Connecting an electron tube instead of an electret mic JFET

At your own risk.


See also next post and previous posts.


An electret condenser mic or capsule has a tiny JFET connected right next to the electret capsule. This is a head amplifier and is usually powered by the device it will be connected to.


For example a Sony Pro Walkman, an iPad, an iPhone, an m-audio micro track etc will give a small positive voltage (1.3 to 2.6V ?) at their mic in terminal through an internal resistance. The internal resistance will be the load resistance where the current output of the JFET (or electron tube) will be converted to voltage. The signal itself.


An electron tube can operate will just 1.5V at the anode if Pleiades bias is used [euroelectron]. This can be just a high Megohm resistor from anode to grid.


We can then supply a heater voltage of 1.3V if a CV2269 electron tube is used or a Nuvistor 2DV4, 2DS4, 2CW4.


Then if the positive terminal of the heater is made ground for the next device (your own risk, there is a short circuit if devices, microphones etc which as at different potential physically contact each other, a way round may be to connect the chassis of the headamp to this potential too, but an isolating input transformer should be used), the recording device phantom voltage will add making for example a total of 3 volts for the anode which can be plenty.


Other electron tubes such as Nuvistor 7586, EF183 triode connected, 12AU7, 6SK7, EF86?, 1H4? can also be used with a 4V for heater.


A simple schematic is the Pleiades V6 and can be made even simpler by this type of direct connection without the need of the output transformer. The input transformer will not be needed if an electric bass, guitar, synth etc is connected directly to the coupling capacitor to grid. Or if a high Z mic is used. Usually hi Z mics have the transformer internaly. For example Shure Unidyne III, Sennhiser MD21 HN, Sony F-96 high etc. The value of the coupling capacitor will dictate how much high pass is implemented.





Some recording devices may have an internal low cut which may be very abrupt.


See also next post.


Reference:


The Pleiades Bias - euroelectron