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RE: BDT Preamp Schematics

Posted by John Swenson on May 14, 2008 at 14:59:43:
Hi Johanes,
the BDT tubes are much more susceptible to magnetic fields than most tubes, so I had to go out of the way to lower those fields. With the metglas shield in place I don't know if the other techniques would be necessary or not. I haven't tried it with the original PS or without the heavy aluminum box and just the shield.

I'm running it without ventilation holes but thats not necessary. The more holes the more you risk fields getting in which is why I didn't ventilate it in the first place. The whole box acts as a heatsink so the air inside really doesn't get all that hot.

See the post on how the regulators work, I hope that will answer the questions. R13 is just a way to vary the cascode bias voltage. The manufacturing tolerances on the threshold of the LND150s are not very tight so the current from the CCS they form can vary a bit from nominal, thus the variable resistor to "tweak" the cascode voltage.

The amplification depends almost entirely on the load impedance. The maximum transconductance is about 1mA/V (1000umhos or 1mS), so with a load of 100K you can get a gain of 100 (40db). You could go even higher if you are feeding into a very high input impedance. I keep on having a death wish and want to try this as a phono preamp!

Thanks for the interest,

John S.