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Old 14th November 2014, 03:38 AM   #1
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Default hum noise on home build speaker.

I am building a portable Bluetooth speaker. I purchased a cebek 20w rms amp and I am powering a single speaker. I am using a 12v battery to supply power. I haven't purchased the Bluetooth module yet and I am just running it direct.

I have a fairly loud hum at low volume or if nothing is plugged in. I have tried a different battery pack and different input sources with no result.

I don't know if it is a bad amp board or what. It sounds good when the volume is up, but the background hum is present.
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Old 14th November 2014, 11:48 AM   #2
AndrewT is offline AndrewT  Scotland
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Sort the grounding errors.

Post a schematic and a set of detailed pics of your assembly.
Then we may see where you have gone wrong.

BTW,
the maximum AC signal from a 12Vdc supply is approximately 4Vac, or a less, sometimes a lot less.
4Vac into a 4ohms speaker is equivalent to 4W, NOT 20W, into 8ohms speaker it is only 2W maximum.
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Old 14th November 2014, 07:46 PM   #3
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A battery powered amp with no external input, shouldn't hum.
Is it expecting a Bluetooth signal or an analog audio signal?
It it's analog, try shorting the input.

A high tech amp could have an internal voltage step-up circuit in it's supply.
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Old 16th November 2014, 02:45 PM   #5
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Here is my setup.
the capacitors are added to the input by recommendation of the manufacture. The email they sent here. hello
to connect to a phone
putting series capacitor 100 nF / 100 V to the signal lines
put a capacitor to each signal wire
regards. cebek.

I am wondering if I need to put a volume knob on. When I connect to my phone or laptop I have the volume on those devices very low.

I notice the hum gets louder if I touch the metal case of the battery, amp heat sink, either input, or output.
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Old 17th November 2014, 06:23 PM   #6
AndrewT is offline AndrewT  Scotland
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pic is 169 x 300 pixels.
It is too small for me to be able to read !
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Old 17th November 2014, 09:36 PM   #7
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sorry. The links below should be larger.
It put several pictures upside down. I'll fix that when I get off work.

If you know a better free image server I can use that as well.
Thank you for the help.
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Old 17th November 2014, 10:03 PM   #8
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i can think of a reason.
when You plug thius thing into Your laptop or whatever, You have the volume on the device set to near zero.
laptops and other things have some noise, and since the volume is turned down a lot, not a lot of signal is mixing with the noise. and the amp does what it is designed to do, amplify whatever is on its inputs.

but, if you use a volume pot, then you can turn the volume on the laptop or whateever device all the way up. thisway, a large signal will be mixed with a small noise.
and the sum of this will be turned down by the pot.

give it a go.
i bet it will help you.
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Old 17th November 2014, 10:24 PM   #9
sreten is offline sreten  United Kingdom
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Location: Brighton UK
Hi,

Any amplifier claiming to be 20W with a 12V supply
is almost certainly a "bridged tied load" type, and
20W is about right with a 13.8V supply and 4ohm.

FWIW any car amplifier works with 13.8V, most
12V chips are car chips and will work with 15V DC.

rgds, sreten.
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Old 17th November 2014, 11:20 PM   #10
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Thank you
I was wondering if this was the problem. I'll try that.
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