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AES Journal Forum: Comment by Scott Dorsey on "A Variable Inductor"

Title: A Variable Inductor
JAES Volume 1 Issue 3 pp. 262-264; July 1953
Comment by: Scott Dorsey

Ingenious idea: a high value ferrite-core inductor that can be moved into saturation by a permanent magnet to reduce the value of inductance.  This acts like a magnetic amplifier but with the control flux provided mechanically rather than electronically.

I am skeptical of the author's claim of 1% distortion at 100mw... it would seem to me that any tiny asymmetry of the core would be magnified as the bias flux is increased and that even harmonic distortion would be a real problem.

It does not look as if this design thread was ever carried on any farther, unfortunately.  It's an interesting design and might have been useful for many things in the days before tunable state-variable filters became available.

Posted on January 28, 2021 at 5:41:33 PM EST

AES Journal Forum: Comment by Scott Dorsey on "Soft Magnetic Materials for Audio Transformers: History, Production, and Applications"

Title: Soft Magnetic Materials for Audio Transformers: History, Production, and Applications
JAES Volume 35 Issue 10 pp. 760-777; October 1987
Comment by: Scott Dorsey

This sort of paper would never be published in the journal today, sadly.  This is Dr. Sowter's basic overview of the history of audio transformers with some interesting notes about audio transformer design.  It's almost a stream of consciousness sort of piece.  It is by no means complete or authoritative, but it is a great overview for someone who wants basic background on audio transformer design and application.

Posted on August 14, 2020 at 7:32:36 PM EDT

AES Journal Forum: Comment by Scott Dorsey on "A High Sensitivity Power Pentode Using the 'Shadow Grid' Technique"

Title: A High Sensitivity Power Pentode Using the 'Shadow Grid' Technique
JAES Volume 9 Issue 2 pp. 152-156; April 1961
Comment by: Scott Dorsey

Why was this published in the JAES rather than in a general electronics journal?  This is a description by some GE engineers of an interesting manufacturing technique for vacuum tubes.  Because the electron beam from the cathode is physically shaped by the grids being made of physical wires rather than a uniform charge, problems occur when tubes have multiple grids whose wire spacing differs considerably, and this paper describes a solution for that to make a more linear small power pentode.

I am pretty sure that the tube described in this article became a production item as the General Electric 6GU5.  It was second-sourced by RCA but the RCA datasheet is very cursory.  Because the shadow grid isolates the control and screen grids  (by making the electron beam between them more uniform) a very high gain control grid can be used (high gain because the spacing between wires is very tight) and consequently the voltage gain of this tube is very high for a small beam power tube.

I can only think that the GE engineers placed it in the JAES hoping to get audio engineers interested in a new upcoming product.  Marketing placement of journal articles has been around since the Royal Academy opened up.

Posted on June 16, 2020 at 6:23:34 PM EDT

AES Journal Forum: Comment by Scott Dorsey on "A Low-Noise High-Output Capacitor Microphone System"

Title: A Low-Noise High-Output Capacitor Microphone System
JAES Volume 25 Issue 5 pp. 278-283; May 1977
Comment by: Scott Dorsey

What is interesting about this new microphone design is the servo circuit used to keep the DC bias point of the input FETs constant.

The parallel fets and very high front end impedances are the big takeaways from this design, and neither one was really very innovative by 1977.  The low noise amplifier is a fine design but any fine points about it are not explained in the text.  One thing that would be really interesting if one tried to build a copy of this is keeping the noise from the DC-DC converter out of the audio path, which is going to be a huge challenge and isn't explained by the author at all.

Posted on June 10, 2020 at 12:48:56 AM EDT

AES Journal Forum: Comment by Scott Dorsey on "Transistor Microphone Small Signal h Parameters"

Title: Transistor Microphone Small Signal h Parameters
JAES Volume 15 Issue 1 pp. 60-62; January 1967
Comment by: Scott Dorsey

Engineers at Bell Labs working on planar silicon transistors discovered that flexing the transistor die changed the beta, and since Bell was always looking for new sources of inexpensive communications microphones for telephone use, a lot of research has been done trying to put this effect to use to create a "Transistor Microphone." There are several other papers in the AES library on this.

This paper comes from Automatic Electric, which was one of the big competitors to the Bell System, showing that they were doing serious research into this microphone design as well.

In the end, the whole thing didn't amount to much, and when Sony started coming out with stable jellybean electret capsules a couple of years afterward, the research in this direction was abandoned.

So this is an interesting look at an effect that was never made into a practical product.

Posted on May 31, 2020 at 2:42:21 PM EDT

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