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Project Notes

#499 HS088 Low-voltage Amplifier

Adding a low-voltage BJT class A amplifier for the HS088 Audio Effects Chip.


Here’s a quick demo..



The HS088 audio effects chip encodes a “ding dong” wave, but like many effects chips it cannot drive a speaker particularly well in its default circuit configuration.

Adding an LM386 is a very simply way to provide amplification, but that requires a power supply of 4-5 volts. But I was interested to explore circuit ideas for providing decent amplification at lower voltages - specifically with a 3V CR2032 coin cell.

There are other amplifier-on-a-chip options that work at these voltage levels (such as the TDA2822), but first I decided to try a descrete BJT design.

My first test was with a simple single-stage common-emitter design (R1=6.8kΩ, R2=4.7kΩ, Rc=50Ω, Re=10Ω with 47µF bypass - designed for Icq=40mA). Unloaded, it achieved excellent amplification, but proved to be very poor at power transfer to the speaker.

Adding a push-pull output stage started to produce some more decent results with an 8Ω speaker. The final circuit described below is an adaptation of a design from deeptronic that was in turn inspired by a design from Bowden’s.

Circuit Design



Testing on a breadboard:


Building an Amplifier Module

After verifying component values on a breadboard, I but the amplifier circuit on a small piece of protoboard.


I’ve taking some measurements of the amplifier performance with a 1kHz sine wave and 3V power supply.

Test 1: Adjusted for minimum distortion

Adjusting the input attenuation for the sweet spot for minimum distortion, gain is around 19.

  • CH1 (yellow) : 400mV input signal (before attenuation)
  • CH2 (blue) : unloaded output (~475mV)
  • CH3 (red) : ~25mV input signal (after attenuation)


Test 2: Maximum Input Without Clipping

About 40mV is the largest input signal that can be handed before the output starts clipping. The output is quite distorted at this point though.

  • CH1 (yellow) : 400mV input signal (before attenuation)
  • CH2 (blue) : unloaded output (~1000mV)
  • CH3 (red) : ~40mV input signal (after attenuation)


Test 3: Overdrive!

The circuit is completely overdriven with a 400mV input. The output is essentially a square wave at this point!

  • CH1 (yellow) : 400mV input signal
  • CH2 (blue) : ~2V unloaded output


Final Build

Finally, putting it all together. First, as always, verify on a breadboard:


I used another strip of protoboard as the mainboard to mount the:

  • HS088 chip and supporting components (trigger switch, output transistor)
  • volume control
  • CR2032 battery holder
  • 3.5mm phone jack
  • amp module mounting


The final build, with volume adjusted for maximum output without distortion procudes an amplified output that is perhaps 3x the apparent volume of the unamplified HS088 module. I don’t have an SPL meter to verify this with a measurement (yet).


Credits and References

Project Source on GitHub Project Gallery Return to the LEAP Catalog

This page is a web-friendly rendering of my project notes shared in the LEAP GitHub repository.

LEAP is just my personal collection of projects. Two main themes have emerged in recent years, sometimes combined:

  • electronics - usually involving an Arduino or other microprocessor in one way or another. Some are full-blown projects, while many are trivial breadboard experiments, intended to learn and explore something interesting
  • scale modelling - I caught the bug after deciding to build a Harrier during covid to demonstrate an electronic jet engine simulation. Let the fun begin..
To be honest, I haven't quite figured out if these two interests belong in the same GitHub repo or not. But for now - they are all here!

Projects are often inspired by things found wild on the net, or ideas from the many great electronics and scale modelling podcasts and YouTube channels. Feel free to borrow liberally, and if you spot any issues do let me know (or send a PR!). See the individual projects for credits where due.