Shoeps CMC style preamp on 9V battery --> portable pcm recorder...

[enricoribbon]

Hello Group Diy people,

I just received a Sony pcm recorder that has mic input (stereo mini jack).
Built-in mics are nice but i'd like to try to use some schoeps MK capsules that I have.

A few years ago I modded two Schoeps KC active cables (with Rean mini-xlr 5 pins connectors at the body edge). I supply the 60V polarization voltage and the 6V for the FET at the capsule side, on 2 separate pins. I also have build a preamp box with 2ch. of CMC clone circuits (to use with a preamp with +48V), based on Russell technologies pcbs. It sounds as good as the CMC5 bodies.

Now I'd like to build a preamp using one or more 9V batteries to feed my Sony pcm-d100. I really care for low noise and audio quality.
Looking at the Schoeps CMC schematics, they suggest a power resistors value (270 Ohm instead of 6k81), and the circuit bridged at C position, for 9V power:

http://www.sdiy.org/oid/mics/Schoeps.gif
What should I expect from using such a low power voltage except short battery life (noise, earlier clipping...)?
Also, in a situation where I don't need a balanced output, could I simplify the CMC circuit without loosing performances?

Thank you for your patience and your replies!
Kind regards
E

 

[abbey road d enfer]

In the Schoeps circuit, most of the circuit operates under a 6.2V rail, the exception being the output transistors, that operate with about 2.5V Vce, which is enough for outputting signals up to about 0dBu, so I believe headroom should not be a big issue.

Schoeps advocate the use of 270r resistors, because it allows the circuit to work even when the battery's voltage has dropped to about 7.5V. However, the consequence is the initial current draw when the battery is fresh is quite high (9mA when the circuit actually needs only 4mA to work).
Since you are not strictly limited by the constraints of adhering to a standard, I would suggest you use a voltage regulator. The actual DC voltage should be adjusted for a current draw of 4-5 mA. The regulator should be of the LDO type, in order to extract the most battery life. It should be a linear type, not switching, since the latter may produce unwanted interference.
LT1121 seems to be the perfect candidate for the role.

 

[e.fiocco]

Thanks for you reply.
Just to be sure, you suggest to regulate the 9V battery to 6.2V (for the input FET, the polarization dc converter, the collectors of the out transistors) and to about 8.4V (for the emitter of the out transistors, through 270 Ohm power resistor)?
thanks, regards

 

[abbey road d enfer]

No. I suggest retaining the 270r resistors (they are necessary) and regulating the voltage at whatever is needed for setting teh total current draw to 4 or 5 mA. The 6.2V will be the usual result of the zener regulation.