Impedance Matching With DACs in Android (How to)
Impedance matching is an important concept in audio electronics that ensures optimal power transfer and sound quality. Impedance is the measure of opposition to AC current flow in a circuit. Every audio component like amplifiers, headphones, and speakers, have an inherent impedance rating. When the source impedance matches the load impedance, it results in maximum power transfer through the circuit.
If source and load impedances are mismatched, issues like signal distortion, reduced volume, noise, and frequency response changes can occur. Impedance matching allows audio signals to be transmitted from sources like smartphone DACs (digital-to-analog converters) to headphones without degradation. Proper impedance matching provides the cleanest and best audio performance.
Impedance Basics
Impedance is defined as the measure of opposition to alternating current in an electrical circuit [1]. It is represented by the symbol Z and measured in ohms (Ω). Impedance arises from resistance, inductive reactance, and capacitive reactance in a circuit [2].
On a basic level, impedance represents how much a circuit or component impedes the flow of current. A high impedance means more opposition to current flow, while a low impedance allows current to flow more freely.
Impedance mismatch occurs when there is a significant difference between the output impedance of a source component and the input impedance of a load component it is driving. This mismatch can lead to power loss, signal reflection, frequency response changes, and other issues [1]. Proper impedance matching helps transfer maximum power and avoid distortion.
DAC Impedance
The output impedance of a DAC refers to the impedance level of the analog output circuitry. This determines how well the DAC can drive headphones or other downstream equipment. Typical output impedance levels for DACs range from less than 1 ohm for dedicated headphone amplifiers, to 75-600 ohms for line-level DACs designed to connect to external amps.
Higher output impedance negatively impacts DAC performance in a few key ways:
- It decreases damping factor, resulting in looser, less controlled bass.
- It increases frequency response variation, especially at low impedances.
- It increases distortion and crosstalk.
To mitigate these effects, a DAC’s output impedance should be <= 1/8 the headphone impedance as a rule of thumb. Lower impedance DACs around 1-2 ohms are considered optimal for the widest headphone compatibility. However, higher impedance DACs may be acceptable depending on the headphones used.
According to audiosciencereview.com, the ideal impedance matching ratio is <= 1/20, but <= 1/10 is still acceptable in most cases (Source).
Headphone Impedance
Headphone impedance can vary quite a bit between different models and is measured in ohms (Ω). The impedance rating essentially indicates how much electrical resistance the headphones have. Lower impedance headphones, such as 16-32 Ω, tend to be designed for portable use with smartphones and portable media players. They require less voltage to reach high volumes and can usually be powered by the device’s internal amplifier. According to the article Understanding Impedance, nowadays 32 Ω is a common impedance rating for portable headphones.
Higher impedance headphones, such as 80-600 Ω, have higher electrical resistance and are typically used with external amplifiers or audio interfaces that can provide the extra voltage needed. As noted in the Beyerdynamic Impedance Guide, headphones above 100 Ω are great for stationary setups with dedicated amplification. The higher impedance causes the headphones to be more power hungry but also leads to several advantages like better damping, reduced distortion and a flatter frequency response.
In summary, lower impedance headphones (sub 100 Ω) are designed for easy mobile use while higher impedance models (100+ Ω) provide benefits like audio quality and precision but require more power. Matching headphone impedance to the appropriate source gear and amplification is important for optimal performance.
Impedance Matching
Impedance matching involves designing the source impedance and load impedance to maximize power transfer and minimize signal reflections. Impedance is represented by Z and is measured in ohms. When there is an impedance mismatch, some of the signal energy is reflected back from the load rather than being absorbed. This can lead to losses and distortions.
Proper impedance matching provides several key benefits:
- Maximizes power transfer from source to load. Almost all the source power is absorbed by the load rather than being reflected back.
- Minimizes signal reflection. This avoids distortions caused by multiple reflections along a transmission line.
- Prevents standing waves. Impedance mismatches cause standing waves that can introduce losses and resonance effects.
- Helps prevent damage to components. Reflected signals can potentially damage amplifiers, transmitters or other components.
- Improves frequency response. With less reflected power there is less variation in frequency response.
- Enables efficient transmitter and antenna design. The antenna is properly matched to the characteristic impedance of the transmission line.
Overall, proper impedance matching leads to efficient and distortion-free power transfer between components in a system.
