A few years ago I made this speaker amplifier circuit for a sound source which was essentially an AC signal -- the two signal pins (d9 and d10, 5v logic) were pulled high and low out of phase to produce sound in the speaker. This was the circuit I made for it (that red box is the speaker):

enter image description here

In an attempt to somewhat salvage it is there any simple way I can get rid of those immensely wasteful (and heat producing) pullup resistors? Maybe add another set of transistors connected to d9 and d10 (the signal lines) to give the speaker line 5v when its respective signal is pulled high?

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    \$\begingroup\$ Don't beat yourself up. Everyone starts from nowhere and has to proceed from there. Just keep moving forward. Besides, it worked enough at the time for you, it seems. Nothing wrong with making something work well enough for you to get by at the time. And finally, what you did is a cheap "bridge-tied load" arrangement. That probably got you a larger swing at the speaker. Now you are asking to learn about something new. That's okay, too. \$\endgroup\$ – jonk Aug 9 '17 at 21:55
  • \$\begingroup\$ Get an LM386 is my advise. \$\endgroup\$ – Andy aka Aug 9 '17 at 21:56
  • \$\begingroup\$ @Andy aka -- I like the amplifier IC route but those are only one channel and there are two signals (one for each speaker pole) out of phase. Are you suggesting I put on one IC for each line? Are there any ICs that have 2 isolated channels (basically two LM386s fused together with shared pins where possible)? \$\endgroup\$ – Murey Tasroc Aug 9 '17 at 22:10
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    \$\begingroup\$ @MureyTasroc You can consider something like the TDA8551. It's 1 W, works off of 5 V, and operates in bridge tied mode, as well. If you are open to an IC. It also includes a volume control that is easy to access with a micro (pulse it.) \$\endgroup\$ – jonk Aug 9 '17 at 23:27
  • \$\begingroup\$ If you replace the big resisters with big currentsources you will save some power and have the basis of a classA BTL amp. \$\endgroup\$ – Autistic Aug 10 '17 at 1:41

Probably the easiest way to improve on what you've done, to get rid of those power-wasting resistors, while still maintaining the cheapness and simplicity of the original, is to do this.

It's not an 'audio' amplifier, just like your original isn't. It's purely designed to take weak logic-level signals, and drive a speaker with them.


simulate this circuit – Schematic created using CircuitLab

Use two, one from each logic pin, and bridge-tied to your loudspeaker.

There is a small loss of voltage due to the transistor VBE. That can be overcome with additional components, but it's probably not worth it at this level.

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