Colour Maximite 2 - Second Generation

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Joined: 07/08/2011
Location: Australia
Posts: 4447
Posted: 10:09pm 02 Jul 2021      

  Mixtel90 said  Can I ask what decided the value of 4k7 for the audio output resistors? That's a pretty high impedance output and probably wouldn't drive any commonly available headphones, although it's fine for active speakers or an amplifier input.
390R would give a maximum current of 8.5mA into a short circuit, so the output would probably be protected. It might also allow the connection of headphones as the audio output would be higher. Ok, it would put DC through the phones, but it probably wouldn't exceed 3mA or so which would be ok for most. It wouldn't give much output, but at least it might be usable and could handle down to 8R headphones safely.

The original design was for line level output and that hasn't changed.
4.7k is the usual impedance for line level.

I did do some tests to see what minimum resistance was needed to prevent distortion at the processor end but can't readily find the info. It's somewhere here on TBS.

To date, the vast majority of users have used PC powered speakers and 4.7k gives a good match to their typical 100 ohm to 2k input impedance while keeping the line level specs satisfied.

There is nothing to stop you bypassing the 4.7k with whatever value you desire but to drive low impedance speakers, a voltage follower or similar would be required.