EEVblog Electronics Community Forum
Electronics => Projects, Designs, and Technical Stuff => Topic started by: IO390 on April 25, 2014, 08:33:41 am
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I'm posting on behalf of a friend.
He's built a TDA2030-based amp (find attached schematics) and it works fine, driving a small speaker when there is an input. When the input is removed, the output becomes high frequency noise at about 3V p-p.
The amp has been birds nested (point-point) nicely so I don't see any issue there. All of the small caps are good film types. I've built the same circuit before and had no issues.
Any help/advice is greatly appreciated.
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I think a picture of the constructed unit is an absolute requirement to troubleshoot this.
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What do you mean by 'removing the input'? Is the pin on the op-amp physically not connected to anything?
Make sure to "tie" all pins to something at all times (ie ground or +Vs) with a high value resistor. "Floating pins" act a bit like antennae on voltage-sensing devices like op-amps: random garbage gets picked up, amongst other things.
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Normally you would have a jack plug that grounds out the input when removed but the input line should still be pulled to ground by a resistor say about 1M. The input should ideally be made with a shielded cable that is earthed only at one end but this is more important in high gain amplifiers.
You could also try adding a filter but as it works with a input I would say this will just cut off sound in the hearing range.
Having a quick look back at the schematic are you using a pot on the input and is one side of it connected to ground?
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Make sure you have that 22k input potentiometer attached, it's the one that ties the input to ground without any input source.
Is it powered from a battery? If not, try it to see if your PSU might have enough ripple/hum to actually couple to the input via that 100k resistor at the input (1). The circuit has some 30 dB gain in that configuration.
Rats nest can also be a fantastic antenna for all kinds of noise like fluorescent desk lamps.
You said film caps, the ones decoupling the supply voltage should be electrolytic and ceramic (or something else with nice low ESR)
What frequency is the output 3Vpp ? try having an additional RC series circuit from the output (4) back to the inverting input (2), something like 15k and 68pF (= 20kHz upper cutoff)