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...?

Phase IV rebuild complete

Over the weekend I finished the rebuild of the Phase-IV baffle. Actually the baffle was done a while ago but since my wife was unhappy having something so ugly in the house, I decided to try to improve how it looked by giving it a coat or ten of paint. There's a I need to learn about finishing.

The mid range and tweeter are now properly located in close proximity so the interference fringing around the crossover frequency should be fixed. By way of comparison, here's what the speaker looked like before. Not great, but a lot less like a building site.

The bass cab is now filled with a fair amount fiberglass insulation - I know there are better materials but this was cheap ($15 and I've used less than a quarter of the roll). Since this changes the bass response, I'll need to rebuild the crossover. So next weekend the measurement equipment comes out again.

Volume estimation

I am still going back and forth over the size of the cabinet; is 5000 cubic inches (about 80 liters) enough for the sealed bass enclosure? In building the Phase IV, I had used the volume information from the DM6 which has in internal volume of "between 51 and 60 liters" according the manual. I decided to look at the original B&W 801; based on the published overall dimensions and some pictures, I estimated the internal volume to be somewhere between 108 and 122 liters depending on the thickness of the cabinet walls (according to B&W's literature, the Nautilus 801 walls are 1.5 inches thick - I used 1 inch and 1.5 inches). Big difference!

The actual air volume is likely considerably smaller since there is a fair amount of internal bracing (the 'matrix' used in the Nautilus could take 10%-15% off this figure). However, since the dimensioning calculation I was doing ignores the internal bracing, I decided to compare only the raw internal volume.

The 801 bass cab volume (~115 liters ) was over double that of my Phase IV (54 liters) and suggested a complete redesign of the cabinet was needed. It probably also contributes to the poor performance below 100 Hz, though the small linear excursion of the Silver Flute drivers is clearly another factor. The Phase V needs a wider baffle to accommodate the 10" AC-250F1 driver, but this alone wasn't nearly enough; several iterations of changes led to a design that maintained the height at 40 inches, increased the depth from 21 to 24 inches (slightly deeper than the 801 which was 22 inches back to front) and kept about the same width (17"). The new bass enclosure will weigh in at 110 liters before bracing. (It will also be fairly heavy)

Still baffling

To fix the destructive interference issue, I building a new baffle. It will also be almost twice as thick, partly to reduce resonance, and partly as a way of loosing all the ugly screws on the front but still allowing access to the inside. The star of the piece is the new circle cutting jig.

(It fits only some Craftsman routers - the plunge router has a larger base, but it does fit the fixed base model). Depending on which way you put in the center locating pin slider thing, you can cute really small circles (<2 inches in diameter) to fairly large ones.

The router jig enabled me to rebate the drivers which I hadn't done on the first baffle. Although I don't understand the physics, there is a strong school of thought that the tweeter in particular needs to be flush with the baffle.  Presumably something to do with diffraction from the sharp edges?

This shows the double layers and the neat rebate the router cuts. The lower section of the baffle will be screwed in close intervals to the cabinet, and the upper section will be attached to the lower plate with the driver mounting  bolts and possibly four screw, one at each corner. 

The AC 250F1's are now in so I want to get this (Phase IV) completed before building he new cabinets.Once the new baffle is built I will likely line the cabinet with SAE-13 or SAE-15 1/4" felt. I didn't do this first time round since I wasn't sure I wanted to blow $80on felt if the sound really sucked. (I also wanted to test the reflective diffusing properties of the cabinet design without any absorbing material). 

Phase V

Current plans are forming around the AC-250F1, an AC-130F1 or MkII, probably the latter (I like the idea of using carbon fiber somewhere) and Hiquphon OW1. The two driver idea will have to wait. Some testing of the implications of the phase change created with any second order or higher filters suggest using only first order. I am assuming the trade-off is with harmonic distortion when significant signal gets to the drivers outside of their ideal operating range. This remains, as they say, an empirical question.  What bothered me was the real mess second order filters seem to make of a square wave.

Above is the simulated result of passing a square wave though the second order crossover designed around the AC-250, AC130 and OW1. Speaker Workshop generates the phase shift data in the total frequency simulation. Using these, I simulated a square wave from odd harmonics, and then applied the phase shifts from the Speaker Workshop output which I had dumped to a file. This (above) was the result and it looks terrible - nothing like a square wave.

This, by way of comparison, is the simulated for the first order crossover I designed for the same drivers. While obviously not square, it seems much better than the signal passed by the second order network.

If the rendition of detail has a lot to do with the accurate reproduction of transients, then the first order network should do much better at this than the second order.

My current plan is to build the speakers (when the wretched drivers get here in March or April) and two pairs of cross overs, one first order, the other second order, and see how they sound.