Analog simulation of analog magnetic tape recording part 14

If a saturating interstage transformer is used it had better saturate with signal rather than DC.


So...


If 2 tubes are used in balanced push pull then the DC component will be canceled.


We have anyway to use an input transformer after the microphone so if the secondary has a center tap phase spliting is already there...


In other words, we might...


Use the following signal path:


Mic - step up transformer - push pull EF183 or K117 JFET - minimum iron output transformer (interstage) to simulate magnetic tape saturation - then for inststeneous playback an electron tube stage with for example 70μs de emphasis network.


Although any second harmonic distortion generated by the tubes will be canceled by the balanced push pull circuit...the transformer core is generously fed and at larger signals will exhibit magnetic saturation. So 2nd, 3rd harmonic etc will be generated like real instruments when they are played loud. This is produced by the waveform becoming gradually peak limited. So we have a limited small objective signal that will fit the A to D conversion and at the same time a subjectively loud signal to the listener's brain.


The idea is to use a less iron as possible.


Thus topology has also the option of automatic gain control by feeding the side chain rectified signsl to the centre tap. See Altec or RCA compressor limiter schematics.


In fact the circuit is very similar to the front end of the RCA BA-6A limiter. The 6SK7 electron tubes behave similarly to the EF183. And they too can be battery operated at a very low anode (plate) voltage. The trick is to connect the cathodes to ground (-) and applying a positive grid electron acceleration bias from Vb to the grid circuit through a high Megohm resistor. This bias neutralizes the self assumed negative grid bias created by electron thermally escaping the cathode.


Thus we can have a healthy 0.5mA anode current at a typical Vb of only 9V?.


The noise is extremely low. Possibly the lowest that may be so far achieved with electron tubes or any device?


The cathodes can be DC heated from the same battery. And underheated as inside the Neumann U47 electron tube prepreamp.


The above configuration should be great for any mic, even a ribbon of low output.


References:


Tubes vs Transistors, is there an audible difference? - Russel O. Hamm- JAES


On preserving the transconductance of electron tubes at an anode potential as low as 3V - euroelectron


Analog simulation of analog magnetic tape recording, other parts - euroelectron