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Ribbledox

Active member
Joined
Oct 15, 2004
Messages
33
Location
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Just a short little question out of curiosity=)

Up to the 1960's, was the vari-mu topology the only topology around?

Because the La2a is supposed to have been revolutionary with its LDR circuitry when it was launched in the early 1960's.

Cheers!
 
Eddie Ciletti has a great piece on this...
http://www.akzent-audio.de/unterseiten/cs-komp-artikel.html
http://www.tangible-technology.com/dynamics/comp_lim_ec_dh_pw2.html
 
> was the vari-mu topology the only topology around?

Pretty much.

Remember that transistors were not available until around 1954, and not good enough for pro audio until after 1960. And even in 1960 they were not very uniform, so circuit design was all about setting the gain with feedback so device variations did not matter. While the base theory of good VCAs was published in the early 1950s, practical understanding (and usable devices) took a while to develop.

It is possible to use plate resistance, same as an FET, but most tubes are unsuitable. Selected 12AX7 work in low-quality applications. GE used plate resistance as feedback around a vari-Mu to extend the deep-GR range where a vari-Mu wants to go nasty.

An incandescent lamp can give slow AGC. Not fast enough to avoid audible transmitter overmodulation.

The earliest boxes seem to have been invented to protect RCA's film-sound system, using ribbon-shutters which would clang and bang horribly on slight overmodulation. Attack times were in microseconds.

While photo conductors have been around since before 1900, good reliable ones don't seem to have been available until the 1950s. Also a high-speed lamp is needed, incandescent is way too slow for film or radio protection, neons are very hard to linearize.

Saturable reactors are old, but not practical until the 1950s, and as reactances they are hard to use in wide-band audio. Also I think they are slow, especially when the control rate is similar to the signal frequencies. And I think distortion is "high". They had a very short fad in big-lamp and some industrial control. I think about the time tubes big enough to pass saturation current arrived, transistors and SCRs were not far behind.

From 1925 to 1960, grid voltage control of gain was THE way to control radio gain, very well known to everybody, pleany fast, and well understood.
 
AGC or automatic gain control, was probably here before compressors/limiters.
In fact, a lot of audio stuff is trickle down from the RF experts. RF design is ten times harder than audio design, so if you really want to know your bussiness as an audio designer, get a phd in old radio design and audio will seem like child's play.
 
[quote author="cjenrick"]... In fact, a lot of audio stuff is trickle down from the RF experts. [/quote]

yep
I think cj may be right. My Dad tells me of many methods that were born of RF transmission solutions.
AGCs are one ... other methods that we might call Diode clamping today. These were not meant to sound good, just save the transmitter and get the message across. Other strange methods with light-globes to dynamically current limit parts of a circuit.
 
> The first rule was, and is, "thou shalt not overmodulate."

So fuzz-boxes are immoral?

Dave, All.... this has been bugging me all day. I can't think of any techniques other than the ones I mentioned, and for quick clean GR it really comes down to grid-control.

There is one other: there is a way to wire a pentode as a poor multiplier. And I think it is to put some negative voltage on the suppressor grid (normally tied to cathode, in many tubes it IS tied to cathode internally). The Gm is nearly linear with G3 voltage. Tying G1 and G2 together gives nearly "squared" plate current. Never seen this done in audio. And frankly, I would call it just another grid-volt technique.

Same with Pentagrids, used as frequency changers in radios. Some of them let you put signal in one grid and control gain with another grid, which sure simplies push-pull drive (don't have to handle GR voltage at the signal grids). Still just grid-volt control.

There is yet another trick, though it seems to have arisen very late 1950s. There is a special beam tube that can be used to demodulate FM by switching current from one plate to another. The idea could be used to steer audio to the output plate or a dummy plate. But for best FM detection it needs to switch very abruptly, and the available parts don't seem to be useful.

One wacky way I never saw: set up a CRT, with two metal areas on the faceplate. Modulate the cathode current with audio. Use electrostatic deflection plates to steer the beam to one plate or the other. Use a defocused beam to get a soft transition. But the bean current in a CRT is very low, so the available output power may be too small to use. (Similar things were tried as computer memory, and generally frustrating because of instability.)

Given film-sound shutters (which can move pretty fast), it should be possible to modulate light with audio, modulate again with AGC, and detect that in a phototube. But phototube sound is generally bad enough even before you play games to reduce it. And most such modulators acted very badly in overload.

> Diode clamping today. These were not meant to sound good, just save the transmitter and get the message across.

In low-quality message systems, GROSS distortion is not a problem. Over 100% THD sounds funny, but after a minute you can understand speech just fine. In fact for a given power limit, grossly clipped speech with some spectrum shaping filters can be understood at much greater range than clean speech. In classic AM transmitter design, overmodulation (especially negative) isn't just clipping, but throws sidebands all over adjacent and distant channels. (It could also cook the transmitter, though that's not so easy on commercial gear.) The B-town cops hear monkey-chatter when A-town's cops splatt their transmitters. That's illegal. Clipping in the audio path then low-passing at 3KHz gives "clean clipping" that gets-out without crapping-up other channels.
 
Thank you for all intersting posts!=)

To go back to the La2a, were there many 'leveling amplifiers' available before its launch? Or were most of the limiter devices fast acting peak limiters?

Because I can imaginge that peak limiter characteristics were the most sought after in radio broadcasting, and that's were most of the develoments were done.

Anyway, thank you!

/R
 
There were simple levelers in many of the tape recorders of the time. Mostly "half-a-vari-mu" types, with biasing voltage applied to a single grid in an amplifier stage..
 

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