#499 HS088 Low-voltage Amplifier

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

Build

Here’s a quick demo..

Notes

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

Schematic

Breadboard

Testing on a breadboard:

Amplified_bb_test

Building an Amplifier Module

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

Amplified_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)

scope_25mV

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)

scope_40mV

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

scope_400mV

Final Build

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

Amplified_protoboard_bb_test

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

Amplified_protoboard_modules

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).