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Compatible PC speaker for DTK Turbo XT motherboard

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First post, by bigwhoop

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I recently acquired a Turbo XT clone motherboard manufactured by DTK. It is using a non-standard header for the PC speaker, namely a two-pin one as opposed to the four-pin with two blanks as is common. I've tried hooking up several known good 8 Ohm speakers that work fine with my 5150 or Apple ][+, but neither generate any sound coming from this system. With my scope and multimeter, I've confirmed that ~5V is seen on the positive pin when the computer beeps.

The header in question is JP1 in the high-res picture of the motherboard here.

I found what I think is the manual for the motherboard, but the only information about the speaker circuitry is that is generates 0.5W.

Does anyone know what kind of speaker is likely to be compatible with this motherboard, or how to empirically test it?

Reply 1 of 2, by majestyk

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I guess you measure 5V~ when there´s no speaker connected.
If there´s a 5V~ AC voltage (U) at the output and the maximum power (P) is 0,5W, the rule is: P=U²/R and hence R=U²/P => R=25/0,5 = 50 Ohm.

If you connect an 8 Ohm speaker you will overload the speaker output. Look for some speaker that has around 50-100 Ohm - maybe a piezo beeper is also worth a try, they also have a higher impedance (AC resistance).

Reply 2 of 2, by bigwhoop

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Thanks for the formula! The piezo buzzer tip was a good one. I dug up one that I had laying around, and got some noise out of the system. Added a 10k Ohm resistor to avoid the neighbors calling the police on me! 😁

Now, the sound coming out of the piezo buzzer isn't particularly pleasing, so I'll keep looking for a higher impedance speaker in the meantime.

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