414EB project - DC-DC converter and transistors

[deb611]

Hi,

I'm looking at building a 414EB clone since I have a spare Teflon 414 capsule laying around, just for kicks. I thought it would be a good hobby project on the side.

here's the schem:

My issues are two-fold:

1) I can get the 2SK30A-R easily enough - but I'm having trouble with the BC413C equivalents. I've been searching datasheets all over, but I can't find anything close with that high of an hfe (350-800).  Anyone that's already done this have a reasonable substitute that they know will work in these circuits from their own build?

2) The DC-DC converter uses  transformer U26a and the 15pF  to form a Hartley oscillator, and then with half-wave rectifiers for the various voltages. I was going to just go down to the 3 patterns, as I have a spare mic chassis I can fit 3 toggle switches into. Anyone know anything about this transformer, or should I drop that section of the circuit and go with the DC-DC converter using a C-W voltage quadrupler and a 2-inductor Hartley oscillator with a 5551 that's so common in the transformerless Schoeps today? (See the right side of the C1 schem below)? I don't see any real downsides to this...

3) Just curious - I know the 10uF and 22nF capacitors across the 9.1V Zener in the circuit are to filter noise, but what's the 100-ohm resistor doing there?

Thanks! I've done several searches for that u26a transformer and come up short, and I don't have any 414EB that I can crack open and measure that transformer...

 

[volker]

1. BC550C will be perfectly fine in this circuit.

2. Use either the Schoeps circuit or the CMOS inverter circuit that is floating around. Audio performance will be no different as long as the converter isn't too power-hungry to make the overall voltage drop too much.

3. It limits the current through the zener. It will also offset the regulated voltage a bit, but that isn't too relevant since zeners aren't very "hard" regulators in the first place (voltage does vary with current quite a bit). And you mostly want a stable voltage, not spot-on 9,1000V.

 

[deb611]

1. BC550C will be perfectly fine in this circuit.

Thanks! I'll take a look around for those. IIRC they subbed in the 550s on the 414EB-P48...

2. Use either the Schoeps circuit or the CMOS inverter circuit that is floating around. Audio performance will be no different as long as the converter isn't too power-hungry to make the overall voltage drop too much.

Yep. I'll look around for that CMOS.

3. It limits the current through the zener. It will also offset the regulated voltage a bit, but that isn't too relevant since zeners aren't very "hard" regulators in the first place (voltage does vary with current quite a bit). And you mostly want a stable voltage, not spot-on 9,1000V.

Correct me if I'm wrong, but aren't Zeners actually noisier with less current?