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Old 8th June 2012, 10:26 PM   #1
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Default Fixing RF and hum in battery system?

Howdy,

Sorry for posting what I know has been discussed elsewhere, but I still struggle with this.

I'm running the tda2003 audio amp with this kit:

Hobbykit - Electronic kits and modules, PCB

It's a battery system-- no high-voltage AC house current involved.

Getting a lot of RF and hum. The hum and RF vanish when I plug in the mp3, or when I touch the heat sinks, or when I'm outside.

I'm planning to install in a metal chassis. Should I connect the chassis to the battery negative?

also, would the "connect just one end of the cable ground" tip be a good idea? That means the audio cable, right? Which end?

Does it matter if the audio cable shield is used for ground, or better to have a separate conductor for ground?

Note, the audio input is a stereo cable connected to a stereo mp3 player, combined into the mono amplifier input, with 5K or 10k resistors on the Left and Right conductors just before they are combined at the amp input, to protect the mp3 outputs from driving each other.

Thanks!

Last edited by johnyradio; 9th June 2012 at 07:32 PM.
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Old 9th June 2012, 08:59 AM   #2
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Hum is usually a problem of getting residues of the mains frequency into the signal due to bad grounding. In your case, where there is no mains connection involved at all, it may however be a sign of the built-in protection system's working. Check is everything is connected correctly.

Sometimes hum is also a consequence of a bad connection, e.g. a bad solder joint or a loose 3,5 mm jack. resolder all joints and check the jack.

It is a good idea to connect the chassis to the battery negative, but it will probably not help you with your issues.

Connecting just one end of the cable shield could solve mains frequency hum, if you had another connection between the MP3-player's ground and the amp ground, e.g. through PE or a separate wire for ground. According to your system description there is no such connection, so you will need to connect the shield on both sides. If you use a seperate wire for ground, then you can lift the shield on the amp side for improved RF shielding, but it is not likely to help with the hum issue.
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Old 9th June 2012, 01:19 PM   #3
johnr66 is offline johnr66  United States
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This is due to the high impedance of the input. There is no external resistor to ground on the input pin, so there is only the internal input impedance of the input and the datasheet shows it to be 150K ohms (typical). Depending what your audio source is, you can add a resistor across the input. If you are amplifying the output from a music player or Walkman, you can use a lower resistance, say around 1K. This should eliminate a lot of the noise and positive feedback oscillations. Other sources may need a much larger value. Be sure this resistor is connected from the external side of input capacitor so as not to disturb bias currents on the input circuit.

You may also need a low pass filter on the input to allow the audio spectrum through but to block RF.

You should twist the input wire and its ground lead together from the board to the input jack you use. You can ground the chassis, but do so at only one point to prevent ground loops. Remember that this is a bridged amplifier. The output leads must float. Do not ground either side. Some jacks may short one side to a metal chassis, so beware.
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Old 9th June 2012, 07:29 PM   #4
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Sorry for not explaining: My circuit works perfectly. The solder joints are good. The hum and RF vanish when I plug in the mp3, or when I touch the heat sinks, or when I'm outside.

How does that change your responses? Really appreciated.
  • Why would lifting one side of ground not help in this situation? Does the fact that there's only hum/RF when mp3 is disconnected and indoors change that?
  • How can I create a "separate wire for ground" from the mp3? Meaning, 3 conductors plus shield? left, right, ground, and shield?
  • What's PE?
  • Why would connecting the chassis to the battery negative not help? Does the fact that there's only hum/RF when mp3 is disconnected and indoors change that?
  • How could hum be " a sign of the built-in protection system's working"?
  • Do I still need to "add a resistor across the input" if there's only hum/RF when mp3 is disconnected and indoors?
  • If I want to "twist the input wire and its ground lead together from the board to the input jack", does that mean I cannot use the shielded audio cable I'm currently using, and does this fix apply if there's only hum/RF when mp3 is disconnected and indoors?
  • How can I ground the chassis "at only one point"? Which point? Does this fix apply if there's only hum/RF when mp3 is disconnected and indoors?
  • There are no jacks to short one side to a metal chassis, just the audio cable wired directly to the circuit board with a male 3.5 mm stereo connector with plastic housing at the other end, going into the mp3.

Many thanks.
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Old 9th June 2012, 08:39 PM   #5
sesebe is offline sesebe  Romania
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Hi,

You very high impedance input (starting from 150K) work like an antenna without mp3 player.
After you change the schematic according to johnr66 recommendations you problem will be solved (at least partially).
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Old 10th June 2012, 11:15 PM   #6
Minion is offline Minion  Canada
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Location: Cowican Bay , vancouver island
Generally speaking most every amp will hum when there is no input connected ..... The input can act as an antennae when it is left floating with no input connected ..... I know with my guitar amp if it dont plug the patch chord into the guitar it picks up hum and noise like crazy .....

Try useing a shorting input jack on the input of the amp , that will make it so the input is automaticly shorted to ground when nothing is plugged into the amp and will remove most all noise ......

I just built a small stereo amp for my MP3 player useing a HA13001 2x5 w chip and it has the same noise problem with no input connected , I just dont run the amp without nothing connected to solve the problem ......
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Old 11th June 2012, 11:52 AM   #7
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Quote:
Originally Posted by sesebe View Post
You very high impedance input (starting from 150K) work like an antenna without mp3 player. After you change the schematic according to johnr66 recommendations you problem will be solved (at least partially).
you mean the drop-down resistor?
will that affect my SPL's or bass response?

what's the other part of the solution?
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Old 12th June 2012, 02:24 PM   #8
sesebe is offline sesebe  Romania
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If you connect the resistor before the input capacitor, the bandwidth will not be afected (suposing that the MP3 player are DC connected to output). The resistor must be connected directly from input not from pin 1 of IC.

You use a phone output of MP3 player?
In this case just use a resistor with a resistance bigger comparing to phone inpedance (5x to 50X bigger) and the bandwidth will not be affected.
For inputs usualy connected to phone outputs I use 220Ω from input to GND. This resistor will work like normal load for MP3 player.

In the same time you can change the value of R1 (decreasing or increasing) and to increase the value of C4 to have the same bandwidth when R1 have lower value.
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Old 12th June 2012, 03:03 PM   #9
mjurban is offline mjurban  United States
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All the comments about the input network are valid, but I think the secret lies in your statement that the hum goes away when you're outside. Do you have florescent lights inside? They are notorius for creating hum in electronic equipment, and the solution, as already mentioned, is a good rf filter on the input.
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Old 29th October 2012, 11:52 PM   #10
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thanks everyone for these great tips! Will report progress in the future...
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