THD+N Meter

Tue, 26 Nov 2024 00:00:00 +0000

THD+N Meter

A THD+N meter measures the harmonic distortion of a device. In audio, this is crucial for assessing the quality of equipment. This project was developed as part of the subject “Instrumentos y Mediciones Electrónicas” at UNTREF. In this course, students design and develop various electronic instruments, and projects are carried forward by successive student groups. This particular project was already underway, and our focus was on developing a phase shifter to align two signals.

Teory of THD measurement

To measure the distortion of a system, a sinusoidal signal with minimal distortion is input to the device under test. The goal is to evaluate how much distortion the device adds to the signal. This is quantified by measuring the power of the original signal and the power of the signal after passing through the device, excluding the fundamental harmonic.

A differential amplifier is used to subtract the device’s output signal from the reference signal, leaving only the higher-order harmonics.

The THD+N is then calculated as a percentage using the following equation:

$$ THD+N = \frac{V_{filt}}{V_{tot}} \cdot 100 $$

Where: $V_{filt}$ is the RMS value of the filtered signal (excluding the fundamental harmonic). $V_{tot}$ is the RMS value of the original signal.

Phase Shifter

To achieve effective cancellation in the differential section, precise phase and gain adjustments of the two signals are required. The gain adjustment had been successfully implemented by the previous group working on the project. However, achieving the necessary 360-degree phase rotation across the entire audible frequency range (20 Hz to 20 kHz) presented a challenge.

To address this, we designed two all-pass filters in series and calculated the component values to achieve the desired phase rotation over the target frequency range.

The design was modular to facilitate seamless integration with the previous stages.

We tested the circuit on a protoboard before designing the PCB using Altium Designer.

The device

The device features several controls for adjusting phase and gain. Both the phase and gain adjustments include fine-tuning potentiometers to ensure maximum precision.

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It is equipped with BNC inputs and outputs, allowing users to visualize the output on an oscilloscope and achieve maximum attenuation.

Results

This device was compared against a commercial THD meter (GW INSTEK GAD-201G), and the results were highly similar. The primary limitation was the base noise level of the measurement environment, which significantly restricted the lowest THD value we could measure.

The specifications of the device are summarized in the following table (in Spanish):

A detailed analysis of the device is available in this final report (written in spanish).

Building and design of a personal loudspeaker

Wed, 20 Nov 2024 00:00:00 +0000

“BassAdo”: A Semi-Portable Low-Cost Home Speaker

This project is part of the Electroacoustics II course at UNTREF within the Sound Engineering program. The task was to design a speaker system from scratch by applying the theory and concepts explained in class. The project was developed over the entire semester, with various stages to complete and present in reports. The speaker is intended for use in large spaces, potentially outdoors, to play music in a social gathering setting. It was named BassAdo to blend the Argentine tradition of “asado” (a typical barbecue gathering) with the word “bass,” emphasizing the speaker’s low-frequency performance.

Design

The goal was to design an accessible home audio system that allowed exploration of topics discussed in the course. The design aimed to emphasize bass response, characteristic of commercial systems, prioritizing low-frequency bandwidth extension over minimal group delay and system time control.

Regarding the transducers, the team had access to Yharo-brand units, which are classified as non-professional, consumer-grade, and suitable for automotive or home systems. The impedance response of the units was evaluated, and an 8” woofer was selected for low frequencies, along with two 4” units for mid/high frequencies.

Measuring the Thiele-Small parameters of the speakers revealed a high Vas (Equivalent Suspension Acoustic Volume), necessitating a large cabinet volume for proper control. To address this, and given the availability of two 8” woofers, the team opted for an isobaric speaker configuration, acoustically coupling the woofers to improve control and reduce cabinet size. Additionally, the cabinet was designed as vented to enhance low-frequency response.

The Thiele-Small parameters were obtained using the software REW. With these parameters, simulations were performed in Basta! to optimize the design for the desired response. A key focus was tuning the port’s resonance frequency to achieve strong low-frequency performance. The transducer had an fs of 45 Hz, and the port was tuned to 40 Hz by adjusting the tube length and cabinet air volume.

Based on these results, a 3D model of the cabinet was created using SolidWorks, and the design was used to cut the materials for construction.

Details of this process are documented in the following design report.

Construction

The wood was cut according to the 3D model, and the cabinet was assembled.

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As shown in the images, rock wool was added as an acoustic absorber. Measurements revealed this was excessive (the port resonance was overly damped), so some rock wool was removed to achieve the desired result.

Measurement and Calibration

Frequency response and directivity measurements were conducted in the university’s laboratory using the following equipment:

Powersoft M50Q amplifier
Earthworks M50 microphone
RME Fireface UCX audio interface
OUTLINE ET250-3D turntable

Using this setup and the Arta software, the acoustic response of individual transducers was characterized (useful for crossover filter simulation). Vertical and horizontal directivity responses were also evaluated to determine the best orientation for use. Frequency response graphs for both transducers were generated.

All measurements and in-depth analysis are included in the following measurement report.

Crossover Filter Design

Finally, the crossover filter stage was designed. Using the previous measurements, data were uploaded to VituixCad to calculate the simulations. The goal of the crossover filter was to achieve a pleasant frequency response for music playback and to enhance low frequencies. Vertical polar response uniformity was also a priority.

Since the power stage required an active supply, an active crossover filter with a Sallen-Key topology was implemented. The number of filters was defined based on space and cost, and adjustments were made in the software to achieve the desired response. For example, the low-frequency driver used the following configuration:

Where:

F1: High-pass fs=30 Hz | Q=0.67
F2: Low-pass fs=480 Hz | Q=0.5
F3: Notch filter at 220 Hz
F4: Notch filter at 400 Hz

Before building the filter, the proposed configuration was tested with a digital filter to practically evaluate the system’s response. Details of this section are provided in the following crossover filter report.

Conclusions

This project allowed us to apply theoretical concepts in practice and gain a deeper understanding of the development and challenges involved in designing an electroacoustic system.

Electronics on Matias Di Bernardo

THD+N Meter

THD+N Meter

Teory of THD measurement

Phase Shifter

The device

Results

Building and design of a personal loudspeaker

“BassAdo”: A Semi-Portable Low-Cost Home Speaker

Design

Construction

Measurement and Calibration

Crossover Filter Design

Conclusions