Voice signal power amplifier.

[ggunn]

160717-1314 EDT

I was not aware of any "hum bucking" coils. There would be no object in bucking 60 Hz since the ripple fundamental frequency is 120 hz.

Humbucking coils are not frequency specific. I know about them as they apply to electric guitar pickups. A pickup with a particular number of turns around a permanent magnet is wired in series with a pickup right next to it with the same number of turns in the opposite direction and with the permanent magnet oriented in the opposite direction. It's called RWRP (reverse wound reverse polarity). The two pickups produce a positively reinforced signal from the strings, but electromagnetic energy from the environment is injected into the signal path by the two pickups with flipped polarity from each other, which cancels it out.

This is because the permanent magnets are involved in producing a signal from the strings; combined with the reverse windings in the second pickup the polarity has been flipped twice, so the signals they produce have the same polarity. With respect to energy picked up from EM interference, however, the permanent magnets are not involved, so the polarity has only been flipped once and the signal thus produced by the two pickups (hum, hence the name "humbucker") are of opposite polarity (sometimes inaccurately called 180 degrees out of phase) and therefore negatively reinforce - cancel. The technical term for it is "common mode rejection". It's why my Les Paul with humbuckers is pretty quiet in most environments but my Strat with single coil pickups sometimes howls like a banshee.

 

[hbiss]

hum would have been annoying

That is why there was a 'humbucker' coil also.

Tens of thousands of turns of small wire with 500Vdc around 10 mA or so and associated ripple, then a separate winding of a few turns of 12 or 14 AWG in series with the 6V tube filaments to 'buck' the 60Hz core flux.

No. The electromagnet on the speaker was either a swinging choke or an output choke for the power supply. As such there was a considerable amount of filtering provided by the large filter caps so the small amount of ripple would not be heard in the speaker, some as large as 15" and quite capable of reproducing 120 and 60 hz. Matter of fact in my youth I often would take one of those old chassis and replace the choke with a resistor so I could use conventional PM speakers. There was no audible difference in hum.

As for that small choke in the filament supply, I have never seen that but I can tell you it was not for hum bucking. Just wouldn't do anything. Likely to fix an RF stability problem. Any designer of the day concerned about hum from the filament supply (usually in the audio preamp stages) would use a DC supply.

-Hal

 

[FionaZuppa]

telecom for transmission of their signals really took off after invention of the P-N junction transistor, commonly known today as a BJT. BJT allowed them to boost signals (essentially voice at that time) to overcome the "resistance" of long transmissions. an amplifier takes a low power signal and dupes that into a higher power signal. energy conservation laws apply, but in general you get amplified signal + operational power losses. but certainly BJT was not the 1st invention of amplifier.

 

[GoldDigger]

Humbucking coils are not frequency specific. I know about them as they apply to electric guitar pickups. A pickup with a particular number of turns around a permanent magnet is wired in series with a pickup right next to it with the same number of turns in the opposite direction and with the permanent magnet oriented in the opposite direction. It's called RWRP (reverse wound reverse polarity). The two pickups produce a positively reinforced signal from the strings, but electromagnetic energy from the environment is injected into the signal path by the two pickups with flipped polarity from each other, which cancels it out.

This is because the permanent magnets are involved in producing a signal from the strings; combined with the reverse windings in the second pickup the polarity has been flipped twice, so the signals they produce have the same polarity. With respect to energy picked up from EM interference, however, the permanent magnets are not involved, so the polarity has only been flipped once and the signal thus produced by the two pickups (hum, hence the name "humbucker") are of opposite polarity (sometimes inaccurately called 180 degrees out of phase) and therefore negatively reinforce - cancel. The technical term for it is "common mode rejection". It's why my Les Paul with humbuckers is pretty quiet in most environments but my Strat with single coil pickups sometimes howls like a banshee.

