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Audio amplifier hum - help!

Discussion in 'Design and technology' started by NewStrings, Jul 11, 2011.

  1. Hi,

    I'm doing audio amplifiers (Rapid electronics project kit) with Y9 and housing in a vac formed case. Everything is going fine, except... half of the circuits have a really loud hum/buzz that completely drowns out the music.

    I've checked all the usual stuff - polarity, joints, bridges, visual inspection of speakers. I've gone at them with a multimetre but can't find any vaults. As far as my eye and my multimeter are concerned, the circuits are perfect. There's a little copper wire behind the joints of the speakers and I thought the problem might have been that some of them were damaged, but even when I'm sure they're perfect the buzz persists.

    Any suggestions which components might be at fault here? I've checked the pins of the chip and they aren't bent or pulled out. We're using the correct value - or rather, the value given in the project pack. That seems to work for those that are fine.

    I'm sure I'm missing something simple but I just can't find the problem, and I really want all of the students to have a high quality amplifier to take home.

    Any suggestions?

  2. bigpedro

    bigpedro New commenter

    I'm not familiar with the circuit but some problems in audio circuits stem from a bad power supply.

    What are you using to power the circuits? If you're using a bench power supply, the output can be as rough as a badgers wotsit. Test with a battery to be sure that this isn't the problem.

    Other than this I can't help much without seeing it, sorry.

  3. Hello Newstrings
    Have been using the circuit for a year or so, but rapid have been know to send out the wrong components, eg resistor and capacitors.
    Sometimes they send C4 100nF as either a ceramic disc or polyester version, I did have a similiar problem with a circuit and the student had mixed up C4 and C2; soldered the 104 (100nF) ceramic disc capacitor into C2, it worked but had the hum and sounded muffled.
    Hope it helps and good luck.
  4. I agree, I have used the Rapid Audio Amp Kit for the last 3 years and it has mostly (1 or 2 exceptions) been excellent, its normally (in my experience) an error with soldering, the components in the wrong way round or wrong component. Its always usefull to order some extra capcitors and resistors for if this happens.
    Just out of curiosity do you charge them for the end product? If so are they willing to pay for it, this is the third year (I have developed a number of different themes for them) that I have used the kits and students are becoming less interested making them (I do work in an all girls school).
    Check out my booklets (click the red_5 above or search for audio amp) feedback on them is appriciated.
  5. Spencer_DT

    Spencer_DT Occasional commenter

    Probably wide of the mark here, so please excuse me..... but my Arcam amp at home started doing the same thing, turns out it the earth to the turntable had come loose, hence annoying buzz. You say the amps are in plastic cases, long shot, but do they need earthing??
  6. I agree, it's difficult to compete with the ones they can buy in the shop and you have to keep on updating it just to keep them interested.
    We did recently get a laser, which has really helped the students make them more personalised, which seems to help sell them.
    I charge £4 which covers the cost of acylic case and circuit, but it's like a catch 22 the better it looks the more students buy it, but the more money you loose when they don't. But by using the lasered designed case it means that a lot of the case parts can be used again.
    I can email some photos of the case, if your interested...

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