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Connecting an MP3 player to a hifi system

A common problem my teenage son has is that the output of his MP3 player is not enough to drive the amplifier input of hifi and not so hifi systems, such as in cars or standalone systems. The result is a flat battery and poor audio because the volume needs to be on full.

Can anything be done to match the output of an MP3 player to the input of a stereo system? R. K., Armidale, NSW).

  • Most MP3 players will deliver a nominal 770mV or even 1V RMS to drive the headphones and this is suitable for connection to a power amplifier without any need to boost the signal. The actual level used should not have much effect on the MP3 player’s battery life.

    The problem you might have is that the MP3 player’s output may require a load that is a lower resistance than the nominal 47kΩ input impedance of a typical power amplifier. The load is required to set up the MP3 amplifier’s DC conditions. With some MP3 players, the audio will not be available (or will be distorted) until a suitable load is connected.

    In practice, you may need to add a load resistor between each of the left and right channel outputs and ground before the MP3 player can be used successfully with a power amplifier. 100Ω resistors should be adequate although you may need to go as low as 47Ω or 39Ω.

    Proposed Layout Changes To Class-A Amplifier

    I am currently purchasing your new Class-A amplifier modules from Altronics. All the parts seem to be available except for the case and the nearest in the catalog is 65mm less in depth at 360mm. As I intend not to include your preamp (I built the separate earlier one), I should have room to move things about. What do I need to keep well spaced? I will keep the general layout as per your article.

    Can I move the left and right power amplifiers off-centre on the heatsinks, closer to the front panel, thus allowing other bits to be moved forward in relation? Can the speaker protector board be moved close to the amplifiers without risk of noise induction? Can the bridge rectifier be moved? All I know for sure is that the power transformer needs to be spaced as much as possible.

    I could alternatively mount the transformer in a separate box. In that case do you have the rectifier in the same box with a DC output or is AC best? I assume the main power board would go in the main amplifier case. (P. G., Orient Point, NSW).

  • While we may seem pedantic, even quite small differences in layout will have a significant effect on the signal-to-noise ratio. For example, just moving the Loudspeaker Protector board out from its location in the corner of the chassis can degrade the signal-to-noise ratio by 10dB or more. Doing the other changes that you propose would make it worse – such changes NEVER improve things.

    The wiring layout we produced for that amplifier was produced after a great deal of trial and error. Believe us, there were times when we were very frustrated in attempting to get the very best performance. So unless you are prepared to accept quite significant degrading of the performance, do not make any changes to the layout.

    If you really must change the layout, then perhaps better results may be obtained by putting the power supply in a separate steel box – ie, with the transformer, bridge rectifier and power PC board. By the way, a complete kit is available from Altronics.

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