AFAIK there are two distinct usages of hum bucking.
1. The one you describe where the goal is to isolate the wanted signal caused by string vibration from the unwanted signal caused by the ambient magnetic field.
2. The one where you have a known direct contribution from stray coupling in the circuitry of equipment powered ultimately 60Hz AC. This is done by injecting a fixed (adjustable) 60 Hz signal derived from the supply voltage.
One example would be a directly heated tube cathode that did not apply center grounded balanced AC to the filament.

 

[junkhound]

No. The electromagnet on the speaker was either a swinging choke or an output choke for the power supply. As such there was a considerable amount of filtering provided by the large filter caps so the small amount of ripple would not be heard in the speaker, some as large as 15" and quite capable of reproducing 120 and 60 hz. Matter of fact in my youth I often would take one of those old chassis and replace the choke with a resistor so I could use conventional PM speakers. There was no audible difference in hum.

As for that small choke in the filament supply, I have never seen that but I can tell you it was not for hum bucking. Just wouldn't do anything. Likely to fix an RF stability problem. Any designer of the day concerned about hum from the filament supply (usually in the audio preamp stages) would use a DC supply.

-Hal

No ?
That is why the internet is so interesting, one finds all types of opinions and misconceptions.
The choke part is right anyway.

 

[gadfly56]

1607162258 EDT

On searching I found a very interesting article that is probably correct in many respects. I somewhat question the time sequence information for microphones in the 1930 to mid 1940 time frame.

See http://microphone-data.com/media/filestore/articles/History-10.pdf

Radios of the 1920s and early 1930s used electromagnets for the speaker. I have a couple different radios with such speakers. I have a radio from 1940 with a permanent magnet speaker. Also bought a cheap microphone in 1942 that used a small PM speaker, 2" diameter, for the microphone.

Alnico was invented in 1932. See

http://link.springer.com/article/10.1007/BF03220870#page-1
You must click on "look inside" to see the article.

My first exposure to broadcast microphones was in 1936. I don't know what was in side. But speach quality was quite good by this time and probably better by 1940.


hbiss:

It is quite common to talk about power amplification in the communication world. A carbon microphone is truely a power amplifer from the standpoint of the input accoustic signal to the microphone to the electrical signal power output. There is a very large multiplying factor here, and that is why the telephone industry was able to develop into a viable industry.

.

.

Assuming your were a precocious lad, that puts you at 90 - 95 years old. Or was 1936 the model year of the microphone?

 

[hbiss]

No ?
That is why the internet is so interesting, one finds all types of opinions and misconceptions.

I got my experience when the internet was a mere glimmer in Al Gore's mothers eye...

-Hal

 

[gar]

hbiss:

With reference to your post numbered #11.

A dynamic (magnetic), crystal, or capacitive microphone in of itself is not a signal amplifier. All of these alone produce less signal output power than acoustic power input.

The carbon microphone is in itself a signal power amplifier, it does require an external source of power. The voice power level at the microphone output can be much larger than the acoustic power input.

The carbon microphone because of its inherent signal power amplification made the telephone system a practical reality because it made communication over a substantial distance possible.

No electronic amplifiers existed in the early days of the telophone industry. When I was a kid, if I called a friend, the telephone operator would would say --- number please ---, then she would pull her patch cord and plug it into my destination number, and my carbon microphone was connected to my friends magnetic earphone. No electronic amplifiers. However, electronic amplifiers were required for long distance.

It was not until I was about 14 that we got rotary dial capability. Apparently it was 1963 when touchtone became available. http://triblive.com/news/allegheny/4291278-74/phone-touch-tone I had thought it was slightly earlier. As soon as touchtone was available I switched to it.

In the 1920s Western Electric manufactured triodes. I have to leave. I have not had time to read this article https://en.wikipedia.org/wiki/Western_Electric .

.

 

[Johnnybob]

160717-1314 EDT

I was not aware of any "hum bucking" coils. There would be no object in bucking 60 Hz since the ripple fundamental frequency is 120 hz. Thus, filament supply current could not be used directly.

