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Class D Switching Power Amplifiers and Power D/A conversion

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Old 20th February 2009, 08:37 AM   #1
revans is offline revans  New Zealand
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Default Buzz from Amp6-Basic

Hey guys,

I'm building a portable boombox (something I gather is fairly rare around here) using an Amp6-Basic and two 4" full-range Tang Band drivers I got from Audiotek168 on eBay (seeing all the stuff about those guys now I feel lucky too have actually received them).

I've been testing the setup using my Sony Ericsson phone as an audio input. However, when I first connect the phone to the 3.5mm cable I'm using as an input cable, there is a short buzz and the speakers move a fair bit for a second (sorry I don't know the technical terminology). Also when I touch the 3.5mm plug itself, there is a continued buzzing.

From my limited (but growing) knowledge of speakers, I'm guessing this all has something to do with 'grounding' and 'interference' but what I was really wondering is if I can stop this happening at all? Best case scenario would be where there is no interference or buzz and the speakers only make noise when there's music coming out.

Sorry if this comes across as stupid or is in the wrong section (I guessed that it was something to do with the amp - maybe not...)

Thanks
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Old 20th February 2009, 12:20 PM   #2
Puffin is offline Puffin  United Kingdom
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I suspect that you are using a 3.5mm jack. The buzz is as you say likely to be a grounding issue as a result of a poor earth connection with the phone. Touching the input lead will ground it and make it buzz. Why dont you try a proper CD player input?

The cone movement is probably as result of high DC offset. Have you measured this at the amps speaker terminals? Anything more than 100ma is too much.
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Old 20th February 2009, 12:42 PM   #3
81bas is offline 81bas  Germany
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Hi!

Shunt the input of each channel of your amplifier with 1-10 kOhm resistor. This means, connect the resistor between ground and input wires of channel. Buzz and pop will not fully go away, but will be much smaller...

Regards
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Old 20th February 2009, 05:57 PM   #4
revans is offline revans  New Zealand
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Thanks for the help guys.

@ 81bas: By that do you mean connecting one resistor across the input and ground wires - like it's in parallel with the speaker?

@ Puffin: Would you please be able to explain what high DC offset is and/or explain how to get rid of it and/or how to measure it at the amp's speaker terminals (with a multimeter presumably).
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Old 20th February 2009, 06:13 PM   #5
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No, that would be the outputs ... not the inputs

Either way, you should absolutely NOT shunt the amp6basic's input.

The real question is, does it actually buzz when you play music? To test this it would be good if you could find an mp3 snip of digital silence (try googling), if there's any noise when playing that, several things could be wrong. But try that first and also try playing the same thing from another source, like your PC.
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Old 20th February 2009, 06:28 PM   #6
revans is offline revans  New Zealand
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Thanks Saturnus,
I just noticed that with only the power pack plugged in it still makes a very quiet buzz that is annoyingly discernible in near silence. Is this an indicator of something? Also, I'll get onto that digital silence thing.
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Old 20th February 2009, 07:20 PM   #7
Puffin is offline Puffin  United Kingdom
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To check for DC offset, get a multimeter and set it to lowest DC setting. Put the black probe on the black or - speaker post and the red probe on the red or+ post. Turn the amp on and take a reading. Do the same for both channels.

Make sure you dont short out the speaker posts.
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Old 21st February 2009, 02:50 PM   #8
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While you've got the multimeter out, check to make sure that the ground (body) channel on the cable is working properly--more or less zero on the lowest "ohms" setting on your meter. Alternatively, try a different cable and see if you get the same behavior. I find plugs, jacks and cables to be very likely points of failure.

--Buckapound
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Old 21st February 2009, 10:21 PM   #9
revans is offline revans  New Zealand
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Thanks for the guidance guys, unfortunately I'm having a bit of trouble.

@ Puffin: I guessed that by "lowest DC setting" you meant lowest DC amps setting seeing I'm meant to be measuring current. I turned the amp on and put probes in said places. Nothing was registered and the meter stayed on 00.0 - I'm guessing I did it wrong... or maybe something wrong with my budget $10 meter...?

@ Buckapound: How do you measure the ground channel on the 3.5mm cable? i.e. Where do the probes go?

Also: I just tried using my laptop an an audio source via the same 3.5mm cable. However whenever I went to plug it into the laptop (i.e. whenever the 3.5mm cable touched the 3.5mm laptop jack, there was a major buzzing and the speakers jumped around in a way that definitely didn't look good for them. Does this explain and/or mean anything?

Thanks again for your guys' patience so far towards my ineptitude.
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Old 21st February 2009, 10:30 PM   #10
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A 3˝mm jack has the correct technical name a TRS (Tip-Ring-Sleeve) plug. Tip is right, Ring is left and Sleeve is ground. That also means that when you plug something with such a plug the channels and ground will cross-connect while you plug it in. It's normal to have heaps of noise and buzzing as well as very dangerous looking cone excursions.

It sounds like you might have too high gain ... what input and feedback resistors do you have?
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