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Medium Frequency Amp

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moicez

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Hello to you all, thanks for reading, so here is my question:

What can I use to amplify frequencies from 500kHz to 3MHz?

The reason is that I am going to multiplex and then demultiplex and in between amplify the sound using AM. So after the AM signals get amplified I will filter then demodulate (probably going to use an envelope detector) and finally connected to a speaker.
 
It would help if we knew what you are tying to do. What's the reason for the multiplexing. Is this AM signal being broadcast? Why such a wide bandwidth? How much amplification?

Inquiring minds want to know. ;)

In general a high speed op amp should provide the amplification you need.
 
I forgot about the wattage (very important), i guess 1000 watts of power should do it. That way I will be able to drive at least 10 speakers, no problem.
The reason that I'm multiplexing is so that I will have a 10 or more channel audio system, actually it will be a MxN channel system. Its just something that could potentially trip your senses in a room with so many channels.
And no, I'm not gonna broadcast. There are two main reasons for modulating, one is for broadcasting (typical) and second for multiplexing :)
 
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If I understand this correctly:

1 You are going to take several audio signals,
2 Amplitude modulate each one on to a discrete carrier frequency in the range 500 to 3000khz,
3 Amplify the resulting set of AM signals to 1000watts
4 Individually demodulate the signals using an envelope detector (a diode)
5 Drive a speaker with the output of the diode detector.

So, after I picked my self up off the floor, where I had collapsed in a gibbering heap paralysed with hysterical laughter...

I can only say that your proposal seems to be seriously flawed in many ways.
Please correct me if my understanding is wrong.
If my understanding is correct, I will reply pointing out the problems with your proposed scheme.

JimB
 
hahahahahaaaa
yup, your understanding is correct Jim. I am eager to know the flaws so I can get this project done faster.
 
hahahahahaaaa
yup, your understanding is correct Jim. I am eager to know the flaws so I can get this project done faster.

The entire idea is full of flaws, but the main one that provides great hilarity is the idea of high power RF amplification.

Assuming that you really want to use low quality AM modulation? (which is another huge flaw), you would do so at only VERY low power (in fact not really 'power' at all, just line level). The high power amplification would be done at the speaker end, where you feed the demultiplexed and demodulated line level audio to the power amplifiers.
 
your understanding is correct Jim. I am eager to know the flaws so I can get this project done faster.
I hope that you would abandon the project in its current form and re-think it.

1 You are going to take several audio signals,
OK so far.

2 Amplitude modulate each one on to a discrete carrier frequency in the range 500 to 3000khz,
That is easily acheivable, but you will have to ensure that the outputs of the modulators are well filtered to remove harmonics and select carrier frequencies so that harmonics and intermodulation products produced by the amplifier will not interfere with the wanted signals.

3 Amplify the resulting set of AM signals to 1000watts
Now we are getting difficult. High power RF amplifiers are not trival devices to design or build, especially ones with a such a large frequency range as 6:1 (500:3000khz).
The amplifier will have to be very linear, otherwise the various carrier frequencies will generate harmonics and intermodulation products which will interfere with the wanted signals.
At best this amplifier will only be 50% efficient and will be running a full power all the time, you will need a 2000watt power supply.
With the best will in the world, even though you are not intending to transmit this over the air, unless the screening of all componentes of this system is excellent, you will be heard on the air over quite a large area.

4 Individually demodulate the signals using an envelope detector (a diode)
Assuming that you have managed to build/buy a suitable amplifier, at each speaker you will have to filter carrier frequency for that speaker from the nine other frequencies.
As this is all happening at the 1000watt power level, let us assume that the distribution system is 50Ω the voltage will be of the order of 225volts. This will need some seriously expensive capacitors.
Now to demodulate the signal you will need some high voltage high current RF diodes.
They probably do exist somewhere, for a price!

5 Drive a speaker with the output of the diode detector.
Now how are we going to connect the 100 watts* output from our diode detector to a 4 or 8Ω speaker?
We could use a matching transformer maybe.
* Did I say 100 watts? Oh silly me!
Yes we did use a 1000watt amplifier for 10 separate signals but what we forgot was that a 100watt carrier with 100% AM modulation has a peak envelope power of 400watts.
So because we need our amplifier to be nicely linear each carrier can only be 25watts.
So we will only be driving our speaker with 25watts**.
**Did I say 25 watts? Oh silly me!
I forgot to mention that the "audio" power in a 100% modulated AM signal is only half the carrier power, we will only have 12.5watts to drive our speaker.

So, do you still want to do this project?
Has my insight into the problems helped you to get it done faster?
Please let us know.

JimB

 
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