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Building a headphone amplifier using the Sparkos Labs discrete SS2590 Pro operational amplifier can take your audio experience to the next level.
This Op Amp is known for its high sound quality and ability to handle complex loads, such as those presented by high-impedance headphones.
Why Use the Sparkos Labs SS2590 Pro?
It may seem unusual to suggest the use of operational amplifiers in a headphone amplifier circuit. And believe me, most operational amplifiers are not designed for this purpose, as they have limited current driving capability. However, the SS2590 is an exception, as it can handle up to a quarter of an ampere and operate with supply voltages of +/-24V, making it very suitable for this application with some simple circuits.
Operational amplifiers are generally designed to handle purely resistive loads, but headphones do not behave that way. In fact, headphones are considered reactive loads, meaning they have significant inductance in the voice coils and capacitance in the wiring, in addition to the resistance of the coil wire. These characteristics are not favored by operational amplifiers. Forcing an operational amplifier to handle such reactive loads often leads to instability and high-frequency oscillations. The trick to stabilizing an operational amplifier when facing a reactive load lies in adding an additional resistor (Riso), as shown in the circuits below. These are the same circuits we use in the output stage of the Sparkos Labs Aries Headphone Amplifier.
The SS2590 is a discrete operational amplifier that offers superior sound quality compared to conventional integrated op-amps. It is designed to handle reactive loads, making it ideal for audio applications, especially in headphone amplifiers. Its ability to deliver high output current ensures that it can handle headphones with different impedances without distortion.
What Do You Need to Build a DIY Headphone Amplifier with the SS2590?
To build a headphone amplifier with the SS2590, you will need the following components:
- SS2590 Pro Discrete Op Amp: The heart of the amplifier.
- Isolation Resistor (Riso): This resistor is placed in series with the output to ensure the amplifier's stability when handling the inductive and capacitive load of the headphones.
- Dual Power Supply: You will need a power supply that provides ±15V or ±18V, depending on the configuration and requirements of the Op Amp.
- High-Quality Capacitors and Resistors: It is crucial to use high-quality passive components to maintain signal integrity.
Non-Inverting Configuration
A non-inverting configuration is one of the most common ways to use the SS2590 in a headphone amplifier. This configuration allows for stable gain and a good frequency response. The Riso is placed between the Op Amp's output and the load (headphones) to prevent oscillations.
In the image below, we can observe a typical non-inverting operational amplifier configuration, with the addition of the Riso resistor to handle headphones. Riso prevents the amplifier's output from seeing the reactive load presented by the headphones, ensuring its stability. It is important to note how the feedback resistor (R1gain) is connected on the other side of Riso, and how Riso is included in the feedback loop. This forces the SS2590 to "overcome" the Riso resistor, preventing it from forming a voltage divider with the headphones. Additionally, it maintains a high damping factor, allowing the amplifier to have firm control over the headphone coils.
Riso should be a power resistor with a capacity of 3 to 5 watts, as it will handle the current demanded by the headphones. Wire-wound resistors are a good option for this.
The Cpole capacitor, which is also used to ensure stability and establish a bandwidth, would normally go in parallel with R1gain. However, with the addition of Riso, it should be connected differently, joining between the Op Amp's negative input on one side and its output on the other. The value of Cpole is usually adjusted to obtain a cutoff frequency of a few hundred kHz. In the circuit shown below, with the given values, this results in approximately 234 kHz.
Headphone Amplifier Circuit Using the SS2590
Some other details of the mentioned circuit are that it has a gain of 3 with the current values. The gain can be easily modified by adjusting the value of R1gain as needed. Since headphones are considered a "demanding load" that requires tens or hundreds of milliamps to operate, you can opt to use 100uF electrolytic capacitors (Cbyp1 and Cbyp2) on the power supply rails. Normally, 0.1uF X7R ceramic capacitors would also be added in parallel with these, but the discrete pro operational amplifier SS2590 already has them incorporated, so it is not necessary to add them externally.
There will be a considerable current flow in the ground of the headphones, as well as in the ground side of R2gain. These ground connections should return to the main power supply capacitors using a star routing scheme.
Implementation of Balanced Mode
For applications requiring higher output power, you might consider implementing a balanced mode. This approach uses two operational amplifiers in a differential configuration, thereby providing greater current handling capability and noise reduction.
Below we show a circuit to implement balanced mode. It is essentially the same circuit described above, but with a second SS2590 added that operates in inverting mode with unity gain to handle the other side of the headphones. This second inverting operational amplifier also includes an Riso resistor and similar gain adjustment components. Since this operational amplifier simply takes the output of the first and inverts it, its gain should be set to unity. The values of R1 and R2 achieve this, as the gain of an inverting amplifier is equal to R1/R2.
A headphone amplifier circuit operating in balanced mode (also called "bridge mode") can double the voltage swing to the headphones. This can be useful if the power supply rails are low or if the headphone impedance is several hundred ohms.
The design of a headphone amplifier with the SS2590 is not complicated, but it requires careful attention to detail, especially in component selection and PCB design. A good design will ensure that the amplifier can handle any type of headphones, providing clean, detailed, and powerful sound. If you need personalized advice, do not hesitate to contact us.
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