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 Post subject: Anyone ever tried base mod. for AM? Circuit..
PostPosted: Sep Fri 24, 2021 10:00 am 
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Posts: 776
The AM broadcast transmitter idea is part15 compatible since input is less then <100mW. The TL01 modulator draws very little current while modulating the base of the 2n2222 clasa A amplifier. Output.matching network can be copied from veronica 1290k circuit. Input needs a pure sinewave, preferably from a LC tuned oscillator 80% modulation is easy with better sound quality thanks to tl071's low distortion and low current. Stupid lm386 not used here. That chip is noisy! Output could reach 95% modulation depth. Sound should be sweet and pleasant to hear to as distortion is very low. Varying the base voltage from 0 to 12V changes its output power from zero to full and that's how AM modulation is achieved.


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 Post subject: Re: Anyone ever tried base mod. for AM? Circuit..
PostPosted: Oct Wed 06, 2021 2:38 pm 
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Location: Equinunk PA 18417
RF oscillator should be sinewave, as you said. Regarding an rf oscillator source, I assume low impedance around 50 ohm is best, right? What p-p rf voltage for 100mw silent carrier?

edit: Am I right to think this won't be very linear at modulation extremes as the base swings closer to ground or supply?

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 Post subject: Re: Anyone ever tried base mod. for AM? Circuit..
PostPosted: Oct Wed 06, 2021 3:10 pm 
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Joined: Mar Thu 01, 2018 1:30 am
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richfair wrote:
RF oscillator should be sinewave, as you said. Regarding an rf oscillator source, I assume low impedance around 50 ohm is best, right?

--- yes a collector tuned oscillator. Low Z output can be from the secondary winding.

What p-p rf voltage for 100mw silent carrier?

6.4v

edit: Am I right to think this won't be very linear at modulation extremes as the base swings closer to ground or supply?

Yes. One can play with biasing voltage and emitter resistor to get the max modulation possible. 80% is easy and doable. Don't know if 100% is possible. I've played with it in 2003. 80% was easy. Advantage Is power savings and very good and clean audio.



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 Post subject: Re: Anyone ever tried base mod. for AM? Circuit..
PostPosted: Oct Wed 06, 2021 11:31 pm 
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Joined: Mar Thu 01, 2018 1:30 am
Posts: 776
6V mod = 7.8ma = 90mW DC IN
2.3V = .1ma through R
9.7V = 15.5ma through R

Is 90-100% possible? Maybe. 80% is good.
Due to chronic back pain issues I'm off my workbench.

Plus one can use TL071. 100 times less distortion than 386!

Power out could be taken from center tap or collector


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