DIY portable headphone amplifier based on an INA1620 chip and TPS65135

Today’s topic is going to be related to audio amplifiers. Particularly a headphone amplifier based on an INA1620 IC. There are going to be two main parts of this post – the first one about the amplifier and its construction, and the second – about my attempt to measure how well it performs.

Amplifier circuit

For an amplifier INA1620 was chosen. I was looking for a physically small solution, which could be used for portable application. As it is in QFN package – it is as small as I personally can solder in. It also has some resistors inside the package, so if, for example, a gain of 2 is enough for you, you can use those resistors and save the PCB space even further.

According to its datasheet, it has some insane parameters like low THD, low noise etc. Of course, they won’t make a lot of difference if the PCB design will be bad, or when used as a portable amplifier with lots of additional noise added to signal and power rails (which is this case).

Also, it can be powered from wide supply voltage range – from +- 2 to +-18V, which allows it to drive high impedance headphones without any problems. Furthermore, low impedance headphones (32 Ohm) can be driven with 150 mW power, which is more than enough to impair your hearing.

The schematic for this amplifier section is quite simple:

INA1620 schematic with gain 2 development board

There you can see many additional resistors, which are optional, as they could be used when I will decide to change amplifiers gain or choose if it will work in inverting or non-inverting mode.

The actual circuit which was soldered together looks a lot simpler:

INA1620 schematic with gain 2 development board simplified

The main parameter, gain, is set to 2. It allows to amplify a signal coming from a phone to acceptable sound level for driving higher impedance (in my case 300 Ohms) headphones. For 32 Ohm headphones such gain might be a little too high, but you can always turn the volume down.

Power circuit

To power all amplifier circuit a TPS65135 was chosen. It takes in from 2.5 to 5.5 V, so it can be either battery or USB powered. It outputs up to -7/+6 V, so it is enough for portable amplifier application, but if you would like to push INA1620 to its maximum, a different power circuit (with higher output voltages) should be used.

This IC is also in a QFN package, so it takes up little PCB space, it needs only a few components. Although it outputs two voltages, the IC uses only one inductor which further saves PCB space.

So, whole schematic is quite simple, taken default from the datasheet:

With the R1, R2 and R3 values shown, real circuit outputs +4.95 and -4.74 voltages.

Whole DIY development PCB

To test out how the combination of power supply and amplifier circuits work, I have made myself a development board using the UV method.

INA1620 DIY development board with power supply circuit based on TPS65135

As this is more of a development board, all components are placed quite far away from each other. If I decide to make a final PCB, it could be made even smaller, so, with some enclosure modifications, it could fit into a 3.5 mm audio jack adapter socket.

Amplifier measurements

I wanted to include distortion measurements, bus as my laptop mic input is not the greatest, quality wise – it has high noise and distortion on its own, I have not done those measurements.

Nevertheless, here are some other performance measurements, which I have managed to do:

Output power

First measurements were done when the input signal is takes from a smartphones output, sinewave at 1kHz frequency. The phone outputs about 1 Vpp signal, so with a gain of 2 at amplifiers output we get 1.15 Vpp with different loads (no load, 32 Ohms and 300 Ohms). Whole circuit (TPS65135 and INA1620) is powered by 5.1 V (taken from USB). Knowing the amplifiers output voltage and load, output power can be calculated. So, here is a table which summarizes everything:

Output voltage (Vpp)	Output voltage (Vrms)	Load, Ohms	Calculated output power (mW)
1.15	0.4	No load	–
1.15	0.4	32	5.2
1.15	0.4	300	0.5

The same measurements were also made using laptop’s output signal. The difference here is that laptop outputs around 1.1 Vpp, so there will be respectively higher voltage on the amplifier’s output:

Output voltage (Vpp)	Output voltage (Vrms)	Load, Ohms	Calculated output power (mW)
2.2	0.78	No load	–
2.0	0.7	32	15.6 (distortion)
1.15	0.4	32	5.2 (no distortion)
2.2	0.78	300	2

In the table above, you can see that when outputting maximum voltage to the 32 Ohm headphones, distortion was observed:

I could guess that this distortion was observed because of insufficient power supply, negative power rail voltage dropped to -2.8 V (instead of -4.7) and, as you can see, the distortion was on the negative sinewave side.

So, the real useable output power to 32 Ohm load is around 5 mW. Which seems not a lot, but actually might damage your hearing.

Power consumption

Also, I measured how much current consumes whole power + amplifier circuit with different output load:

Output voltage, Vpp	Output load, Ohm	Output Power, mW	Current consumption, mA (@ 5.1V input voltage)
1.15; 2.2	No load	–	15
1.15	300	0.5	17
2.2	300	2	20
1.15	32	5.2	34
2.0	32	15.6 (with distortion)	50

Overall, the power consumption is low. Circuit is suitable to be powered either from a battery or USB port.

Summary and listening experience

The whole idea was to have tiny portable headphone amplifier solution and this circuit is exactly that. It takes not much PCB space and has enough power to drive either high or low impedance headphones. Of course, it can be tweaked to be better suited for particular headphone impedance (mainly by changing the gain), but as tested, with gain of 2, it behaves as a quite universal circuit.

The distortion with low impedance load could be probably improved with a better power supply circuit, but it could also make whole solution bigger PCB space-wise.

To my taste, this amp sounds awesome. Well, I don’t think that there is a lot to tell, because I personally don’t hear that the sound would be somehow distorted, especially with high impedance headphones. Also, I imagined that there should be some HF noise on the amp’s output from the noisy power supply chain (USB, DC-DC converter, etc.). But, to my surprise, I haven’t managed to hear any electrical noise. Which makes me believe that this IC has good power rail noise rejection.

Because power circuit can be supplied from 2.5 -5.5 V voltage, it is suitable to be powered from either Li-Ion battery or an USB port. My personal preference is USB port. In this case, amplifier could take not only power from the smartphones USB socket but also audio signal, if, for example, an USB-C DAC is used such as this one or similar solutions. Then whole circuit could be whole USB-C to 3.5 mm jack adapter with integrated USB DAC and headphone amplifier into one solution.

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