a zener with a lower voltage in parallel with D7 (62 V.) will do the same. I think a 47 V zener would be a good starting point.
As far as I can see, T9 regulates the voltage on the DC converter stage.

 

[baloubass]

If you use the Matador voltage converter, I suppose a single resistor in parallel with R5 is sufficient to reduce the output voltage.
In this case you could use a simple on/off switch, to switch the parallel resistor in- or out of the circuit.
If you are following the original U87ai converter circuit, I suppose that a zener with a lower voltage in parallel with D7 (62 V.) will do the same. I think a 47 V zener would be a good starting point.
As far as I can see, T9 regulates the voltage on the DC converter stage.

Thanks i will try for the resistor method (matador board).
The correct steps.
-Mount the board like the matador thread
-put the resistor (???) in parallel with the switch

 

[ln76d]

In fact, polarisation voltage isn't only difference - we have different arrangement of the input, including use of one of two backplates vs. one backplate for cardiod, 1G vs. 500M of input impedance, 220pF vs. 160pF for HF filtering etc.

 

[Matador]

Indeed - there are many other differences between the two microphones other than the polarization voltage.

If you only want the polarization voltage switchable, then, yes, you can make it go easily between normal 46V mode and 60V mode with a SPDT switch.  Would behave like a pad.

 

[baloubass]

Thank you so much, in fact it's not really interesting to have just the polarization switch. The really interesting goal for me is to have a real switching of the ai booster board, like a "true bypass".
With different answer in the last post it's seems to me that impossible.

 

[RuudNL]

In fact, polarisation voltage isn't only difference - we have different arrangement of the input, including use of one of two backplates vs. one backplate for cardiod, 1G vs. 500M of input impedance, 220pF vs. 160pF for HF filtering etc.

In the U87 this is R6//R7 , both 1 G.ohm, so 500 M.ohm
In the U87ai this is R105 // (R103 // R104), all 1 G.ohm, so ~333 M.ohm

 

[JMPGuitars]

Hi,

Is anybody successfully using these boards? I ordered and made a couple of them a long time ago, but they don't fit in my u87 mic bodies. I want more headroom though.

Thanks,
Josh

 

[rogs]

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..

 

[Matador]

Hi,

Is anybody successfully using these boards? I ordered and made a couple of them a long time ago, but they don't fit in my u87 mic bodies. I want more headroom though.

Thanks,
Josh

The board was sized for Chunger's full-sized U87-like bodies:  is this what you are using?

 

[JMPGuitars]

The board was sized for Chunger's full-sized U87-like bodies:  is this what you are using?

I have the "budget" YT5 body along with the blue version of the boards.