Design Idea:
Tube Microphone Preamplifier   

  Your disdain for negative power supplies is understandable, particularly, if you wish to use tube rectifiers. Nonetheless,  I do not see the FET working. You see FETs have a resistive region where the drain impedance is significant. This occurs up to a few volts and then the FET enters its saturation region, with its nearly infinite drain impedance. The 6DJ8 requires about a 2 volt cathode bias to draw 10 mA of current. In other words, the point at which the FET becomes a current source is too high to bias up a 6DJ8. Even a small negative power supply voltage (-5 volts) would greatly help.

    Will a differential input plate follower configuration create a low Z in, low  enough to work as a transformerless input pre? Also instead of using a split rail I want to use a JFET in a grounded gate setup with something like 500 ohms on the drain. So far it sounds terrific. Tube CAD helps a lot but some things need the attention of the algorithmic genius himself. Your response will be appreciated. By the way I made the February Circuit of the Month regulator and all I can say is it's a serious thing indeed.


   Thanks for the new title. I like it, but it will not fit on a personalized license plate.
   In spite of the title, I have to admit that I know very little about the requirements for a microphone preamplifier. How much gain is needed? Balanced outputs or a single-ended output? So those in the know do please E-mail the required specifications.
   Still, a microphone preamplifier cannot differ too radically from an MC pre-preamplifier. Low noise is
the requirement. 
   A Plate Follower feedback arrangement would provide a low impedance input, but one triode would not provide sufficient gain to both sustain the feedback and provide a high gain output. This means we must cascade two triodes to increase the total gain from the preamp.

   Finding the right FET can also be problematic, as a high IDSS will be needed so that the idle current can be cut back by using a source resistor. The only FET I can think of that would prove quiet and beefy enough is the 2N4391. (Look for the metal can version: TO-18.)

   On the other hand, using a negative power supply has some real advantages. The most important of which is that the ground is freed up from having to support both the signal  and amplifier circuit currents. This is a much better arrangement, as all the active circuitry is maintained by the plus and minus legs of the power supplies and the input and output signals have the ground bus to themselves. Add to this advantage the fact that active current sources really are not needed, as large valued cathode resistors are easy to implement.
   One advantage to split-rail power supplies is never mentioned, but should be. A split-rail power supply is 1/4 as dangerous as the same total voltage single rail power supply. How so? Let us say that you are fiddling with a chassis with a +/1 200 volts power supply. Your left hand is turning the metal volume knob, while your right hand is attaching a scope probe to a resistor. But back of your right hand also hits a part charged to +200 volts. A shocking experience.

pg. 14

<PREVIOUS   Copyright © 1999 GlassWare. All Rights Reserved