Converting a D-87 to ORS87

[joulupukki]

I've got a D-87 (Dany Bouchard) that I've built into an Aurycle A460 mic body. I originally had it built using an Arienne Audio K87 capsule but I've since swapped it out for a JLI TSC-2 capsule (because I used the Arienne for my Fuchs U87a). With both cases I've been underwhelmed by the lack of sensitivity of the D-87 and would like to change it if possible so it's closer to what the Fuchs U87a would be (and my other condenser mics).

As is, the -10dB pad and low cut filter switches are functional.

I'm currently using a 33V Zener with a 47K at R17.

It's been suggested that the ORS87 would yield better sensitivity so I wouldn't have to turn up my preamp so high. I'm starting this thread so I don't pollute the D87 build thread anymore. Has anyone else done this conversion with satisfying results? As pointed out by @micolas I think it's just going to be a matter of tracing both circuits and removing what no longer belongs on my existing circuit board. Right?

 

[Khron]

You've been warned you'd be giving up the omni & fig-8 patterns, right?

 

[joulupukki]

You've been warned you'd be giving up the omni & fig-8 patterns, right?

When I switched from the AA K87 to the JLI TSC-2 I already gave up one of those. I've never used omni or fig-8 with this mic anyway. I've got a modded AKG P420 and the Fuchs U87a for that purpose if needed.

These are the U87 schematics I've been referencing with the D-87 build:

http://recordinghacks.com/images/mic_extras/neumann/U87-schematic-1972.png
http://recordinghacks.com/images/mic_extras/neumann/U87-schematic.png

 

[joulupukki]

If I’m looking at the ORS87 schematic right, I think if I were to make the following changes, that’d change it to where it’d be in the right territory. Right?

Is the main reason for the increase in sensitivity the fact that C4/470pF is removed in the ORS87?

 

[Khron]

I think if I were to make the following changes, that’d change it to where it’d be in the right territory. Right

If you replaced C1 with a jumper, you'd hard-wire the mic to omni.

C4 can indeed be bypassed; might just be simpler to just move the capsule wire from the turret where C4/R6 were joined, to where R7 & the JFET gate are joined.

 

[micolas]

Is the main reason for the increase in sensitivity the fact that C4/470pF is removed in the ORS87?
View attachment 144011

Not.
C4=470pF is just a coupling capacitor, it can only affect the passband (low frequencies) but it is big enough for the sound of the U87.
Bypassing C4 causes the microphone phase to be reversed so you will need to reverse the transformer primary or secondary wires or reverse pins 2 and 3 on the XLR output.

C3=10pf greatly reduces the sensitivity of U87i.So you have to remove it from the circuit.
I also think that it is possible to affect the shape of the deemphasis curve.

Edit:
Some have replaced C4 with 1000pF and claim that the microphone sounds with more bass. I didn't hear any differences

 

[micolas]

If you replaced C1 with a jumper, you'd hard-wire the mic to omni.

C4 can indeed be bypassed; might just be simpler to just move the capsule wire from the turret where C4/R6 were joined, to where R7 & the JFET gate are joined.

Hey @joulupukki

@Khron is absolutely right. You can very easily get an ORS87 that only lacks
figure 8.

 

[joulupukki]

Ah, I see now how the back diaphragm would be connected by jumpering C1. And, the JLI TSC-2, if I’m remembering correctly, has a shared backplate wire.

So doing something like this would be more correct for the ORS87 wiring?

Not.
C4=470pF is just a coupling capacitor, it can only affect the passband (low frequencies) but it is big enough for the sound of the U87.
Bypassing C4 causes the microphone phase to be reversed so you will need to reverse the transformer primary or secondary wires or reverse pins 2 and 3 on the XLR output.

Ah, so I could just leave it in.

C3=10pf greatly reduces the sensitivity of U87i.So you have to remove it from the circuit.
I also think that it is possible to affect the shape of the deemphasis curve.

If the basic ORS87 uses 10pF for C3 in the circuit and it has greater sensitivity than a normal U87/D87, what main difference in the ORS87 contributes to it being more sensitive? If there’s some other tweak I could make to the D-87 circuit that bumps the sensitivity up without all the other changes, I’d be game to try that. I mean, I could take out C10 and just see how that sounds or potentially lower it to 1/2.2pF.

@Khron is absolutely right. You can very easily get an ORS87 that only lacks
figure 8.

Because of the shared backplate on the JLI TSC-2 I think that may be what I already have.

 

[micolas]

View attachment 144026

Ah, so I could just leave it in.

If the basic ORS87 uses 10pF for C3 in the circuit and it has greater sensitivity than a normal U87/D87, what main difference in the ORS87 contributes to it being more sensitive?

In ORS87, the 10pF capacitor is not installed at all.

The scheme in blue was just a computer simulation. I posted it just to see the values with the 33v Zener.

The more realistic schematic is the one in the picture here, but it was originally made with a 24v Zener.
***
So we have to put:
Zener GR1 = 33v
R16 less than 47Kohm, it is tested
(a Zener to stabilize effectively needs to be applied a few extra volts)

Don't use that capacitor (10pF)

 

[joulupukki]

Ah, that makes more sense. Hopefully I'll get time to try it out today.

 

[micolas]

Ah, that makes more sense. Hopefully I'll get time to try it out today.

I finally overcame the language barriers

 

[jp8]

Quickly simulated the ORS87 with and without 10pF feedback capacitor and a 9.5:1 transformer with a 3k load.

JFET is BF256B from LTspice lib
With 10pF: Gain = -8.2 dB @ 1kHz
Without 10pF: Gain = 6.3 dB @ 1 kHz.

A major difference....

A feedback capacitor will definitely affect frequency response, as explained in my KM84 description. More LF and RF roll-off with lower values. And distortion and max SPL are affected too, of course.