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Author Topic: Do RF amps behave like Audio amps?  (Read 1874 times)
KA1MDA
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Posts: 548




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« on: December 10, 2008, 11:42:52 AM »

I have a strange (and hopefully not too silly)  question I was hoping to have answered by some of the resident Amp experts here.

I've been involved in audio/high fidelity for at least 30 years. Back in the 70's, the hi-fi industry played fast and loose with highly exagerrated power ratings of their audio amplifiers, causing the Federal Trade Commission to step in and set up standards for measuring power output and advertising.

If I remember correctly, one of the FTC standards calls for measuring power output (using RMS) with both channels driven, after a preconditioning period of 1 hour, during which the amp is driven at 30% of it's rated power output with a sine wave. The reason 30% output was chosen was that it put the greatest thermal stress (dissipation?) on the amplifier's output devices.

Since AF and RF amps are both linear and operate in similar classes (B, A/B) does the same thing happen to an RF amp being driven at 30% of its rated output? For example, would a 1KW linear operating at 30% rated output run hotter than if it were running at 50% of rated output? Common sense says driving an amp at less than full output will place less stress on the components, but is there a point where running an amp "too low" causes more stress, as in the audio amp example above?

Thanks for all comments/replies.
Tom, KA1MDA
www.ka1mda.org
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K0ZL
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« Reply #1 on: December 10, 2008, 12:13:48 PM »

I think the FTC was messed up, if I am reading the specs correctly here; While a class AB1 or AB2 amp will tend to be more efficient at the higher power levels (due to the idle or bias current taking up less percentage of the total input power at the higher power levels), any class AB1 or AB2 amp will have more heat to get rid of at higher power levels, than at lower levels. A possible justification for testing at less than 100 percent output, might be that at moderate power levels, an audio amp tends to be "cleaner" than at lower or higher levels.

A push-pull audio amp should be driven to the level of distortion above which would be unacceptible, then the power output is measured in RMS. You can the play games with peak or p-p power if you want to make the amp seem more powerful than it is. I have seen "100 watt" guitar amps put out only about 30 watts RMS before they start clipping.

RF amps can be run up to whatever criteria you choose, such as compression or loss of linearity, to find their maximum output within an acceptible distortion level.

Class A amps are different; RF or AF, the class A amp will ideally be dissipating the same average power all the time since the device is biased well up into conduction in order to take advantage of the most linear part of the device transconductance curve. Additional power showing up at the load as the amp is turned up comes from the power supply; plate, collector or drain dissipation should not change very much in the real world regardless of the drive level.

Class D and e amps are completely different animals. The idea here is that the device is either all the way off (open) or all the way on (shorted). In either ideal case, the energy dissipated as waste heat in the device is zero; all energy developed in the circuit is sent out to the load. Realistically, devices never turn all the way on, there is always a Vsat or saturation voltage that, when multiplied by the current through the device, gives the total heat dissipation when the device is on.

So class D and E amps, using external lumped components to help restore the original waveforms, have gained great acceptance in both RF and AF circles due to their great efficiency (85 percent and up). We even like them in our motor speed controllers; a 60 HP 3 phase AC motor can be driven from an inverter about the size of your small window AC unit now. You can get a 2000 watt RMS audio amp in a 2-rack  unit space that weighs 27 pounds. The neighbors might not approve.

Looking at your linear mode power supplies, which is just a DC amp, yes indeed, running them at less than full output can actually cause them to run warmer than at full output; depends on the voltage drop in your power transformer.

Class C RF amps are much like class D and E in that they are in conduction only for a small part of the input cycle waveform; they "kick" a resonant circuit like a flywheel; again, the tube (typically, a tube) is biased completely off; along comes the positive-going waveform of the input to the grid, suddenly the tube turns on like a mother, pulls a bunch of current from the power supply, the change of current is transferred to the tank circuit where a largish circulating current is set up. If the circuit is resonant, the tube kicks the flywheel at just the right spot and just hard enough (called "loading"); if it's coupled effectively to the load, you can see 70 to 85 percent efficiency.

Some RF amps are actually combination class C rf amp and class AB1 audio amp, using "heavy iron" to modulate the plate source to the class C part. These are commonly called "boat anchor AM transmitters". If you want, I'll 'splain how you can take 30 watts RMS of audio power, combine it with 100 watts from a pair of 6146 tubes, and come up with 440 watts PEP of AM modulated signal. It's the magic of plate modulation...

73, Bill K0ZL
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KE3WD
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« Reply #2 on: December 10, 2008, 04:58:01 PM »

The lower drive rating for audio amps is likely to guarantee that the amp is not driven into clipping, which would yield an inflated power reading but tells us nothing about the audio amp's ability to reproduce the input with fidelity.  

30% would be about right to preclude that condition.  Give or take.  


KE3WD
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K7KBN
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« Reply #3 on: December 12, 2008, 11:05:29 AM »

Shortly after the FTC stepped in to specify just how audio amplifiers were to be rated so as to put them all on an even playing field, I stopped in at a local Radio Shack.  Here were all sorts of amplifiers that had been advertised as "500 WATTS (instantaneous) POWER" and such, now proudly bearing markings of "30 WATTS PEP".

Very innocently, I asked one of the clerks, "What's this 'RMS' stuff?".

His reply?  "Oh!  RMS stands for "Real Music Sound!"

I thanked him, and left.
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73
Pat K7KBN
CWO4 USNR Ret.
W8JI
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« Reply #4 on: December 14, 2008, 06:42:18 AM »

There is no such thing as RMS power. It is technical nonsense to say "RMS power".

There is only peak or average power.

Let's not allow the audio phools to mix into radio any more than they already have.

The steady state or carrier power is the average power, not RMS power (a meaningless term). A sine wave's power is the average power. It is also the peak power of the sine wave. It is by definition nothing but the power.

A complex waveform or a time-varying waveform can have different values of peak and average power.
It has no "RMS power". If you do the math you will see why.

73 Tom
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