Phase IV Mk2 construction underway

After very little thought, I decided to see if I could improve on the Phase IVs. A new cab design was created in Sketchup (which I'm now getting better at using) in fairly short order. Judith was asked for her thoughts on the three variants I generated (each with slightly different baffle widths) and she chose the right one.

A weekend of fairly hard work, and not all of it plain sailing. I built part of a new pair of cabinets (this time I decided to build a pair). I'm not cabinet maker so I'm learning as I go, and almost all is trial and error. On Sunday there was a lot of error. The plate joiner joints between the side panels and the corners weren't at all clean and didn't have any structural strength, something which became very clear when I kicked one of them (to test the gluing, not out of frustration that the join wasn't mating cleanly - honest guv).

I'm going back and forth between the real artifact and Sketchup, updating the drawing as I make small adjustments in the process of building. For example, I added the cabinet bracing to the drawing and found that there may be a problem with the brace hitting the back of the mid range driver. We'll see soon enough. I'm quite pleased with structure in the back half of the cabinet. Now, I need to think about how best to build the fronts.

I ordered the drivers today (two 8" Silver Flutes for the bass @ $36, a 5 1/2" Silver Flute mid $20, and I've got some Vifa D27TG-45 tweeters to use up). I also ordered some ferrofluid in case the poor HF response (6db drop >15kHz) of the D27TG-45s has something to do with the old ferrofluid. I've no idea but it's worth a try. If that doesn't work I'll try replacing the voice coil.

One nice thing though; I was looking at either the Vifa XT25BG60-04 or the XT25TG30-04 as tweeter if I can't the the D27TG-45s to brighten up at the top and the mountings are identical. So if I can't get a pleasing sound from the D27TG-45s, I can swap them for either of the XTs without rebuilding the baffle (which is good because this time they're going to be glued not screwed in place).

The other change I'm anticipating is putting the crossovers inside the cab (a bit radical, I know). The drivers will be here by the weekend (I hope), an incentive to get the cabinets finished.

One week on

I've been listening to the Phase IV with the second order crossover for about a week now. The sound is still fairly pleasing, but isn't as crisp at the top as I'd like. Sounds a little like the tweeter is covered with a pillow. The bass is tight but does struggle at the very low end. So I'm about to put the first order cross over in and try that for the coming week.

A tale of two crossovers

Friday saw the arrival of two boxes of components from Erse. Definitely the place to go; their components are cheaper and have narrower tolerance bands than those I was buying from Madisound. The 4.0mH 14 gauge coil is, in the vernacular, "stonking".

The two crossover designs were both based on the same near-field measurements. The aim was to test the theory that 1st order networks did less harm to the phase than 2nd order network. Both should have fairly flat response curves if the simulation is to be believed.

Measurements in Speaker Workshop suggested that the frequency response wasn't too far from simulation for both. However, phase was another thing altogether; although the 1st order network looked better in theory between 500Hz and 5kHz, empirically there was little to tell the two apart. The 2nd order had a slightly flatter response so for the moment I'm staying with that one. 

The 1st order seemed to have slightly better performance below 50 Hz, and the 2nd order has a 9dB peak at about 250Hz - so on reflection I think I'll fry a 1st order on the Phase Vs (not yet built). The upper panel is the 1st order network the lower panel, the 2nd order.The black lines is the simulated response, red is measured. The dotted lines show the phase.

Another factor favoring first order is that there are fewer parts, particularly coils (copper isn't cheap, particularly when you're talking about 14 gauge). If I can damp the mid and bass driver oddities at the top end of their ranges by stuffing the cabs, then perhaps the slower roll off isn't such a bad thing...

So it looks like I can get fairly close with the measurement setup as it stands. Clearly measuring the driver performance in the cabs they'll be working in and building the crossovers accordingly is (quite obviously) the way to go. So for the phase V's I'll spend more time measuring the raw driver performance in situ and tweaking the physical design before turning to the simulation and the electrical design. 

Sitting here listening to the results, I'm surprised by what one can get out of the two Silver Flutes which were the cheapest cast frame woofers Madisound had.  Had I not already bought the AC 250s I'd be tempted to build the second cab and stop here. Interestingly I spent more on crossover parts (having now built four variants) than on the drivers, although the parts in the final 2nd order network I'm currently listening to came to about $55. Excluding experimentation and waste the total for half a pair was about $260. Since I think they're now at least as good as my DM6s (which I've seen advertised on the web for $1,000 the pair) which I bought second hand in 1979 for about $2,000 in today's money, I'm feeling fairly smug. 

Finally the philosophical question for the evening (post a smallish celebratory Blue Moon beer). Should I do the measurements in the room in which I will be using them (my home office) where there are all sorts of reflections that confuse things? Or, alternatively I could relocate all the measurement stuff (PC, amp, mic and pre-amp) to the shed and do the measurements in the open air - no walls to mess things up...?