Sunday, 25 August 2013

Beyma Coax 15XA38Nd Option - Ported Box Plan

The following is the outline drawing of the box with the port and using solid panels. I have re-arranged the bracing as well as the damping and insulation. The front panel is still made of two pieces of 12mm ply wood. There are two main reasons for this. The first being that I can get pre-veneered 12mm ply, but not 24mm. And the second being that the driver depth is 12mm, and thus I can flush mount the driver without have to rebate the edge.

Note that the design also allows for the port to be closed with a port cover. So that I can convert this between a ported and closed box design. In addition, I would like to experiment with varying the port between fully open and closed, that is, by stuffing it at various amounts. The estimated weight of this cabinet is around 46 kg without the driver and the binding posts. The speaker weights about 6.8 kg, and therefore the total estimate weight would be in the vicinity of 53 kg per speaker.

Beyma Coax 15XA38Nd Option - Ported Box

The enclosure speaker design requires the use of a Linkwitz Transform circuit to get a predicted flat response down to the low 30 Hz range. As an alternative, I modelled a ported version using the same enclosure size. The ported has an area of 250 cm3, and is back mounted about 150mm from the base of the cabinet. See below on the comparison between the two on Basta. I have chosen to predict the output at 8W power amp.
Comparison Between Closed an Ported.
The ported response is quite similar until 50 Hz, where it starts to diverge from the closed box scenario. It should be noted that the efficiency of the ported box can be seen, as the cone excursion is quite a bit less than the closed box when it gets to 20 Hz. I then further took the box design and put it into MJK's MathCAD spreadsheet. The spreadsheet used is the ML TQWT dated 11/07/07. The following are some screenshots of the results. The enclosure parameters are the same as per the drawing design in the previous blog entry.
Basic predicted response. 
Predicted response with Baffle step loss accounted. 
The final result with the BSC added, with the BSC being 20R and 12 mH.
I also modelled the same BSC in Basta and got the following result. It should be noted that in all the modelling, 8V or 8W was inputted as the amplifier power.
Basta simulation of the ported box with the BSC modelled as well to match MJK's spreadsheet.
Currently, I am working on an alternative cabinet using a 24mm solid thick wall, with dual 12mm front panel. This should make construction easier. In both designs, there is an assumption of internal box stuffing. Whilst Basta only allows for a percentage, MJK's spreadsheet allows for a more accurate assessment. In the design, I started at 8 kg per cubic meter. The resultant total weight of insulation was 1 kg and resulted in a reduction in the ripples in the response. I had originally place a thick layer of insulation between the bottom of the port and the base. But in MJK's simulation, it reduced the bass response significantly. And hence, the layer of insulation to the bottom of the enclosure has been removed. It also appears that over stuffing the box will result in reduced bass output.

The dilemma of practical construction will be ensure that the correct amount of stuff is placed, to reduced internal ripples and resonances but also not too much as to reduce the bass output. There will be lots of tweaking and construction sequence issues that will need to be resolved as this build progresses.

Saturday, 24 August 2013

Beyma Coax 15XA38Nd Option - Closed Box

This is my design option for a closed box design for the coaxial 15XA38Nd speaker. I have purchased this for over two years, I have not yet had the chance to open them since my departure from home. As I am relocating back home, I have decided that this is the first speaker that I should be building.
Cabinet Plan.
The following is the bill of materials. I have elected to use two layers of 12mm plywood as means of construction and only allowed for dampening around the inside faces of the cabinet. By using two layers of ply with a suitably, softer (i.e., less stiff) glue, I hope to reduce the panel resonance.

Bill of Materials
The results in a box with an approximate volume of 115 litres not included any of the dampening. I have run the simulation in Basta for this design and the following are the simulation results. The crossover is the same one as the standard Beyma crossover that can be bought separate from the speakers. The design is a relatively simple 2nd order crossover with a zobel network on the 15" low frequency driver. The approximate crossover point is 1.8 kHz. The components for the crossover that I have chosen is Jantzen Superior Z-cap, Jantzen Wax coil inductors and Jantzen Superes range.

The low frequency driver has a inductance of 1.5 mH and capacitance of 15 uF. The high frequency driver has an inductance of 1.5 mH and capacitance of 2.2 uF. The LF zobel is 8r2 and 8.2 uF.
Base Scenario.
Base Scenario with Linkwitz Transform.
Base Scenario with Linkwitz Transform and Room Gain. With this scenario, the source voltage is 6V.

So far, the predicted performance is pretty good. So the process now is to order the crossover components. The next step is to order the plywood cut to size followed by the damping materials. With the LT bass boost, the Xmax of the driver limits the power output to 4.5 W, which produces about 6 V and a cone excursion of 4mm. The estimated SPL at 1m would be around 103 dB.

