LM386 Bridged Stereo Amplifier Project — 1-2W Circuits

Our main objective in this article is to build an audio amplifier circuit for a stereo setup. Just a small and simple stereo amplifier circuit using classic amplifier ICs, such as LM386. We’d want it to have an output power of at least 1-2W per channel — 2W would be ideal.

Building small audio amplifiers has always been one of the most enjoyable parts of DIY electronics. The LM386, a classic low-voltage audio amplifier IC, is often the first choice for hobbyists thanks to its simplicity and low component count.

The Original Idea Behind this Project

The original idea for this stereo amplifier circuit originated from a rather intriguing circuit idea we saw in an old electronics magazine. It is an LM386 stereo amplifier circuit with a 2+2W output power. 

It’s a relatively basic amplifier circuit that uses a bridged (BTL) configuration, but the interesting bit is that it only has 3 amplifier units instead of the 4 we would expect. Normally, 1 channel is served by 2 amplifiers in a bridge configuration; logically, 2 channels mean 4 amplifiers in total.

So, to find out if it really works, we decided to build the circuit. However, the result was that this amplifier didn’t perform as well as hoped — the sound became distorted, and the gain wasn’t as stable as theory suggests.

In the end, we came up with a traditional amplifier circuit that takes a regular, non-bridged approach. It uses one LM386 per channel, so the output power may not have been as high as a bridged circuit. But the circuit turned out to be more stable, easier to assemble, and still powerful enough for small speakers or desktop use.

Before we begin looking at the amplifier circuit, let’s brush up on the basics of power amplifiers and the bridged amplifier (BTL) configuration.

The Basic Theory Behind Amplifiers

If we want to amplify a signal for a speaker, we can add a power amplifier between the signal source and the speaker. This is the simplest way to go about doing it, and the signal before the speaker is the original signal times whatever the gain of the amplifier we are currently using.

Suppose that the amplifier we are using has a gain of 2 (that is, a 2× multiplier), and the input signal has an amplitude of 1V. We can calculate the output very easily as follows:

1V × 2 = 2VSimple enough, right? But what if we want more gain? We can either add another 2-gain amplifier front to back with the old one to get a total of 4-gain amplification, or we could have used a bridged amplifier instead.

Bridged Amplifier (BTL)

A bridged amplifier — sometimes called a bridged transformerless (BTL) or bridged mono — is a type of amplifier configuration. A bridged amplifier uses one normal non-inverted amplifier and another inverted amplifier side by side.

Unlike the normal amplifier shown above, which connected the speaker’s negative to ground, the bridged amplifier instead connected the speaker’s negative to the inverted amplifier and positive to the non-inverted amplifier.

This circuit, in particular, has a theoretical total gain of 4 (4× multiplier). The amplifier 1 has a non-inverted gain of 2, and the amplifier 2 has an inverted gain of 2. So, from the perspective of the speaker, the potential difference between the two amplifiers is 4 gain (2 → 4 gain).

This can also be thought of as lowering the zero point from 0 to -2 rather than increasing the amplitude gain by a further 2 (from 2× to 4×). The system’s total gain is 4 since the difference between 2 and -2 is 4.

Why Bother With a Bridged Amplifier?

Notice that connecting another of the same 2-gain amplifiers in series with the old one would have achieved the same result, namely, increasing the gain from 2 to 4. So, what is the benefit of a bridged amplifier?

The thing with a bridged amplifier is that the output speaker is floating — i.e., it is not connected to the ground or voltage supply. Meaning it is less susceptible to DC noises, rendering the output DC blocking capacitor rather redundant.

These DC blocking capacitors are often bulky and space-consuming. Additionally, they can also lower the total output power at low frequencies. 

Another benefit of a bridged amplifier is that it does not need many big components. Hypothetically speaking, we want to amplify a 12V signal by 4 times (12V × 4 = 48V). For a series of amplifiers, we need the first to increase 12V to 24V and the second to increase 24V to 48V, which may not be possible in some scenarios.

On the other hand, for a bridged amplifier, we need one amplifier to do 12V to 24V and another to do 12V to -24V. The potential difference between those two values is 48V, and we did not even need the 24V to 48V amplifier.

The Failed 3-LM386s Stereo BTL Amplifier Circuit

Let’s first take a look at the failed LM386 stereo amplifier circuit we saw in an old electronics magazine, and then try to figure out what went wrong. They claimed the circuit is a 2+2W amplifier using three LM386 power amplifier chips.

This circuit somewhat resembles a bridged topology amplifier, with an inverting and non-inverting amplifier and floating speakers. Apart from that, it’s a rudimentary LM386 amplifier circuit.

IC1 is a non-inverting amplifier for the left channel, and IC3 is an inverting amplifier for the right channel. However, the intriguing part is IC2; it seems to act as a bridged complementary amplifier for both left and right channels.

IC2’s positive input is connected to the right channel input, but its negative input is connected to the left channel input. And the output of IC2 is used by both left and right speakers.

