Newbie Q: Audio amp as Power Supply

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Newbie Q: Audio amp as Power Supply

Postby Brian K1LI » Sun Nov 24, 2013 9:22 am

From an EER neophyte: why can't a simple audio power amp be used as the "supply" for an EER system? It's just delivering audio-modulated power into a low-impedance load, right?
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Re: Newbie Q: Audio amp as Power Supply

Postby Gilly » Tue Dec 03, 2013 8:42 am

Hi Brian

Your question is probably not in the same area that this WEB site is about....
Here we use Class E for AM transmitters for their high efficiency in the frequency range of 1.8Mhz to around 7Mhz only....
Steve has developed Class H and PWM ( High efficiency ) Modulators to superimpose the audio into the TX for our AM signals....
In the EER systems ( don't know too much about them really ) the power / voltage comes from a high efficiency power supply to the Class E amplifier...then a wide band amplifier modulates the Amplifier via a shunt feed method....also the opamp used for the modulator seems to be a OPA357 that can operate up to 250Mhz
The inductors and capacitors in this power supply seem to be small in value indicating to me that the modulating signal is varied and high in frequency as well...
So an audio amplifier is unsuitable for this type of system..I think ?
Maybe I'm wrong....


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Re: Newbie Q: Audio amp as Power Supply

Postby Brian K1LI » Wed Dec 04, 2013 8:09 am

Wayne,

Thanks for your thoughtful response, but I'm still not sure I see the light.

I would be interested to know more about the EER system in which you found reference to the OPA357. My guess is that its 150V/us slew rate and 100MHz GBW are intended for signal bandwidths associated with sophisticated data modes, not the low-kHz bandwidths of audio signals.

Yes, Class H is a more efficient method for producing the drain voltage that modulates the carrier signal provided to the gate, thus effectively complementing the overall goal of delivering a power-efficient transmitter. I found this in Steve's "modulators and power supplies" section:

"Such a modulator can use any technology - class A analog, high efficiency analog, or digital (Pulse Width), as long as the result is modulated DC applied to the RF amplifier."

So, for purposes of experimentation, I'm wondering why one couldn't simply connect the speaker outputs of a garden-variety "stereo" to the drain circuit of a class E amplifier for purposes of modulation.
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Re: Newbie Q: Audio amp as Power Supply

Postby kf1z » Wed Dec 04, 2013 6:46 pm

That depends on the MOSFETs used and the voltage/current output of the amplifier.

there is no reason I can think of that a normal stereo amp couldn't be the power supply for a class-e stage.

The output would have to be shifted ( biased?) to all V+ though.
As a normal stereo audio signal is AC not DC I believe ( excursions above and below 0 volts.)

Could be wrong of course. :-) and that may be amp type dependent of course
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Re: Newbie Q: Audio amp as Power Supply

Postby Gilly » Thu Dec 05, 2013 12:20 am

Hi Brian

Hi to you Bruce...hope your well...see you have moved across to the other side of the States....

Well not sure if you meant Stereo but certainly can use a mono amplifier and connect it to the Power Supply feed to the TX via a series capacitor...
The critical component is the Audio Choke which will greatly effect the Audio Quality of the AM signal..
On my first Class E TX that I built this was my first method of Modulation and worked well really...

Just wondering what your intentions are for your experimentations....
Are you wanting to build a reasonably high power TX ? with in time a PWM...for the lower Amateur HF bands ?


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Re: Newbie Q: Audio amp as Power Supply

Postby Brian K1LI » Thu Dec 05, 2013 6:05 pm

Wayne,

Thanks for weighing in and affirming my hunch. While I am new to the practical side of Class E, I've read about it with great interest for many years and I'm interested in experimenting with high power transmitters, rather than adding an amplifier to the output of an existing rig. With the availability of low-cost FPGA and DSP technologies for signal generation, I think our hobby is ripe to replace the expensive, cumbersome and inefficient amplify-filter-amplify-filter-amplify-filter architecture with a more elegant approach. Being "fun per dollar" driven, I want to experiment with stuff I have lying around as much as possible and add what is necessary as I go. In addition to having an extra audio amplifier ("stereo"), I have the (still unbuilt) IRF510 amplifier kit from March 1999 QST, which I plan to use as the starting point for my experiments.

I'm particularly interested in the Microsemi app notes written by Dick Frey, K4XU, using the ARF-line of high-voltage MOSFETs. These seem especially amenable to experimentation by amateurs. I'm an EE by education, have a reasonably well-equipped workbench and plan to begin with LTSpice simulations.

I look forward to many interesting exchanges with the group.

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