Stealth 2 OB Speakers

28/6/08
This is the second iteration of original Stealth Open Baffle speakers. It is practically the same as Stealth OB, aside from additional woofers and obvious baffle size plus a bit better documentation ;). It is a culmination of my work/experimentation with various designs of loudspeakers.

Nothing can compare the naturalness and non-coloration of dipole bass, and the 4 x 10" woofers provide plenty of them. The system acts as dipole up to 6kHz which is considered very high.

How much bass? Well if you have system that goes down low enough and permit my digicam's distortions these clips may give some ideas :)
http://www.youtube.com/watch?v=x61YMdowJbQ
http://www.youtube.com/watch?v=Tb4KPk3WVx0

Baffle

The baffles are 45 x 60 x 18mm MDF. In Australia go to Mitre 10 and get the 1,200mm one and ask them to cut in half - simple. It cost about $15. I designed them to be placed on top of my study room table. If you want them on the floor simply do a taller baffle. The rule is very simple with OB: The bigger, the better.

I painted them flat-black and it looks good. It's how I got the "Stealth" name from as it does remind me of F117. Yes it also "dissapears" on good recording.

Bass

Once you hear dipole or open baffle bass, it is hard to "unlearn" the experience. One would instantly recognise what "box coloration" is and will be irritated by it. Unfortunately dipole bass is hard to produce due to cancellation.

This can be rectified by displacing more air and the way to do it is by adding more woofer, bigger woofers, and woofer selection. The woofer must have Qts value of 0.7 or more (which means less damping, smaller magnet, etc). This tends to be cheaper woofer too! The Stealth 2 uses 4x 10" woofers. They have Qts of 0.7 and cost only $29 from Jaycar.

Second step is to equalize it. The equalising circuit is very simple as shown below. It can be built using three op-amps such as TL072 ($1 a piece). The signal goes through 120Hz low-pass filter and then shelved at 6db/octave to compensate the cancellation. The signal phase is inverted so from the amplifier the cables must be swapped between positive and negative.

It uses regulated PSU using 7815 and 7915, something like this. Very easy to make.

29/7/08
Note: Revision 3. Changed passive xo to 1st order for phase coherence. This is after comparing the female voice with Linkwitz Pluto. The original schematics is still available here, also Rev1, Rev2





I'm too lazy to draw PCB and use a breadboard instead. Don't worry the electrons don't mind... That extra op-amp IC was just for experimentation with notch filter.

Mid & High

I am a follower of simpler-is-better philosophy so driver selection is critical for the Mid & High section. Simple full-ranger like Fostek, Lowther, or Visaton B200 is the obvious choice but they are expensive and hard to find in Australia. So I went with the following:

For midbass use P13WH-00-08 woofer which has very smooth rollof at 6kHz. This driver has been praised by many for it's natural rollof and don't require crossover at all. No crossover is the best thing after active crossovers. Additionally, since it goes up to 6KHz the system will behave as dipole up to that frequency range - which is very good.

Tweeter selection is no longer critical at 6khz. Tweeters usually become expensive when they need to deal with low-frequencies. Since the P13WH midbass can go up to 6kHz the tweeter can use Vifa TC20TD05-06.

As shown on the diagram above it is crossed passively using simple 1st order crossover. Since this tweeter sensivity is 90dB we need to attenuate it using -4db L-pad resistor circuit. Simple.

What can be improved

I have few measurements using ARTA but could not find anything offending the frequency response. Being dipole it's quite hard to nail down true response due to many factors. I simply find music recordings become very enjoyable and I can listen to it for hours and hours.

Try it. It is actually very hard to get things wrong with OB. As the masters say:

"It is difficult to screw up an open baffle speaker design to where it sounds worse than your typical box speaker. " - Sigfried Linkwitz

"Being that open baffles are so easy to design and construct, the crossover is by far the biggest hurdle." - Nelson Pass

But obviously I have my further desires :) ... like:

• Bigger woofers. I wish I had bought 12" ones as there are still some space on the baffle. Or perhaps 2x 12" + 2x 10".
  
  
• Perhaps a full-ranger instead of midbass+tweeter. It's difficult to place the tweeter at ear level -- if I have the money that is
  
  
• what else... I don't know actually. Perhaps prettier baffles ...
  
  
• Stealth 3 ? ... it will be 4x 15" Eminence Alpha with fullrange drivers for mid & high. If I have the room ! perhaps even a CS2 clone.