• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Early history of High Fidelity amplifiers

First two historical facts:
  1. On 1927 H. A. Hartley coined the phrase "High Fidelity" which meaned "... a type of sound reproduction that might be taken rather seriously by a music lover".
  2. On 1937 Harold Stephen Black invented how negative feedback could be used in amplifiers without making them oscillators.
That means there was a ten year period of early Hi-Fi amplifiers while use of negative feedback was absolutely out of question. How was audio amplifier development within that 1927–1937 period? I am sure that patents, books and scientific articles about improved amplifiers were published during (and before) that ten years period.
 

20to20

Member
2010-06-23 9:25 pm
First two historical facts:
  1. On 1927 H. A. Hartley coined the phrase "High Fidelity" which meaned "... a type of sound reproduction that might be taken rather seriously by a music lover".
  2. On 1937 Harold Stephen Black invented how negative feedback could be used in amplifiers without making them oscillators.
That means there was a ten year period of early Hi-Fi amplifiers while use of negative feedback was absolutely out of question. How was audio amplifier development within that 1927–1937 period? I am sure that patents, books and scientific articles about improved amplifiers were published during (and before) that ten years period.

Since the first electronic phonographs used radio amps you could research the development of 1930's radio circuit output stages to find what you are looking for.
 

ChrisA

Member
2008-01-08 12:22 am
..
[/LIST] That means there was a ten year period of early Hi-Fi amplifiers while use of negative feedback was absolutely out of question.

You likely can't imagine how poor the sources were in the 20's. There were no tape recorders, no LPs not even decent phono cartridges or even good microphones.
HiFi back then was live AM radio, not what we'd call HiFi but compared to what was before it must have sounded great.
 

kevinkr

Administrator
Paid Member
Also take a look at what Western Electric and RCA were doing with theater sound technology in the early 1930's - this was where a lot real innovation was occurring, and arguably marked the appearance of the first speakers, amplifiers and a source even vaguely capable of performance close to what we would consider high fidelity.

Comment: The use of (global) negative feedback is not universally considered a requirement for high fidelity. Look at both tube and solid state (FET) SE amps for examples.
 
You likely can't imagine how poor the sources were in the 20's. There were no tape recorders, no LPs not even decent phono cartridges or even good microphones.
HiFi back then was live AM radio, not what we'd call HiFi but compared to what was before it must have sounded great.
The voice was often captured acoustically with great realism from the earliest days .The first commercial electrical recordings from 1925 had teething problems,mainly,I think, due to resonances in the early Western Electric microphones. However by the late 1920's/early 30's consistantly excellent results were being obtained in instrumental music which would,I imagine, shock many listeners today if they were to hear them properly reproduced-and I do not mean filtered beyond recognition to eliminate surface noise. AM radio can also be a very high quality source. Surely the reason for the great linearity of some of the early triodes was because they had to rely on this quality without feedback to help out. The fact that most people still listened on wind up gramophones with steel needles or cheap radios was an economic matter not because high quality reproduction was impossible.
 
It wasn't unusual not too long ago (I worked in another Telco Research Lab) to work on a concept for several years before even filing a patent application, and then between corporate legal and the patent office for it to then take a long time for the patent to be granted.

Note the usage "Patent Pending" and "Patent Applied For" which is a warning to potential infringers.

So the patent may have been in process for a long time after the Black's work was common knowledge, and possibly licensed to others for their use.

My internet connection is acting up tonight, didn't get to see the entire .pdf, but the patent linked to sure looks to me to be GFB related.
 
Yes, early gfb patent.

Still takes about three years for a patent to go through. Fastest I've ever seen was two years, and that was around 1984.

Hey Gimp, I suspect you are correct on the average time. I have 5 US patents and I have one go through in less than 4 months and one that took almost 5 years to complete the process. It all depends upon how much prior art is involved and how many claims are being made that require research. The quick one I got through was a patent for printhead normalization.

Mickeystan
 
Just a thought, but it seems to me that with the active devices available being triodes and the loudspeaker drivers rather well damped mechanically I suspect that the need for FB was not really all that great except for long distance telephony where multiple repeaters were necessary.

Pentodes started to supplant triodes in radio output stages by the mid 1930's and global negative feedback followed within less than a decade..
 
I noticed that the classic article by Harold Stephen Black is available here:
http://125.71.228.222/wlxt/ncourse/ecb/web/files/02_Stabilized_feedback_amplifiers.pdf

Seems that the year was 1934.

So, there was approximately seven years period (1927–1934) of High Fidelity without feedback amplifiers.

I get a "Not Found" when I click on the above link..

Also feedback was not widely employed in commercial products made in the USA until about the mid 1940s, the first commercial U.S. HiFi amps I know of that definitely used NFB were the Brooke 12A, and the 10C.. (Post war/late 1940s) Leak introduced a model in 1945 that might have had NFB - need to check. For the DIY realm that was probably the original Williamson design in 1946. There might have been others, but I'm not aware of them - after 1950 it became progressively more and more common except in very cheap radios and phonographs.
 
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