Analyzing What Went Wrong

As we’ve said earlier, we’d build and test this exact circuit on a breadboard. And the result isn’t exactly lined up with what the original author claimed. The output has quite a bit of distortion, and the output power is not 2+2W.

So what went wrong? We suspect this was due to the asymmetrical bridged setup. IC2 basically only works as a bridged complement to the channel with the higher input at that moment. So, in the case that the two inputs receive different signals, one of the channels is guaranteed to lose its bridged complement.

In the end, the stereo amplifier circuit from an old electronics magazine doesn’t work as well as we expected. Now, let’s take a few steps back and find a more conventional solution for the small stereo amplifier circuit.

Basic Functional Stereo Amplifier Circuit using LM386

This circuit is a basic stereo amplifier using two LM386 power amplifier chips. However, it isn’t a bridged amplifier; it’s just two simple LM386 amplifier circuits side by side, each circuit outputting to its own speaker.

In the schematic above, we can see that both LM386 circuits are identical. The top circuit (IC1) controls the left channel, while the bottom circuit (IC2) controls the right channel.

Because this circuit no longer uses the bridged setup, the output power drops to about 1W per channel (paired with an 8Ω speaker) instead of the 2W of the previously mentioned failed circuit. Furthermore, this circuit has a gain no higher than 200 and consumes only 10mA of current.

Can We Increase the Power to 2 Watts?

A 1W amplifier should have adequate power for a small desktop speaker. However, we set out on this project to build a 2W stereo amplifier circuit. What can we do to increase the circuit’s output power to 2W? The quick answers are relatively simple:

• Use a bridged (BTL) amplifier circuit. Remember that earlier we discussed using a bridged setup to double the gain? As the gain (voltage) increases, the power also goes up. In theory, by adding two more LM386 to form bridged amplifiers, we can double the power from 1W to 2W per channel.
  
• Use a power transistor to increase the output power. It’s an excellent way to increase the output. Using a power transistor, we could even increase the power to much higher than 2W, such as 5W or 10W.
  
• Another option is to replace the LM386 with higher-power chips, such as TDA2030, LM1875, or PAM8403 (Class D).

Today, we’ll play around with a bridged (BTL) amplifier to see if it works with the LM386 chip we are using. Then, in the future, we may look into alternative solutions for stereo amplifiers with 2 or more watts, such as those using a power transistor.

2W Bridged (BTL) Amplifier Using LM386

We begin the experiment by cobbling up a bridged amplifier circuit from two LM386s. The reason for testing only one channel (mono) is that we currently have only three LM386s, and two BTL bridges require four ICs.

Even so, it should not have made any difference because a stereo is essentially two mono channels placed side by side. In order to make a bridged LM386 stereo amplifier, you can just build two copies of the following circuit. However, you might want to see how it performs first.

However, when we turn the volume up, the IC starts to heat up, and the sound becomes slightly unclear. It may be because of the IC’s limitation and the 12V battery.

How Much Current Should the LM386 Bridged Amplifier Output?

Let’s do a quick calculation of what the maximum output current of the LM386 bridged (BTL) amplifier is when driving an 8Ω speaker and a 12V voltage supply.

The simple answer is about 3-4W RMS, which is derived from the voltage supply constraint.

• The supply voltage, which for this chip is limited to about 15V.
• When we use a 12V supply, the actual Vrms value is around 5-5.5V (safe).

Next, we can find the output power from the following formula:

Additionally, the LM386 has an output current limit of approximately 200mA, so the output power is definitely not high.

From our own conclusion:

• A BTL amplifier using an LM386, powered by a 12V supply and driving an 8Ω speaker, will yield a maximum power output of approximately 3–4W RMS.
• For distortion-free sound quality, the output power should be set to ≤ 3W RMS, a safe margin. This ensures clear sound without clipping and prevents the IC from overheating.

Wrong Bridged Amplifier

We tested the following circuit (wrong bridged amplifier) and found that there was a very loud whistling sound coming from the speaker, and the current draw was almost 200mA. It is as if it were working as a frequency generator circuit and not an amplifier anymore.

In the schematic below, we feed back the output of IC1 to the non-inverting input of IC2. This type of signal feedback causes oscillation that creates a high-frequency signal to the speaker. Another mistake is that pin 2 of IC2 is connected to ground, just like IC1, so the signal phase is the same for both chips.

Anyhow, mistakes are opportunities for us to learn from them. If we wire the circuit correctly, we’ll get a proper bridged amplifier, like the one shown earlier.
Additionally, in our opinion, the LM386 chip prefers the OTL amplifier (class AB) circuit. The mid-range sound is smooth and classic, and you can listen to it all day long. It uses very low battery power and is almost free from all the annoying noises.

Here are a few related posts you may find helpful, too:

• TDA7052 amplifier | Low voltage 3V, 5V | 1 W BTL
• TDA2030 Bridge amplifier circuit
• TDA2822 stereo amplifier circuit with PCB & datasheet

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