VTF-3 MK3 Measurements & Analysis

Figure 1a, b.: System Impedance at left, amplifier transfer function, at right.

The impedance curve of a system (seen above, right) can provide useful information about the system, including the presence of any pathologies that might effect its performance. The impedance scans were done with the VTF-3 Mk3’s power amp disconnected, the cable connecting the driver to the LMS card fed through one of the system’s ducts. As always, the 4-wire measurement method was used.

The red curve is the system’s impedance with both ports open (max. output mode) and the blue curve that with one port blocked (max. extension mode). At left we see the typical max-min-max curves presented by a bass-reflex or ported system. The blue curve’s local minimum at just over 15 Hz and the red curve’s local minimum at just over 21 Hz indicate the system’s native (driver/cabinet with no power amp in the circuit) tuning frequencies when set to operate in either max. extension mode or max. output mode. Further examination of the curves present no noteworthy pathologies.

At right, Figure 1b presents 3 amplifier transfer functions. The blue curve is the transfer function of the current amp, set to run in max output mode. The green curve is again for the current amp. this time set to run in max. extension mode. For comparison purposes, the transfer function of the old, original power amp (red curve) and no longer used by Hsu Research, is included.

All three measurements were made driving a single line-level input channel directly with the LMS card’s line out. (Driving both of the VTF-3 Mk3’s line-level inputs will result in a 6 dB increase in the amp’s output. (You are getting more voltage gain for a given input but the overall power limit of the amp will be unchanged).

The left most peaks for both the max extension and max output modes of the current amp have been shifted below and above the old amp’s LF speak (respectively). Both of the new amp’s curves feature a gentle roll-off immediately above their respective peaks, hitting a local minimum at around 80 Hz. Capturing data plots of a powered sub’s amplifier transfer function is a good to gain further insight to just how a particular sub functions. As a side note, if you know what a powered sub’s amplifier transfer function and you know what said sub’s dB spl response plot is, with just a bit of math you can derive thesub’s native dB spl plot.

Figure 2a, b. Amplitude response plots, VTF-3 Mk3’s. At left, max. output mode) & max. extension mode) at right. 2m, on-axis, dB re 20 µPa., ground plane, no scaling. All measurements done outdoors.

In Figure 2a & b we see (at left) the amplitude response plot of the VTF-3 Mk3 when operating in max. output mode (both ports open) and max. extension mode at right (one port open). The outdoor ground plane measurement approach was used, with the LinearX M31 microphone placed 2m, on axis from the sub. No scaling or smoothing has been used. Ground plane at 2m produces results very similar to that of measurements made under anechoic conditions at 1m.

The scans in both cases were started at a drive level sufficient to produce approximately 90 dB in the passband and re-run with the drive level increased by ~ 3dB at each turn until compression significantly affected the system’s output. The phase switch was set at 0°, the crossover was bypassed and mode switch was set, of course, at either max output or extension mode. Note that at no point during measurement or listening was the optional turbo unit used (none were provided for the review). Adding the turbo, had one been available, would have resulted in a system capable of as much port-noise free sub- 20 Hz output as two VTF-3 MK3’s!

The measurements fairly well speak for themselves. We see the compression kicking in as the curves (in the mid-bandpass region) approach the 105 dB spl mark. At the higher output levels, particularly when the VTF-3 Mk3 was operating in max. extension mode (and much as Hsu Research would prefer it not to) at 16 Hz, the MK3 without turbo has port noise creeping in well before the amplifier runs out of steam. Adding the turbo (had one been available) would have effectively given about 6 dB more port-noise free output at 16 Hz. Overall, the VTF-3 Mk3 proved it self an excellent example of a well engineered product, purpose built with the intention of wringing every last drop of performance possible from a single 12” driver in a vented cabinet. Let’s see you top this one, Dr Hsu! ;-)