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Radio Breakthrough

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Graham Slee View Drop Down
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    Posted: 26 Sep 2017 at 5:16am
When the fridge thermostat goes to off do you hear a click through your speakers or headphones?

If you do it's radio breakthrough and this 1976 article still applies.

(download http://www.americanradiohistory.com/Archive-Practical/Wireless/70s/PW-1976-02.pdf and find page 39 in scan order which is actually page numbered 853)

41 years ago this article criticised audio circuits for being too sensitive to radio frequency interference (RFI) and offered some solutions.

Back then in the UK you were protected from the wrath of interference by the government Home Office. The things they would do to stop you and I getting our hands on a transmitter. They even passed a law to stop radio ships broadcasting pop music from the sea to the UK mainland, making it a criminal act to do so.

Today though the world has laxed and most households have a number of radio transmitters. The 'pushers' have made us reliant on them, and we can no longer conduct our lives or businesses without them.

The mobile phone in its many forms; wi-fi; bluetooth; DECT phones; even Ethernet over mains; the car key; the garage door opener...

And therefore radio interference on audio circuitry has mushroomed. Not that it makes itself known on every hi-fi system, and that's because of things like FET inputs, but some circuits are still highly sensitive, such as phono stage inputs.

Back in 1976 you could take the lid off the amp and find the first transistor to apply the 'cure' which was the addition of a small series resistor, and a capacitor which shunted the offending RFI away from the base emitter junction which is the equivalent of a radio set's rectifier diode.

The problem today is it simply kicks it down the road. It causes an oscillation-in-waiting where an even higher frequency (which wasn't around then but you can be sure it is today) can come along and modulate the signal. It will not be rectified and appear as radio reception, but it will muddy the sound.

And another problem today is finding that transistor. It's often an operational amplifier: you can find the base connection, but not its emitter. You can however shunt it to the op-amp's other input which does the same job. Unfortunately that can send the op-amp unstable, and if the op-amp is configured to be inverting, it will definitely oscillate!

So what's the answer? For that you'll need part two of the article in http://www.americanradiohistory.com/Archive-Practical/Wireless/70s/PW-1976-03.pdf

There it talks about effective shielding. But these days in hi-fi people have been convinced to have unshielded cables. Most hi-fi owners believe it to be a good thing (and most hi-fi owners have been bullsh*tted).


Edited by Graham Slee - 26 Sep 2017 at 5:18am
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Graham Slee View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 27 Sep 2017 at 7:53am
I think that when the billion dollar semiconductor manufacturers start making EMC hardened op-amps, that is when we should realise that the problem of interference is serious.

They have started making them.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 27 Sep 2017 at 10:42pm
Originally posted by Graham Slee Graham Slee wrote:



Back in 1976 you could take the lid off the amp and find the first transistor to apply the 'cure' which was the addition of a small series resistor, and a capacitor which shunted the offending RFI away from the base emitter junction which is the equivalent of a radio set's rectifier diode.

The problem today is it simply kicks it down the road. It causes an oscillation-in-waiting where an even higher frequency (which wasn't around then but you can be sure it is today) can come along and modulate the signal. It will not be rectified and appear as radio reception, but it will muddy the sound.



With a little inspiration from the techniques used on EMC hardened op-amps I simulated the shunt capacitor idea again but this time with a twice value cap going to ground. The oscillation was gone. Now, what can I design using discrete transistors...? Ermm
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