Diode mixing digital audio

[cowana]

I have two digital audio sources that should not be on at the same time as each other. One gives out a 5v square wave, the other a 3v square wave. Will I be OK to diode mix these into the amplifier (a logic level FET)?

Thanks

Andrew

 

[audioguru]

1) You do not mix digital sources.
2) A logic-level FET probably will not work with a 3.3V signal that is reduced to 2.6V with a diode. It needs at least 4.5V.

 

[cowana]

Sorry - I should not really have called it mixing. It is simply that one is on while the other is off, and I don't want to blow up the output of the other.

I'm using a LDO schottky diode, and I've found that my FET does turn on at the 2.9v there is on the gate. It is a small speaker, so even thought the Rds of the FET is ~1.5 ohms, that's no problem in this case.

Andrew

 

[ronv]

Here is a link from another post with some low Rds on at low gate voltages.

https://www.fairchildsemi.com/collateral/new/920001_PortableSolutions.pdf#page=11

 

[audioguru]

You are feeding DC pulses to a speaker?
But a speaker uses AC, not DC.

An audio amplifier has a very low output impedance (0.04 ohms or less) that damps the resonance of a speaker. Your FET has 1.5 ohms in one direction and infinity (boooom) in the other direction. The speaker will sound like a bongo drum.

 

[cowana]

That's fine - It's just a buzzer. I'm not looking for nice sounds And the correct term for what I want would be an OR gate, rather than a mixer.

Andrew