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Old 27th Aug 2010, 12:53 pm   #1
Herald1360
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Default Interesting amplification techniques

Given that "Oldest working...." seems to be digressing (not entirely without my help )..... I thought a new thread on interesting and obscure types of amplifier might prove informative.

Après moi, who's got anything to add to the deluge?
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Old 27th Aug 2010, 1:14 pm   #2
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Default Re: Interesting amplification techniques

We could be in Auxetophone territory here: http://www.auxetophone.com/Welcome.html

I once had a play with magnetic amplifiers when I was in the 6th form at school. Actually made a working light dimmer out of odds and ends in the physics lab.
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Old 27th Aug 2010, 1:55 pm   #3
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Default Re: Interesting amplification techniques

Magnetic amplifiers are very much alive - lots of multi-output switch-mode power supplies us them as the control element to regulate the outputs independently of each other. Much lower loss than a transistor, even operating as a chopper.

Auxetophone looks fun (I want one), I'm surprised that the compressor is not in a separate enclosure though, with interconnecting air pipe, for quietness - or was it really so loud it drowned out the motor?

S G Brown made a carbon-granule based AF amplifier.
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Old 27th Aug 2010, 4:11 pm   #4
Herald1360
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Default Re: Interesting amplification techniques

Not quite a magamp, but similar (I think) was the use of saturable reactors (series chokes with an additional dc winding) for theatrical light dimming.

As a technique it didn't last long- expensive compared with big rheostats and soon superseded by thyristors.

There was such a board installed in the IC Union theatre in the mid fifties which I spent some time fixing in the early seventies. AFAIK it's long gone, now.
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Old 27th Aug 2010, 4:23 pm   #5
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Default Re: Interesting amplification techniques

I suppose what I was doing all those years ago was a saturable reactor. I think they are a specific case of a magnetic amplifier.
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Old 27th Aug 2010, 9:05 pm   #6
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Default Re: Interesting amplification techniques

I worked on a newspaper as an electronics engineer during a sojurn out of railways and the AEI motor drives on our nine Crabtree "Viceroy" presses were all of the saturable reactor type with small SCR's used to control the multiple 150HP motors on each press. The AEI system dated form the mid 1960's when the presses were installed and lasted reliably into the mid 90's when three Goss presses were installed - and the Allan Bradley drives in those went bang more often!

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Old 8th Sep 2010, 11:48 pm   #7
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Default Re: Interesting amplification techniques

Seem to remember certain submarine systems power feeding using a saturation method to regulate the voltage - from memory the core was partially saturated with DC to control the output -if it rose ,more saturation ,if it dropped -less . From memory this was Cantat 1 and it's fore runner the Anglo swedish system .
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Old 9th Sep 2010, 12:27 am   #8
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Default Re: Interesting amplification techniques

Saturable reactors made an appearance in most power control applications at some time. Advantages included very high reliability and long-term stability, offset against disadvantages such as peculiar hysteresis effects and waveform distortion.

But while we are on the subject of vintage sub-AF amplifier technologies:

Mercury arc rectifiers with grid control: Grid controlled bulbs were used for variable speed drives and regulated supplies. A single bulb can work in two quadrants and serve as an inverter too.

The Ward-Leonard drive: uses a specially-built generator as an amplifier, on which the armature voltage can be reduced to zero while maintaining low output impedance. Control input is applied to the field winding, power input via driving shaft from separate motor.

Amplidyne & Metadyne: A modified form of dynamo that cascades two stages of electrodynamic amplification in one magnetic system. The control input is applied to a field winding, giving rise to a voltage in the armature that drives a high current through a pair of short-circuited brushes. The resulting armature reaction flux passes through the specially designed pole pieces and generates an armature voltage in mechanical quadrature that is separately collected by a perpendicular pair of brushes. Amplidynes offer high power gain and were popular for artillery and radar servo drives during WW2. Metadynes were used for railway traction speed control.

In the workshop, we have a 7hp DC jig-boring machine drive built in 1956 using a Ward-Leonard set controlled by an Amplidyne controlled by a saturable reactor. All four rotating machines making up the amplifier are mounted on one shaft - AC motor, main generator, exciter and Amplidyne. The rectified control transformer output drives the Amplidyne, which is differentially connected with the exciter that energises the main dynamo field. It works well although set-up involves lots of fiddly interactive adjustments to tune the open-loop transfer function. The power gain is approximately 40dB, the small manual speed control pot working at about 20V and the output to the spindle motor being 0-370V DC & 0-30A

If the OP was specifically about audio amplification, I don't think any of these would be of much use!

Lucien
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Old 9th Sep 2010, 1:40 am   #9
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Default Re: Interesting amplification techniques

Hello,

Mr Edison's 'Electro-motograph circa 1879?

http://www.diclib.com/Electro-motogr...electrical/938

Yours, Richard
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Old 9th Sep 2010, 9:42 am   #10
Lucien Nunes
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Default Re: Interesting amplification techniques

I wonder whether the Electromotograph actually had gain, or merely less loss than ordinary electromagnetic tranducers of its day. Without audio power gain, I think it would only qualify as a power-assisted transducer rather than an amplifier.

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