High Performance Low Voltage 1.5W Amplifier

Printable Detailed Build Instructions



This is a small amplifier module based on the # SSM2211 # IC from Analog Devices. Connect power, audio, and a speaker and you are good to go.

This is our go-to amplifier for small projects, and there are several reason why we love it:

  • Very low background noise
  • Low and single supply power requirement
  • No output filters or decoupling needed
  • Liner Class-AB output
  • Good headroom (400mV)
  • Gain setting similar to Op Amp
  • Small, yet easy to hand solder, SO8 package.
  • Up to 1.5W with decent THDN
  • Good datasheet with examples


This example uses a smartphone for audio input, a USB charger for powering the SSM2211, and a small speaker for the audio output. The smartphone volume control controls the audio volume at the speaker.



Overall Specifications

0.005% THD+N Test equipment THD+N limit
0.005% THD+N Battery, No load, No RF shield
0.016% THD+N USB Charger, No load, No RF shield
<0.2% THD+N 1W load (Datasheet)
<1% THD+N 1.5W load (Datasheet)
Power Supply 2.7V - 5.5V
Voltage Noise 85nV/√Hz
PSRR 66 dB @ 5V Vdd
Output Z 0.1 Ω
Current Draw 100 nA (Shutdown) / Up to 20 (On)


Schematic Description

Audio input is through the AIN "IN-" port. Amplified audio output is taken over the VOA and VOB output. Please not that this is a "Bridge Tied Load" amplifier, i.e. VOA provides an inverted signal of "IN-". VOB provides the non-inverted signal. A "Bridge Tied Load (BTL)"" configuration has a key advantage over a "normal" amplifier (like the LM386): You do not need (typically large) coupling capacitors on the amplifier output, you can simply connec the speaker directly to VOA/VOB. A side effect is that you get twice the amplification you would expect from a normal inverting amplifier: Your gain will be -2*R2/R1 instead of simply -R2/R1.

Warning: With a BTL config DO NOT connect measuring equpiment directly over VOA/VOB - unless you are sure there is no common connection to GND. Dangerous cases include having a wall connected oscilloscope and powering the amplifier with a USB charger: If you connect the oscilloscope probe ground clip to VOA/VOB, you'll end up with a short circuit from VOA/VOB to GND. Or if you try to measure AIN (IN-,GND) and OUT (VOA/VOB) using one oscilloscope you'll have a common GND over the oscilloscope probes GND wire and through the oscilloscope.
Both VOA and VOB are supplying a voltage, so neither should be shorted to GND.
Dave/EEVblog has a detailed explanation here: YouTube

Tip: If you want to be on the safe side, don't measure output over VOA/VOB. Instead measure over VOA and GND - then double that value to get the correct measured value.

R1, R2 sets the amplification of the amplifier, so that Output Voltage = R2/R1 * Input Voltage. However, since we have a BTL configuration amplifier, the output voltage will actually be double that:

\(\boxed { \text { Output Voltage } = { 2 \cdot {R2 \over R1} \cdot \text { Input Voltage } } }\)

C1 acts as a coupling capacitor between the "IN-" port and the amplifier. This is necessary since we do not want to amplify any DC level in the input (audio) signal.

Consider for example if the input signal was a 1 kHz sine biased at 1V and with an amplitude of 0.5V. This is an input signal that varies between 0.5V and 1.5V. If we amplify that signal 5x, you'll get an output signal between 2.5V and 7.5V. This has two main negative implications: First - You will get garbled audio since you get clipping above 5V (assuming you are powering the amplifier with a 5V power source. Second - You can potentially damage the speaker.
C1 thus removes the DC component from the input signal and passes only the AC signal (which contains all the audio information).

A side effect of this is that C1 ends up forming a high-pass filter together with R1 with a cut-off frequency of \(\boxed { 1 \over { 2\pi \cdot R1 \cdot C1}}\)

This means that you'll need to choose C1 depending on the value of R1. Details on how to calculate all the values of the components to fit your required input impedance and amplification is given below in the component dimensioning section.

C2 stabilizes the IN+ and BYPASS pins of the IC, holding them at GND. This helps reduce power supply noise in the audio path. C2 is dependent on the values of C1, R1, refer to component dimensioning

C3, C4 stabilizes the Vdd power supply for the IC. This reduces noise/fluctuations (from the power supply), which may otherwise leak into the audio path. Recommended values are 100nF and 100uF.

R3 defaults/holds the IC "Shutdown (SD)" Pin to GND - this powers on the IC. If you want to power off the IC you simply connect Vdd (i.e. 5V) to the SD Port.



For consumer audio equipment, the standard line out level is 894mV (peak-to-peak). This is what you expect if you are using the "line out" port from a computer soundcard.

