VTF-1 Design and Construction

Introduction

After having spent some time with another Hsu subwoofer, the STF-1, I was asked to spend some time with another, the VTF-1. During the review of the STF-1, I spoke with Dr. Hsu on several occasions about his design philosophy and approach to product engineering. The approach, as I gathered, seemed to focus on balance, which has allowed Hsu Research to develop a reputation for engineering and manufacturing high quality, value oriented subwoofers that consider a broad range of performance parameters and are widely considered very competitive against similarly priced products.

The VTF product designation is an acronym for Variable Tuning Frequency, which differs from their other subwoofer line, Single Tuning Frequency, by using a dual ported cabinet tuned to two different resonant frequencies. Within both of these product lines, Hsu uses a simple naming convention to distinguish between models of increasing quality and capability with increasing numbering, and with the occasional Mark number, to indicate when significant improvements to the product design have been applied to a particular model.

The design intent of the VTF series is to provide a flexible product that puts the decision for the trade off between louder or deeper, inherent in a budget subwoofer, into the hands of the user. Depending on the application and demand, the owner can decide what is preferred. For example, Hsu suggests one possibility is to use the higher SPL configuration for LFE and the lower extension configuration with dedicated music listening, where peak output demands are typically lower, to maintain bottom end fidelity. Another alternative, perhaps the owner is throwing a party and wants to crank it up and does not need to approach pipe organ lows, just switch the sub around; handy.

Design and Construction

The VTF-1 is a moderately sized powered subwoofer that is slightly wider but noticeably heavier than the STF-1 I reviewed previously, with the latter weighing in at 35 pounds verses the former at 56 pounds. The cabinet is the standard sub rectangle but features radiused corners with reveal lines just below and above the top and bottom panel and the unit is adorned on the front at the bottom with a badge bearing the Hsu logo. The bottom of the unit has metal threaded inserts at the corners to accept either plastic cones or rubber feat, both of which are provided to support the unit on either carpet or hard flooring.

The VTF-1 is available with two vinyl finishes. I was sent the matte black version of the sub for the evaluation but the VTF-1 is also available in a ‘woodish’ Maple finish. The vinyl finish is plain but competently applied; Hsu’s budget minded subs spend money on performance rather than flashy appearance. Remember, most people stuff subs behind or beneath other furniture anyways. For those who are décor conscious, the best bet is to buy something nice for camouflage. This will serve double duty for WAF when she gets something nice with the new sub.

The subwoofer design is downward firing with dual rear tuning ports that utilizes a 10 inch cone driver with a video shielded motor. The dual tuning port comes with a low permeability foam plug that, when placed in or removed from one of the ports, will alter the cabinet tuning, allowing the owner to adjust between a higher SPL or a deeper frequency extension configuration. With one port sealed, the VTF-1 is said to reach 25 Hz at the –3 dB point. When both ports open, the sub is said to be able to develop higher output but can only reach down to 32 Hz before roll off becomes significant.

The 10” driver used for the VTF-1 is custom manufactured for Hsu using a polymer treated paper cone, butyl rubber surround, and a poly-cotton spider. In speaking with Dr. Hsu, he has said that this driver design was chosen to provide a long, reliable service life while maintaining cost. When applying a philosophy of high performance/high value, Dr. Hsu has said that he has not found a significant enough advantage in using drivers made from exotic materials to justify the added cost to his designs.

The various controls, connections, and the internal amplifier are plate mounted at the back of the VTF-1, as is standard for most subwoofers. Towards the top of the back plate, the user will find toggle switches provided for power with off/auto/on settings, a phase switch that provides 0/180 degree settings, a switch for one or two port operation, and a crossover bypass switch. Volume and crossover frequency knobs provide continuously variable settings for system balance with the crossover frequency variable between 30 and 90 Hz using a 24 dB/octave Linkwitz-Riley low pass filter. Line level connections can be made using stereo RCA type line-in or at speaker level with binding posts that include speaker in/out connections.

At the bottom of the plate are the input voltage selection switch, a grounded power input, and access to the main fuse. The input voltage switch is protected behind a clear plastic cover attached with Phillips screws and the fuse is externally accessible with a standard screw mounting. The power input will accept any standard grounded computer power plug in addition to the supplied power cord.

After some disassembly, I found the cabinet construction consisted of 3/4” MDF panels with fiber glass damping material adhered to five sides on the interior. The tuning ports are run nearly the full depth of the cabinet and are flared at both ends. The driver assemble is built on a stamped metal frame that supports a decent sized motor structure. The frame struts are embossed at the center, which improves their stiffness and the motor has a perforated metal disc at its center apparently to improve thermal venting.

Amplifier section is mounted to the back plate on two separate circuit boards. The upper board serves as the preamplifier stage, with all the input, volume, and signal processing functions correspondingly located with the external controls. The board for the power amplifier stage is mounted at the bottom of the plate near the external power input.

The power amplifier is based on a BASH design (Bridged Amplifier Switching Hybrid), a patented amplifier topology licensed from Indigo Manufacturing, Incorporated. BASH amplifier designs are intended to be a compromise between class A/B amplifier fidelity and class D amplifier efficiency, capable of high current output. The amplifier section of the VTF-1 is rated at 200 watts RMS, slightly higher than the STF-1 at 150 watts, and 600 watts of dynamic power.