Systems List › DIY › Focusworks GUJ18V1(DB18V Cab)
NotesThe impedance curve for the GUJ18v1 loaded into the DBV18 test enclosure with a single port open shows an impedance minima way down near 14Hz of about 7.2 ohms, an in band peak impedance of 57 ohms at 30Hz followed by the second impedance minimum of about 9.5 ohms at 50Hz. The effective vent tuning appears to be just a hair below 15Hz. The overall raw response shape is reasonably flat and extended and is within a 6dB total window from 22-175Hz. The outoput then gradually tapers off towards 14Hz before transitioning below the vent tune towards 10Hz. The sensitivity with a 2.83v input at 1m is about 89dB above 60Hz but registers around 93-94dB from 30-40Hz. Sensitivity at 16Hz is about 83.5dB. Up around 180Hz the vent resonance can be seen which would limit the useful operating range The time domain results show nothing of note. The group delay never even reaches 1 cycle even with the 12dB/Octave HPF used for the high power measurements. The high pass filter used was a 16hz 2nd order which may not have been quite aggressive enough. A 3rd order at 15Hz would have been preferred but the equipment used did not have the ability to quickly set one below 20Hz. It can be seen that the 2nd order filter had some effect on frequencies well above 30hz.
The long term output measurements start with 5.1 volts to produce the usual 90dB@50Hz@2m output level. This may seem high but this is a true 8 ohm system. things progress quite well up through 29 volts applied which produces around 105dB at 50Hz. At this point the vent compression is just reaching 1 to 1.5dB. By the time the system is asked for 10dB more output with 92 volts applied we start to see the first real signs of significant output compression. The vent is now compressing by about 3dB at its highest. From 50Hz on up the compression is just 0.5dB. At this level there were some hints of vent noise and perhaps some driver distortion so the next input level was only 4dB higher instead of the full 5dB. This required 145 volts from the K20. At this point the driver made some over excursion distress noises during the sweep and we see that the output compression in the deep bass has reached 4dB. Above 40Hz 1.5dB of output compresion is observed which isn't much comparatively. However the repeat 90dB 5.1 volt measurement exhibited a significant amount of sensitivity loss due to heating with 1dB loss above 50Hz so this was likely a good stopping point.
Distortion results captured at the same drive levels as used for the long term testing reveal that during the 16.3 volt measurement the system was producing less than 2% THD above 22Hz which is excellent. The 29v or 105dB level produces a bit more distortion but it remains below 3% above 16Hz. The 51v or 110dB sweep produces much the same result with THD held to 4.5% or less above 16Hz. THD in the deep bass remains below 20% from 10-16Hz. The 92v or 115dB sweep produced THD of 7% or less above 16Hz which is an excellent result. THD has started to balloon below 16Hz indicating that driver excursion is getting high and the HPF is not quite aggressive enough to keep cone excursion down. The final 145v or 119dB sweep the THD jumps up quite a bit and reaches 14% from 25-30Hz which should correspond to the in band excursion maximum for the driver. Meanwhile the THD below 13Hz has gotten out of hand with the driver showing signs of reaching the excursion limits near 10Hz.
The short term burst output with distortion limits indicates that the system has a good amount of headroom even accomplishing a passing score of 90dB at 10Hz which is well below the vent tuning. Output is strong at 16Hz and above. The system was distortion limited below 50Hz and amplifier voltage limited above that point.