An interesting article is http://www.edisontechcenter.org/speakers.html . My 1928 GE superheterodyne, two TRF stages, has an electromagnet speaker. I believe that speaker would reproduce 60 Hz.

The following article is quite wrong in timing, and somewhat wrong in other ways. https://en.wikipedia.org/wiki/Electrodynamic_loudspeaker

This next article has some useful information, but again timing is wrong. I have a Utah speaker from just before WWII with an Alnico magnet. Speakers as such were readily available. https://en.wikipedia.org/wiki/Loudspeaker

Google as a useful search engine is getting quite bad. I have not found a reference to a hum bucking coil.

.

Humbucker (one word) coils are common on electric guitars to minimize distortion. See;
https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&cad=rja&uact=8&ved=0ahUKEwiSrMDprILOAhVEpJQKHV3aBSUQFggcMAA&url=http%3A%2F%2Fthehub.musiciansfriend.com%2Ftech-tips%2Fhumbucker-vs-single-coil-pickups-explained&usg=AFQjCNGO2UQFEskeQCBSuKFKMCLIMjylRQ&bvm=bv.127521224,d.dGo

 

[hbiss]

A dynamic (magnetic), crystal, or capacitive microphone in of itself is not a signal amplifier. All of these alone produce less signal output power than acoustic power input.

The carbon microphone is in itself a signal power amplifier, it does require an external source of power. The voice power level at the microphone output can be much larger than the acoustic power input.

I agree with you though I have never heard of that property of a carbon microphone termed an amplifier. That might be because it inputs acoustic energy and outputs electrical energy. And it's what that electrical energy can do that that would determine if there was an advantage or not. Hard to quantatize back in those days. It also might be because it was foreshadowed by the triode which takes a small electrical signal and outputs a corresponding large electrical signal. Apples to apples.

-Hal

 

[gadfly56]

I agree with you though I have never heard of that property of a carbon microphone termed an amplifier. That might be because it inputs acoustic energy and outputs electrical energy. And it's what that electrical energy can do that that would determine if there was an advantage or not. Hard to quantatize back in those days. It also might be because it was foreshadowed by the triode which takes a small electrical signal and outputs a corresponding large electrical signal. Apples to apples.

-Hal

I thought carbon mics worked on the principle that the rarefaction and compression waves of your voice changed the resistance across the carbon bed, modulating the DC applied thereto. Have I been misinformed all these years?

 

[ggunn]

Humbucker (one word) coils are common on electric guitars to minimize distortion.

Not really. The purpose of humbuckers is to buck hum (hence the name) not to reduce distortion. Any EM noise picked up and injected into the circuit from the environment by coil A is canceled by the reverse polarity injection of the same noise by coil B. There are also minor cancellations of higher harmonics because the two pickups are not "hearing" exactly the same spot on the strings, which actually is perceived as more distortion in the tone rather than less. My Strat has a sweeter, purer tone with a single coil turned on than does my Les Paul with one of its humbuckers turned on, but the Strat is much more vulnerable to EM noise.

 

[hbiss]

I thought carbon mics worked on the principle that the rarefaction and compression waves of your voice changed the resistance across the carbon bed, modulating the DC applied thereto. Have I been misinformed all these years?

Correct. If you put a carbon mic in series with a battery and a earphone you will have the equivalent of two cans and a string- one way communication though. I suppose it's possible to use a speaker instead of an earphone and, with a high enough voltage battery, achieve a sound output from the speaker that is equal to or louder than the sound presented to the microphone. I should note however that the carbon granules in the mic can't handle much current so that is going to limit the output or even make this not possible.

-Hal

 

[GoldDigger]

The tin can and string has no power gain and so will be limited in application.
The carbon microphone and earphone or speaker can definitely produce a higher sound energy output than the tin can and string, other things being equal.
If you stand back from the loudspeaker in that configuration the sound may not be as loud as when standing next to the talker, but that is because the mic captures only a small fraction of the overall acoustic energy.