[AMRadio] Modulator question.

Al ae6cm ae6cm at sonic.net
Wed May 12 04:28:14 EDT 2010

Greetings from a lurker --

Dick has an interesting idea here and I enjoy pretending that I know how to 
play the numbers <g> so here goes...

Let's use the 200w figure with a car amp with a bridge amplifier powered by 
the car's 12v battery, and for the moment we will neglect the losses.

The amp's output transistors will produce a maximum output voltage swing of 
6v peak (12 v  peak to peak.)  With a sine wave, the rms voltage will be 
about 0.707 x 6vpk =  4.24vrms.  The AC rms current at the 200w output level 
will be about 200w/4.24v = 47.2 amps to drive an impedance of 4.24v/47.2a = 
0.0898 ohms.

So now all we need is a couple or three big honkin' car batteries, several 
feet of car battery/starter-motor cable,a 200w rms car amp that is actually 
capable of 200w (which will be much larger than what most manufacturers CALL 
a "200w" amp), and a 200w modulation transformer with a primary impedance of 
0.0898 ohms.  Oh, and don't forget what Larry said about DC current through 
the secondary of the transformer... and we still haven't figured-in the 

So, is it doable?  How much money do you have Dick? <g>  The guys have 
presented some good options, that are most definately doable. But if your 
heart is set on ss high-level modulation, there might be a better way to 

Here is an amp that uses +65v/-65v for power instead of +12v/0v:


With this amp, the max sinewave output voltage is 65v peak, or .707*65vpk = 
46vrms.  For 200w output the rms current will be 200w/46v = 4.35arms to 
drive a load of 46v/4.35a = 10.6 ohms.  When you figure in the losses, the 
load will probably need to be closer to 8 ohms -- just perfect for that 
off-the-shelf 200w audio output transformer. <g>

In reality, I suspect the transformer will be a problem -- but not an 

I think it's worth a shot Dick!


disclaimer:  I have no connection with apexjr.com other than being a 
satisfied customer.

Dick wrote:

>I have over the years built several low power AM transmitters. I would like 
>to build something a little bigger say in the 400-500 watt plate modulated. 
>I have been kicking around the idea of using one of the solid state 
>amplifiers made for car audio driving a reverse connected audio 
>transformer. I have seen these audio output transformers available in the 
>200 watt range. I understand that the RF output stage would have to be 
>designed to fit the impedence of the output transformer.  Does this sound 
>like it could be done?
> K6KWQ Dick
> Amps by "MORE POWER"

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