The Halo 5 Transmission Line Mini Monitor
Mini monitor and transmission line seems like a misnomer as anybody who knows transmission line loudspeakers can attest. But let's call them mini monitors anyway as these are fairly small in transmission line terms. Why Halo 5? Well I had to call them something. 5 indicates the 5 inch mid/woofers that I used and Halo is close to the sound stage that I sit in when I am in front of my PC based sound studio.
Choice of Drivers
I have used Vifa drivers before and have a lot of respect for most of what this Danish company manufactures. They have some very serious audiophile drivers for the financially impaired speaker builder. I have read some very good reviews on the Vifa 5" mid/woofers on the Net and they are often used in very expensive commercial loudspeakers. First choice was the P13WH00 (also used in the highly acclaimed Ariels) but the close proximity to my PC's CRT monitor dictated the use of a magnetically shielded unit. I finally chose the MH13SG which is essentially the same unit but with a shielded magnet. The high end is handled by the D27SG05, a magnetically shielded relative of the D27TG35 that I have very successfully used in other designs. To achieve a higher SPL from the mid/woofer (to match the very efficient tweeter) I chose to use two drivers in parallel and use a typical MTM baffle layout. The increased cone area and reduced impedance adds 6 dB's to the bass and mid sound pressure levels.
Transmission lines are essentially open ended pipes tuned to the natural frequency of the driver (traditional TL design theory). The main advantage it that it uses the low frequency backwave from the driver, delays this through the line and adds it to the driver's front wave via the open end (port), thus increasing the efficiency of the speaker. Higher frequencies and standing waves are damped by lining the critical areas in the line with damping material, The MH13SG has an FS of 54Hz and the line length was thus chosen at 1/4 of the wave length for this frequency. The result is a line length of 1583mm. Groan.... so much for small! The line is tapered to reduce standing waves and improve bass response. I played around with various tapers on my transmission line spreadsheet and eventually settled on a 4:1 ratio with the closed end at 324cm^2 (2 times the total drivers' piston area). The open end (port) is at 80 cm^2. This is half the total driver piston area and smaller than most traditional theories dictate. However I settled on this as size was important (they had to fit on my desk) and Martin King's MathCad spreadsheets showed very little difference in response between this and the more traditional 1xSD (for these drivers at least).
Cross-Over and Headaches
The first version of the cross-over was a 4th order Linkwitz-Reilly design that looked very good on paper, but sounded horrible. The sound was coloured and "boxy", something that I did not expect from either the drivers or the transmission line. Something was seriously wrong and I played around with damping material for weeks on end. The small pipe depth was blamed and I eventually settled in to listening to them with the optimum damping. Listening fatigue (a harshness somewhere close to 1000Hz) eventually forced me to analyze the problem more closely. Say hallo to tweeter resonance.
The tweeter's FS sits at 1000Hz and it finally dawned on me that this might be the culprit. So I went back to my spreadsheet and designed an RLC circuit to get rid of the tweeter's resonance. I also discovered that I did not have sufficient spare components to add this to my existing X-over so the entire thing was redesigned. The mid/woofer ended up with a second order Bessel filter and the tweeter with a first order filter. However, the chosen X-over point and the tweeter's RLC allowed for a steeper (2nd order) cutoff.
R1 and C2 in the woofer circuit compensates for the impedance rise in the woofers and R3, C4 and L2 supresses the tweeter resonance. The tweeter is a little bit too efficient and R2 pulls the tweeter dBs down to the woofer/mid levels. The component values are not exactly as predicted by theory, but was derived (lots of calculation and resoldering the X-overs about 20 times) to cancel some of the box width associated standing waves and edge diffraction effects. PS: inductors are air core and caps are Solen polyprops, quality components do make a difference. The tweeter is wired out of phase else their is a dip in the frequency response in the crossover overlapping region.
Hallo Halo! And there it was, everything that I expected. The colouration was gone, the soundtage huge, the dynamics provided all the slam in the kick drums that I desired and I was a happy man (yes I used to be a drummer and knows that the kick drum makes a thwack and not a boom). Not only can I properly mix my own tracks with these babies but I can now happily listen to my favourite CD's while working at my PC. Obviously the bass does not go down very low (one can only expect so much from 5" woofers) but it is extremely accurate. I eventually ended up by adding a subwoofer to the system with great results. The rest of the system is a Terratec 24 bit soundcard in the PC, a rather old Philips CD player and Rotel pre and power amps. My PC speakers will put many Hi Fi systems to shame, if I have to say so myself.