Re: New DIY Mid High
What’s probably worth mentioning is that I have been using the 4594 for a while in another box. It has 2 x 10” Eighteen Sound drivers and is very similar to JTR's 3TX. It runs two-way active crossing over to the 4594 at 630Hz. Prior to using the 4594 it used an Eighteen Sound NSD1480N compression driver and crossed at 900Hz.
The version with the 4594 goes louder and sounds better…. It’s that simple.
The 1480 is an excellent driver but wasn’t happy going as low as I needed, If you crossed it at 1200Hz to 1600Hz it was great but there were issues with the two 10’s not wanting to play nicely together above about 800Hz.
http://www.eighteensound.it/PRODUCTS/Products/CatID/3/ProdID=151#.VMQto2kbrq4
In this case the double 12” has probably 10dB or more output than the double 10” because of the horn loading and more powerful drivers used in the mids.
I have been concerned how the 4594 would keep up and I have after a little bit of testing raised the crossover frequency of the HF to 800Hz. It sounds slightly better crossed at 650Hz; and for theatre applications that’s probably where I will run it.
BMS’s can provide a clever HF – VHF passive crossover, it use two capacitors and one inductor providing 12dB low pass for the HF and 18dB for the VHF …. and the correct phase / time alignment. The crossover needs to be asymmetric because of the steep roll off the VHF has below 7kHz.
I suspect that in order to achieve its high efficiency BMS has minimised the diaphragm to phase plug clearance and is using a very high resonate frequency / low mass diaphragm. This however limits the low frequency capability and it rolls of sharply a little below 7KHz as noted above.
I was concerned because I need a lot more from the 4594 than I did with the double 10 that the passive HF – VHF crossover may not provide enough protection for the VHF… so I have run this box all active crossing at 6900Hz at about 54 dB per octave. The Lake FIR crossover allows me to do this and still sum flat though the crossover region, it also allows me to put some appropriate limiting for both the HF and VHF sections.
The advantage with the two-way HF – VHF design is its peak ability on complex waves forms. For example if you imagine a 1KHZ pulse (2.83 volts) with a 10KHz pulse (2.83 volts) imposed directly on top of it. The peak magnitude is 5.66 volts.
We can pass it through a crossover to get two waves 1 KHz & 10 KHz - 2.83 volts each. Into 8ohms this would be 1 watt each i.e 2 watts total. If we do this without the crossover then it’s 5.66 x 5.66 / 8 = 4 watts. This is peak power, the RMS continuous power or heating value remains the same – 2 watts.
Acoustically it’s more or less the same, but in terms of peak mechanical stress there is a significant amount more when one driver has to do both. In large compression drivers that are designed to go low, this can often result in none linear behaviour of the diaphragm. This is why I was using the 1480N with the Titanium Nitride Diaphragm previously – it’s much stiffer and operates as a piston to a much higher frequency. As a result it has less intermodulation distortion.
Turbosound have been doing this 2-way trick for years using a 6” cone diver/horn and VHF driver (EV DH3a) in their Flashlight and Floodlights and now in their Flex and Flash Line. They claim 6dB more usable output compared to a tradition compression driver with their combination.
In BMS case it’s a little trickier combining them in one driver and getting it all to work nicely in the same throat.
In the past when I have driven DH1A’s with extreme VHF levels I have neatly sheared the diaphragm off around where it attaches to the voice despite having a limiter. I once destroying 10 in one go; after that I always low passed them at about 15 KHz every time I needed them to go loud.
Bottom line … this little DIY goes loud and sounds great, Im yet to find anything better in this size and weight. If anyone can suggest a better driver for this application, I would love to try it.:nod:
... the suggestions so far have help me quite a bit with this design, thanks to everyone ... further suggestions a still very welcome.
