Systems List › DIY › Focusworks GUJ18V1(DB18V Cab)
NotesThe GUJ18v1 loaded into the DBV18 cab and run with 2 ports open produces an impedance curve with a minimum of 7.2ohms at about 18.7Hz and an in band maximum of about 55ohms at 32Hz. The second impedance minimum is roughly 9.5 ohms at 50Hz. Overall the impedance is that of a true 8 ohm system. The effective vent tuning with 2 vents open appears to be about 19Hz. The frequency response fits within an 8db total window from 21-155Hz and drops off to about 12dB down from the 36Hz peak by 16.5Hz. Below 16Hz we see the steeper roll of below the vent tuning. Up near 160-180Hz we see some issues related to the vent length resonance and also internal standing waves related to the cabinet dimensions. The amount of lining in the enclosure may not be quite enough to quell some of the resonances. The 2.83v sensitivity of the GUJ18v1 in this vented alignment averages about 89dB from 68-130Hz, peaks at about 96dB at 36Hz which is strong for such a low frequency and then drops off to about 83dB at 16Hz. The time domain measurements show that the group delay never exceeds 1 cycle over the effective range of operation even with the high pass filter in line. No significant ringing is encountered over the useful bandwidth either.
The high power long term output measurements and distortion measurements have a 2nd order high pass filter at approximately 20Hz employed in order to prevent driver over excursion below the vent tuning. A 3rd order filter may have been a better match in retrospect but the 2nd order was used in an effort to preserve some output below the vent tuning.
The long term output measurements start with a 3.25v input. The SPL was referenced to 90dB at 40Hz instead of 50Hz by mistake for this test. No appreciable output compression occurs until the 32.5v measurement where compression near the vent tuning just reaches 1-1.5dB. The output compresses only moderately at each 5dB increase in signal and only reaches about 3dB near the vent tuning during the final 145v sweep which corresponds to what should be a 123dB at 40Hz nominal level. Up above 30Hz the output is compressing only 1 to 1.5dB. As with the single vent test there is some sensitivity loss apparent in the follow up 3.25v measurement. During the 103v sweep some driver excursion noise was noted as was a bit of vent noise. The 145v sweep produced obvious over excursion distress noise from the GUJ18v1 driver and increased air chuffing noises from the vents so the testing was stopped. The 145v level is only a 3dB increase over the 103v measurement due to the noises previously mentioned during the 103v sweep.
THD for the GUJ18v1 in the DB18V cab with2 vents open shows less than 3% THD above 21Hz with the 18.3 volt measurement. The 32.5 volt measurement does nothing of note to increase THD with THD remaining below 5% above 20Hz. and rising to 13-15% from 13-20Hz. The 57.8v measurement is more of the same with THD 6% or less above 20Hz and reaching 20-24% from 12-17Hz most of which is below tuning. The next 5dB increase in signal to 103v produces a bigger increase in THD as the driver starts to become driven very hard. At this level the THD is at or below 8% above 20Hz which is good. THD is now reaching 35% at 16Hz which is slightly under the vent tuning and we can see that the THD has grown very high below that point. The driver has started to reach it's excursion limits at this level indicating that the high pass was not quite aggressive enough of a slope. The final loudest measurement at 145v causes the GUJ18v1 driver to reach its excursion limits in the deep bass below vent tuning. THD at 16Hz is now about 48% but at 20hz and above the THD is still around 10% or lower. Of note is that there appears to be a spike is THD related to either the vent resonance or a cabinet panel itself perhaps. This was the first driver tested in the DB18V cab so this will be investigated later with an effort towards eliminating or minimising this thorugh extra bracing, lining or fill materials.
Short term distortion limited burst output for this system is generous from 16Hz on up. The 115.5dB at 20Hz is particularly noteworthy. The system could not quite produce passing results above the noise floor at 10 or 12.5Hz but this is well below the vent tuning which is closer to 20Hz. At 63Hz and above the system was limited by the amplifier voltage. Below that point the distortion and driver excursion were the limits with the K20 amplifier.