Measuring Impedance
Measuring the impedance of headphones accurately can be challenging, but there are a few common approaches:
Using a multimeter or ohmmeter is a simple way to get a rough measurement. You connect the leads across the headphone terminals and take a reading. However, this only provides the DC resistance and does not account for the full impedance curve across frequencies (Source 1).
More advanced testing uses an audio generator, series resistor, and voltmeter to measure the voltage drop across a range of frequencies. This allows you to map out the full impedance curve. However, it requires specialized equipment (Source 2).
The impedance can also vary based on the amplitude of the signal, so loudness needs to be controlled in testing. Obtaining a fully accurate reading across the entire audible frequency range is difficult without laboratory-grade tools.
Software-Based Impedance Matching
One way to match the impedance between a DAC and headphones on Android is through software and digital signal processing (DSP). Modern Android devices have powerful DSP capabilities that allow impedance matching to be handled in the digital domain before the audio signal reaches the DAC.
The Android AudioManager API provides methods like setOutputDeviceOutProfile() that can be used to specify audio device parameters like impedance. Apps can use this to optimize the output impedance based on the connected headphones [1].
More advanced audio engines on Android like AudioTrack allow direct control over audio playback parameters. Developers can leverage these low-level APIs to implement custom DSP algorithms that alter the output impedance. This approach avoids the need for manual hardware adjustments.
Matching impedance in the digital domain has advantages over analog methods. It allows real-time switching between different headphone impedances, and doesn’t require additional analog components. The main downside is that the accuracy depends on the DSP processing power and capabilities of the Android device.
Hardware Considerations
When driving high impedance headphones, having an amplifier with enough power and voltage is key. As impedance increases, the voltage required to reach the same output power also increases. Most smartphone and laptop headphone outputs may struggle to provide sufficient voltage to properly drive headphones over 100 ohms or so.
For example, the headphone output on many smartphones and laptops provides around 1-2Vrms maximum. To produce 100mW of power into 300 ohm headphones would require around 2.45Vrms. So in this scenario, the device may not be able to reach optimal volume levels before distortion kicks in. An external headphone amplifier can provide higher output voltage to properly drive the headphones. Some dedicated amps can output well over 5Vrms.
Additionally, the output impedance of the audio source can impact the frequency response of headphones if mismatched. A low output impedance (under 1/8th the headphone impedance) is ideal. Smartphone and laptop outputs often have an output impedance of a few ohms to over 10 ohms. So high impedance headphones may see more variance in frequency response.
In summary, hardware limitations on output power, voltage and impedance from typical mobile device audio outputs can make properly driving high impedance headphones difficult. Using an external headphone amp provides greater power, voltage, and lower output impedance for optimal performance.
Recommendations
When it comes to impedance matching between a DAC and headphones, there are some suggested impedance pairings to aim for optimal performance:
Low impedance headphones (<32 ohms) should be paired with a lower output impedance DAC, around 1-2 ohms. This helps ensure proper damping and avoids alteration of the headphone's frequency response (as cited from https://eepower.com/technical-articles/understanding-impedance-matching/).
Medium impedance headphones (32-150 ohms) can be paired with a slightly higher output impedance DAC in the 2-5 ohm range. Going above 10% of the headphone impedance starts to impact frequency response (as cited from https://en.wikipedia.org/wiki/Impedance_matching).
High impedance headphones (>150 ohms) benefit from being paired with an appropriately high output impedance DAC, but there is more flexibility. Aim for a damping factor of 4-8 for optimal control and dynamics (as cited from https://blog.minicircuits.com/impedance-matching-devices/).
When in doubt, choose a lower output impedance DAC for maximum compatibility. Evaluate the headphone’s intended frequency response and sound signature when fine tuning impedance pairing.
Conclusion
To summarize, understanding impedance is crucial for audiophiles using external DACs with Android devices. A DAC’s output impedance interacts with a headphone’s input impedance, which can alter the frequency response and dynamics. Impedance matching through lower output impedances on the DAC allows headphones to perform as intended. Measuring impedances, enabling software-based gain adjustments, and selecting appropriate hardware are key steps.
Impedance matching enables headphones and earphones to render smooth frequency responses with proper damping and dynamics. Without proper matching, key details in the sound may be lost or altered. For those pursuing hi-fi quality audio from their Android device, considering impedance levels and how to optimize the pairing of DAC and headphones is highly recommended. When done properly, impedance matching provides the optimal listening experience.