Saturday, 3 August 2013

Re-direction Part 3 - First Verification

This will be a long entry. Last weekend I was back home for a few days, and I took the opportunity to set up my speaker system. The main purpose of this was to verify two things - listening and volume and bi-pole speakers. The set-up was quite simple and there was no effort to optimise cabling, speaker position, etc. The amps were my previously completed LM3886 chip amp and the second was the Decware Zen kit. Both of these amps have been completed for over 3 years, but have had very little chance to run due to life circumstances. The speakers were a 'closed' bi-pole FE-127e towers.

A few notes about the amps. The Chipamp was made from the stock with no upgrades or modifications of any kind, had it was not a dual mono. The Zen was significantly upgraded from the kit. All resistors and most capacitors were changed. It has been modified to use a 5U4G rectifier and has a large 40uF ASC oil cap. The tubes used were all Russian 'EV' grade 6n1p and 6p15p.

The first arrangement was the Chipamp with the speakers set up as dipole. That is, both of them moving in the same direction (even though they are mounted back to back). This would be akin to an Open Baffle arrangement, as the dispersion pattern is a figure 8. Previously, I preferred this arrangement, and my starting point was from here. The source used was by QA350 running on battery power and with a range of CD ripped or HD Tracks converted WAV files.

First time, everything worked. Which was quite a relief. I will now talk about the Chipamp and Dipole in a whole. The sound field was strange, and with the nulling effect at 90 degrees of the speakers was quite prevalent. The sweet spot was indeed a spot, both vertically and horizontally. The speakers are mounted a little lower, around 800mm off the ground, and was lower than my ear level. I found that I had to be pretty level with the speakers and in the middle for the sound to be good. But when I was in the right spot the sound was pretty good. After listening to a few familiar tracks, I realised that this particular combination didn't sound right to my ears. So I decided to switch it to a bi-pole configuration.

In a bi-pole configuration, the speakers are wired so that they both move in the same direction relative to the speaker baffle face that they are mounted on. That is, they both move in to the speaker and out of the speaker at the same time. The net effect of this, was a dispersion pattern that is very omni-directional. This was quite obvious when standing 90 degrees to the speakers. Unlike the dipole, there was no reduction in volume.

Sitting back and listening, I found two things. Firstly, the sweet spot is much larger and more forgiving. I could imagine a few people being in that spot. Secondly, the sound stage and sound field was much larger and there was more immersion. After a little more listening, I quickly realised that this was the sound that I preferred.

After I got to this point with the speakers, I decided it was time to try the tube amp. I cranked it up with the Chipamp and did some casual measurement on my iPhone. The levels at the listening position was around the 80 dB marked, and hovered around there with peaks just shy of 90 dB. At this point, I realised that my goal of 100 dB plus at the listening position is not required for me. Therefore, I have to revised my design criteria, and it seems that I would like the amp and speaker combination to have total capacity to reach somewhere in the 90 dB range at the listening position. At this point, I am thinking specially between 96 and 96 dB. This would give some additional headroom, and would significantly open up the speaker and amp combinations.

The spec of the FE127e is 91 dB, and in a bi-pole configuration, it should be around 94 dB, which, in newly revised, should only need a 2-4W amp to reach the desired levels. I did notice that listening at loud levels, I did get a slight headache, and it was fatiguing. Hence my next move to change the amp.

Plugging in the Zen did not alter the sound dramatically. However, having only listened to these in the span of a few days, I imagine it'd be difficult to find all the differences. However there were two important differences that I noticed with the tubes. One is that it was not fatiguing at all at any levels. Second, the sound was more full, whereas the Chipamp was more sterile. So in this short sessions, I have decided that Chipamp (and solid state) was not the way for me moving forward.  Once I settled on this, I spent probably 6 to 8 hours in total listening to this setup. And it was really a delight.

Overall, the sonic presentation was natural, easy and spacious. The bass was not loud, but was present and without authority. This is one area that is becoming more and more important. That I need to really find a good speaker to supplement to provide the missing bass presence. With the Zen, two observations was made. The first, is that almost at full volume, the levels were around the very low 90 dB, maybe hitting 92 dB. This was probably as loud as I would want to listen to for a period of time (more than peaks or more than a track). Secondly, I found that classical music, which to to date, has been a little disappointing in my other setups, was very pleasurable. It didn't seem constrained and didn't feel like it was running out of steam.

So, the little effort that I made this trip provide to be very valuable. The following items have been confirmed.

  1. Peak levels would be in the low 90 dB range, and thus total system response should be at least 96-98 dB at the listening position.
  2. I prefer bi-poles to dipole. I didn't get to chance to wire it as a monopole - though I was thinking about, but I was just enjoying the sound so darn much.
  3. I prefer the Zen to the Chipamp. And thus, I could make the extrapolation that I prefer tubes to solid state sound, or Class A to Class D. 

This has been both important and enjoyable, and now I have a few months to prepare my plan of attack before I am back at home permanently.