However, the output level of a headphone jack (e.g. iPhone, Android) is unspecified. We did some empirical measurements of the output from a few common Android phones / iPhones, and during playback from Spotify or YouTube we measured a peak-to-peak of ~800mV at maximum volume. We also tested an iPhone audio analyzer App (called 'Analyzer') and here we saw a higher output voltage: 1400mV peak-to-peak.

In any case, this is roughly inline with the default dimensioning from the SSM2211 datasheet - which dimensions for 1V peak-to-peak input, 8Ω speaker, and a 5V power supply. We will therefore use the datasheet dimensioning as our default.

How to dimension for any value is given below.


Component Dimensioning

$$\boxed { R1 = \text{Wanted Input Impedance} }$$ $$\boxed { R2 = { \text {Wanted Gain} \cdot R1 \over 2} }$$ $$\boxed { C1 \gt { 1 \over {2\pi \cdot R1 \cdot \left ( { 20 \over 4.14 } \right )} } }$$ $$\boxed { C2 \gt { C1 \cdot R1 \over \text { 25 kΩ } } }$$ $$ { \text { } }$$

General Gain Formula

$$\boxed { Gain = { 2{R2 \over R1} } }$$

Example: Default Dimensioning

$$\boxed { \text{ Wanted Input Impedance} = 20k \Rightarrow \space R1=20k }$$ $$\boxed { \text{ Wanted Gain = 2.8 } \Rightarrow \space R2 = { 2.8 \cdot 20k \over 2 } = 28k }$$ $$\boxed { C1 \gt { 1 \over {2\pi \cdot 20k \cdot \left ( { 20 \over 4.14 } \right )} } \Rightarrow \space C1 \gt 1.65uF}$$ $$ { \text { Lets choose C1 = 2.2uF } }$$ $$\boxed { C2 \gt { 2.2uF \cdot 20k \over \text { 25 kΩ } } \Rightarrow \space C2 \gt 1.76uF }$$ $$ { \text { Lets choose C2 = 2.2uF } }$$ $$ { \text { } }$$ $$ { \text {So for Gain=2.8x and Impedance=20k,} }$$ $$ { \text {we have R1=20k, R2=28k, C1=C2=2.2uF} }$$


PCB, Schematic, and other Files

PCB and Schematic files are hosted on EasyEDA: Link
Or if you prefer, you can grab the complete Gerber File here: Link to ZIP Archive

Bill of Materials

Name Quantity Marking Type Value Manufacturer Manufacturer Part
PCB 1 - PCB - Protoriam SSM221M PCB
U1 1 SSM2211 Amplifier IC SSM2211 Analog SSM2211SZ
R1 1 2002 Resistor 20k Vishay RT1206BRD0720KL
R2 1 4701 Resistor 28k Vishay RT1206BRD0728KL
R3 1 103 Resistor 10k Vishay RT1206BRD0710KL
C1, C2 2 Red Capacitor 2.2u Yageo CC1206KKX7R8BB225
C3 1 Green Capacitor 100n Samsung CL31B104MBCNNNC
C4 1 Blue Capacitor 10u Samsung CL31A106KAHNNNE


Step 1: Entire Board


Solder Op Amp U1
Solder remaining components

This populates the entire board. To verify the board: Connect power (5V) to the PWR port, Audio to audio-in, and a Speaker to audio-out


Hardware Setup


The PCB was mounted on a breadboard, and connected to the UR242 via two unbalanced 6.35mm 10 cm long patch cables.

Power was provided either using a standard 5V USB Charger, or using a standard 9V Battery regulated to 5V using a 7805 IC.

Software Setup


The test setup was a 24-bit, 192 kHz, USB Audio Interface "Steinberg UR242" and the free "Room EQ Wizard" software.

The UR242 Software was configured to use: -10 dBV Input Setting, DAW "Solo" mode.

The UR242 USB Audio interface "Output" control has been calibrated so that the UR242 Line output is -10 dBV when -10 dBV is configured as the output voltage in the REW software. Calibration was done using an RMS voltmeter and verified with an oscilloscope (Siglent SDS2354X).

The REW software has been configured to use ASIO drivers, 192 kHz sampling rate, and -10 dBV output voltage.

REW Software Setup


The REW "Signal Generator" and "RTA" (Spectrum Analyzer) where used to derive THD+N @ 1kHz.

The REW "Signal Generator" was setup to produce a -10 dBV sinewave @ 1kHz.
The REW "RTA" function needed to be calibrated by adjusting the "FS sine VRms" from 1.0V to 1.216V.
The REW "RTA" function was configured to use a Hann window, 512k FFT samples, and no averages. The Span was adjusted to 20 - 20kHz.

Test Result: THD+N @ 1kHz with Test Equipment Only (Only UR242)


Test Result: THD+N @ 1kHz with Battery 5V (9V Battery regulated to 5V using 7805 Voltage Reg IC)


Test Result: THD+N @ 1kHz with 5V USB Charger