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I wish to hook up the line-out from a digital piano to a surplus 4 ohm 15w speaker unit. I bought a simple Amplifier module and connected it up.

My plan was to roughly assemble the circuit and, if it worked ok, transfer to strip-board with RCA connector for Audio and a DC power socket and some sort of quick connector for speaker cable.

When I play a note on the piano I get a good level of sound from the speaker but the quality is very poor.

I guess I need to use better wiring on the input side of the amp?

Circuit:

enter image description here

Here's a photo:

enter image description here

  • Battery: 12V SLA 3.3Ah
  • Speaker: 4 Ohm 15W
  • Amplifier: Kemo M031N 4.5-12V, 3.5W "Music power", for 4-15 ohm speaker, 40Hz-20kHz,
  • Source: Line-out 1/4" TR connector (left channel, set to mono).
  • Pot: 10K Linear

The speaker is a rear satellite speaker unit from an old TV that had 5.1 surround-sound. The speaker enclosure contains two sounders (approx 5" and 1").

The digital piano is powered through a mains adapter (100-240 VAC 0.2A 50-60Hz) connected using a USB cable (5V 1A). It is light so assume switched mode.

Notes sound rather buzzy. There is mains hum in the background.

What steps can I take to improve the quality of the sound?

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    \$\begingroup\$ Try making a single point ground (bring all the returns to one point) then bring that single point to the battery. Also shorten up the wires as much as you can, and try adding a ceramic capacitor across the amplifier supply + and - as close to the amplifier as you can get. \$\endgroup\$ – Adam Lawrence Nov 4 '11 at 19:08
  • \$\begingroup\$ @Madmanguruman: Thanks, can you suggest a value for the cap? \$\endgroup\$ – RedGrittyBrick Nov 4 '11 at 19:11
  • \$\begingroup\$ I would consider a ceramic type, from a few hundred nanofarads up to as high as a microfarad. You may also try an electrolytic capacitor of a few tens of microfarads close to the amplifier. \$\endgroup\$ – Adam Lawrence Nov 4 '11 at 20:45
  • \$\begingroup\$ What do you mean the quality is poor? Is there usually mains hum in the output of the piano? Do you have this connected to anything other than the piano? \$\endgroup\$ – endolith Nov 5 '11 at 3:33
  • \$\begingroup\$ @endolith, I tried the piano on my HiFi amp, there is a little mains hum but the sound is much clearer. The noise is hard to describe, when I press a chord at higher volumes, there is a periodic crackling in the background, If I use a chord an octave up or down, the crackling changes note accordingly. I have solved the problem by using a 12W amp instead of a 3.5W amp (and using better connectors). I did try connecting the 3.5W amp to a 1 KHz signal generator but this was inconclusive - my cheap scope didn't show any noise and I couldn't hear any. \$\endgroup\$ – RedGrittyBrick Nov 6 '11 at 12:40
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It's really hard to tell from your description. A good description of the noise and when it happens would be a good start.

You already said that the noise happens when you play a note. What happens if the volume is turned down? Either using the control on the electronic piano or by using the pot in your circuit. If the noise goes away then you can assume that the problem is either with the signal level on the input, or that you're asking the amp to put out more than the rated 3.5 watts.

If I were a betting man, I would guess this is the case. You're using a 3.5 watt amp to power a 15 watt speaker. Normally for a 15 watt speaker you would be using a 15 to 30 watt amp. So your amp is undersized.

Adding a cap like what @Manmanguruman suggests isn't a bad idea, but I would hope that there is already some caps inside the amp module. Still, you would use the largest cap that is somewhat practical. Start with about 470 uF and higher.

The pot should have a log taper, not a linear taper. Changing the pot will make the volume control more useful-- but will do nothing for your noise problem.

It is also possible that you have a signal loading problem with the piano. Basically, the piano might not be able to handle having an amp plugged into it. I would hope the piano was designed better, but I have seen some products do some stupid things. Turning the volume down with your linear/log pot should help this issue.

Other than that, the only thing I can think of would be a component failure. Something like the speaker, amp, battery, or piano actually being broken in some way.

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  • \$\begingroup\$ I replaced the amplifier with a 12W amplifier (same make), used a 1/4" TR to phono adapter with a phono lead and powered it from a 240V adapter - sound quality is much better. I also discovered that powering the piano through it's 9V input, rather than through it's USB connector, increased the line-output volume significantly. \$\endgroup\$ – RedGrittyBrick Nov 6 '11 at 12:34
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The input needs to have shielded audio cable from source to potentiometer to amplifier with the shield connected to the common and center wire as the signal in. This will eliminate stray noises on the input. Battery or good DC regulated PSU is also important.

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