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OP amp radio

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Rigel

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Hello, I've built the op-amp radio pictured at this site: Op Amp Radio, but I'm not hearing anything on the speaker. I'm using a 10-180pf variable capacitor and an 11uH inductor (about 100 turns around a paper towel roll), which, if I know what I'm doing, which I don't, should put me right in the middle of the shortwave spectrum. My antenna is about 2 feet long. Any ideas?
 
My current internet connection will not allow access to the aaroncake link.

But at a quick guess, this is just a crystal radio with an op-amp audio amplifier.
Its performance will be very poor.
Make the antenna at least 20feet long and you may be in with a chance, you will need an earth connection as well.
The selectivity will be very poor, you are likely to hear lots of stations all at the same time.

JimB
 
In theory you could use low noise, high speed op amps with active filters, to build an AM radio (1.5MHz max in the US), but I doubt it would have any advantage over a standard transistor design.
 
My current internet connection will not allow access to the aaroncake link.

I couldn't get on either, but most of the circuits there are pretty useless.

But at a quick guess, this is just a crystal radio with an op-amp audio amplifier.
Its performance will be very poor.
Make the antenna at least 20feet long and you may be in with a chance, you will need an earth connection as well.
The selectivity will be very poor, you are likely to hear lots of stations all at the same time.

I seem to remember someone asking about it before, and the opamp wasn't used just for the audio - it's truely a 'design' without the slightest clue what they were doing.
 
**broken link removed****broken link removed**


My estimate of your inductance depends on the dimensions of your coil. My paper towel roll is 38 mm diameter. Assuming you wound your coil with an overall length of 75mm, your inductance would be 154uH. If you coil length is 150mm then inductance is 86uH. So, I'm not sure how you arrived at only 11 uH. Please explain.
With your variable cap, let's estimate the tuning range. It depends a lot on the impedance of your antenna wire, and the capacitance of the diode+op amp load. However, we'll estimate the ball park boundary with the 86uH coil to be 1Mhz to 5Mhz. That's ok for starters but you'll have to tune things empirically to account for the other factors.

If you make your coil large enough, you don't need an antenna if just receiving AM broadcast up to 1.7MHz. Large enough = diameter anywhere from 20mm to 1000mm and coil length of 20 mm (ie. the wires overlap each other).

The op amp is not biased correctly. You can correct this with two different methods: 1) disconnect pin 4 from ground and attach it instead to -9V or 2) disconnect pin 3 from ground and then attach two 100K ohm resistors to pin 3. Attach the other end of one of the resistors to +9V and attach the other end of the other resistor to ground.

You should not use a normal 8ohm speaker in this circuit. The op amp cannot drive enough current to power the speaker. You can use a crystal earphone as the application circuit shows, or you can attach another amplifier after this one to interface to a speaker.

I recommend that you tune the input to receive local AM broadcast signals first because they are much stronger than most shortwave signals and so will be easier to find.

Edit: Audioguru notes something in the next post that is very important also. You must have some sort of DC load on the right side of the diode for the detector to work. Here is an explanation of how the diode (or envelope) detector works:
https://www.st-andrews.ac.uk/~www_pa/Scots_Guide/RadCom/part9/page2.html
so you must add a resistor and capacitor to ground on the right side of the diode. The resistor should probably be about 22K ohms and the capacitor is a small value, like 470 pF
 
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The opamp radio is an old crystal AM radio that uses the tuned circuit from a "real" AM radio.
Its detector diode has no load resistance so probably does nothing and its 741 opamp has no input bias violtage or no negative supply voltage so it won't amplify any audio. Maybe the signal is detected inside the opamp.

Besides, an old crystal radio needs a long wire antenna and an earth ground. Many years ago when crystal radios were used there were hadly any stations so there was no interference between them on a simple crystal radio that has only one tuned circuit.
But today there are many AM stations that need a good radio circuit that has many tuned circuits to select the one that you want and reject all the others.
 

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