At your risk.
Always use a fuse in series with any battery for safety.
This is on connecting a moving coil mic to an electron tube and the electron tube to the iPad or other device electret mic input. Driving with a hotter signal from a low noise mic booster amp will reduce hiss. The sound quality should be exceptional with a good mic. The iPad low cut which seems to be quite steep below 300Hz may be a problem. But it can be an advantage for bass heavy mics omni or directional ones. For example the Seenheiser MD21 can sound great for vocals. MD21 HN has already been tried with the Pleiades K117 JFET booster cable directly connected to iPad.
Normally too much gain is not needed or the iPad input stage may overload.
The signal path is:
Singing voice at close distance from mic.
Moving coil mic, omni or directional, (including ribbon figure of 8).
A step up transformer if the mic does not already have one. The impedance driving the low noise electron tube can be above 5KΩ.
A coupling capacitor to the electron tube grid.
The electron tube can be a Nuvistor 2DV4 which has a 2V heater which will run greatly at 1.3V. Other electron tubes can be a Nuvistor 7586, heater supply can be 3.6V. Or EF183 triode connected having a higher heater current consumption at 3.6V. Or 6SK7 etc...
The circuit to be used is a variation of the Pleiades V4 which is really a Pleiades V6 without an output transformer. The load resitor can be the resitor inside the iPad mic input circuit.
Concentrating for this example on the Nuvistor 2DV4...
Heater can be supplied by a 1.5V battery in series with a fuse and an on off switch. This would be gradually turn on or off the system as cathode heats up and emits electrons.
The cathode connected to (-).
The battery (+) becoming ground. This should should create 1.5V + what ever iPad gives for electret FET supply. So if iPad gives 2V, total would be 3.5V which is plenty if Pleiades bias is used giving normally more than 25μA anode current. This insures low noise, not very high gain and world class A sound. Otherwise battery (-) can be ground and total plate supply can be for example 2V. All these values are Vb. actual plate or anode potential will be less by the load resistor violated drop as through it anode current flows, (ohm's law). Across the load resitor is developed the amplified AC music voltage. As electrons dance to the variations of the music signal. Inside and the outside of the electron tube.
The mic transformer is capacitor coupled to grid. This is not to upset the quiescent grid potential.
A 1.5MΩ to 8MΩ Pleiades bias resitor can be used between anode and grid to make the electron tube conduct or operate at such low anode potential.
Anode is connected to iPad mic signal terminal. iPad ground is connected to booster amp ground.
This amplifier like many other great amplifiers operates in class A meaning electrons flow all the time. The electron steam or current is modulated by the music signal to be amplified. This schematic is similar in many ways to the Neumann U47 internal VF14 electron tube head amp as: There is just one triode connected electron tube with cathode deliberately underheated possibly for lowest noise. Grid is fed by a high impedance. Anode is resistance capacitance coupled.
As soon as cathode is heated in an electron tube, grid becomes many hundred millivolts negative with respect to cathode by the electron cloud. Pleiades or pull up bias makes the grid less negative so that the operating point is moved away from cutoff at a low plate (anode) electric potential ie a small electric field. The 2DV4 can operate with just a 1.3V battery (in series with fuse) power supplying both heater and anode, see Pleiades V6 2DV4 posts.
See also:
On preserving transconductance of electron tubes at anode potential as low as 3 volt - euroelectron
Operating features of the Audion - E. H. Armstrong
No comments:
Post a Comment