With regards to class AB audio amplifiers there is really nothing new under the sun, and this design is no exception.
I have a collection of amateur radio transceivers I built as kits from QRP Labs that the audio ouput can only drive small headphones (earbuds). Personally I cannot stand having earbuds in my ears for long durations, so I wanted to make an audio amplifier to drive a speaker I reprovisioned from a commercial mobile radio.
My requirements for the audio amplifier was:
- Run off of a 12 VDC, single rail, power supply
- Drive a 4 Ohm speaker
- Loud enough for a typical size room with enough volume to overcome any incidental noise/sounds
- Needs to be an analog class of operation - please, no switching...
- I don't want to waste power - needs efficiency
- Power switch
- Power input on a 2.1x5.5 mm panel jack
- Power indicator
- Volume control
Loudness requirements is a bit nebulous to me as I have no clue as to how the sound pressure levels from the speaker relate to the wattage applied to the speaker. In this case I decided to just go for the maximum that I could get from a 12 VDC single rail supply, which I know is not much really in my way of thinking.
My general conclusion is to use a class AB amplfier, and the ultimate circuit I ended up with is based on the design of The Tube Roaster channel on YouTube. The largest difference was the supply voltage he was using and my 12 VDC supply requirement. I ended up entering the circuit in QSpice, tweaking the bias resistors till I got even clipping on the output. The resulting circuit looks like:
The net simulate results were maximum unclipped output voltage across the 4 Ohm load of 8.1 Vp-p which results in around 2 Watts being delivered to the load. The simulated unclipped gain was 23.1 dB at 1 KHz.
The only oddity of this circuit is the boostrapping capacitor C4. Based on what little I know about boostrapping it has the effect of reducing the AC current through R7 which effectively raises the collector AC resistance on the input common-emitter amplifier (Q1). Running the simulation with and without C4 show the gain to increase around 3 dB with the bootstrapping circuit in place.
After building this circuit I ended up with about 8.23 Vp-p unclipped maximum output voltage which will deliver 2.1 Watts to the 4 Ohm speaker.
| Qty | Description | Distributor | Part Number | Notes |
|---|---|---|---|---|
| 1 | POT 1K OHM 1/2W PLASTIC LINEAR | Digi-Key | 53AAD-B28-B10L-ND | Overkill - lol |
| 1 | Toggle Switch SPST Panel Mount | Digi-Key | 360-MN11ES1W01-ND | |
| 1 | LED 3MM PANEL MNT W/24AWG LEADS | Digi-Key | SSI-LXR3612ID-150-ND | |
| 1 | CONN JACK MONO 3.5MM PNL MNT | Digi-Key | SC1455-ND | |
| 1 | CONN RCA JACK MONO 3.2MM PNL MT | Digi-Key | CP-1412-ND | No locating features |
| 1 | CONN PWR JACK 2.0X5.5MM SOLDER | Digi-Key | CP-5-ND |
The case was printed using ABS filament. I always have a problem with determining hole size for the threaded heat inserts. In my case the standoff hole sizes were a tad small and I needed to increase the hole size so that the head insert would go in without deforming the standoff.
- Mute button
- Headphone jack integration
- Use surface mount components to reduce size
- Smaller output transistors and heatsinks. In the current design these are barely getting warm at full power output