Re: Schoeps Blue Dot Sensitivity CMC5

[Sean B]

http://www.sdiy.org/oid/mics/Schoeps.gif

So it's a cathodyne-style phase inverter realized with a jfet. Are they just dumping one phase to AC ground to get a sensitivity reduction? I don't get the connection coming off the DC-DC converter going back to the jfet's drain. Can anyone shed some light?

Thanks,

Sean B

[Phil Allison[_4_]]

Sean B wrote:
>
> http://www.sdiy.org/oid/mics/Schoeps.gif
>
> So it's a cathodyne-style phase inverter realized with a jfet.
>

** Aka "concertina circuit" and other titles PLUS a pair of emitter followers with common collectors producing a 6.2V DC supply for the RF oscillator that boosts 60VDC. At least one of the Rode FET condenser mics ( the NT-2) use a near identical circuit - strange about that.


>
>
> Are they just dumping one phase to AC ground to get a sensitivity
> reduction?
>

** Nope.

> I don't get the connection coming off the DC-DC converter going back
> to the jfet's drain. Can anyone shed some light?
>

** In standard connection (B), inverted audio signal from the FET drain is coupled to the +60V supply feeding the capsule producing a 5dB gain loss through negative feedback.

In the (A) position, that link is simply removed and C1 is used as a final noise filter.


..... Phil

[Sean B]

Thanks!

The only other thing I don't get is the function of Zeners D3 and D4. Any ideas? The test point voltages seem to indicate that there is a Vce of only 1 volt or so across the output BJTs. Seems kind of low for good linearity.


Sean B

[Phil Allison[_4_]]

On Tuesday, July 10, 2018 at 3:47:20 PM UTC+10, Sean B wrote:
> Thanks!
>
> The only other thing I don't get is the function of Zeners D3 and D4.
> Any ideas?
>


** PNP transistors T2 & T3 need protection against C-E reverse polarity, which happens when one mic signal line ever shorts to ground. Such a short allows C9 to momentarily supply quite high current, enough to cause immediate damage. Zeners D3 & D4 prevent this by forward conduction. When the mic is first powered up - the same zeners provide voltage limiting in breakdown mode.

FYI: The Rode NT-2 uses 1N4148 diodes in the same position providing only reverse breakdown protection.



> The test point voltages seem to indicate that there is a Vce of only
> 1 volt or so across the output BJTs.
>
> Seems kind of low for good linearity.
>

** It does, but the JFET input stage has little voltage swing available and poorer linearity than emitter followers - so JFET distortion likely dominates.

Max output from the pre-amp is given as 1V rms which corresponds to a SPL around 132dB at the capsule ( calculated using a typical capsule sensitivity figure of 13mV per Pascal ) which is plenty of headroom for a studio mic.




..... Phil