Revised Assumption

Having read and reviewed more about my desired listening levels, I have come to the conclusion that 100 dB with 6 dB of overhead is probably a little too much. I believe that a value of 90 dB at the listening position is more sensible. Therefore, my assumption would change from a required combined speaker and amp able to deliver around 96 dB at 1 W at 1 m. Given a small buffer, the target range should be 95 to 98 dB. Of course it would be nice to have additional headroom of say 6 to 9 dB, but given my basic premise, I believe that the revised SPL levels are more realistic.

Verification of thought process

Since I will have a short trip home in the coming weeks, it is my plan to verify my second assumption, that is the 104 dB levels. My goal is to take out my pair of closed FE127e towers (yes, those with the front dust cap crumpled) and play them using my Decware Zen and then LM3886 Chipamp in the 100 cubic meter room. To hopefully negate the effects of the crumpled dust caps (yes, they are both now crumpled…) I will rotate them around and use the 'back speakers'. Wonderful thing about dipole and bipoles! The towers are made from 19mm thick Tasmanian Oak and have light filling. But the top and bottom are loose, leaving plenty of opportunity for tweaking.

The drivers are all terminated separately so that I can experiment with dipole, dipole and the parallel and series. I will listen to my favourite music, and see what levels are acceptable. If, and I stress if, they are suitable with the Zen, then I would see so many options opening up in both speaker and amp combos. If they are no loud enough with the LM3886, then I really need bigger amps and/or higher sensitivity speakers.

Through this, I will also experiment (hopefully) with the monopole, dipole and bipole arrangement. Which should give some further guidance as to my preferred arrangement. The source will be my QLS350 straight into the amps. I might try bringing in my HT sub, to see the effects of low end reinforcement. I will consider this my most important experiment. Speaker configuration, low-end reinforcement and power level combinations.

Note - to be honest, I am keen to see if I can get these to work well with the Zen, and then I can upgrade them to the F120A or even the new Jordan full rangers!

Re-direction Part 1

After much pondering about my audio system, especially on my imminent return home, I have been through many of the same processes as I have done in the years prior to my departure from home. At the end of the day, it always came back to the same things as I have covered in my previous blog entries. Namely, back and forth between an all tube amp system or a tube plus SS system (for more power) and that of the speaker system. Which has oscillate between a high efficient system using horns, or a lesser (but still quite) efficient system using a traditional full range speaker driver.

Thus, I have come to a different process of deciding. A little background about my gear generally. In all my interests and hobbies, I plan and want to just have one - what is perceived by me as the ultimate, and then that is it. Not that it has to be the most expensive or best, but something in my mind that is no compromise. That is case for my cameras, computers, watches, cars, bass guitar, etc.. I have found that going around from low end to high end, ends up costing more money and time. But in the audio field, as I started with very little knowledge, and wanted the DIY route, it was natural that I take a longer journey.

In terms of headphones, I have I have almost reached that point. With my (soon to be built) C3g amp with HD650. Even with my existing range of built amps (about 6 in total) and headphones (over 4 high-quality pairs), I am more than a happy camper.

With speakers however, it has been a (very) long journey. Mainly due to moving houses, moving country and having children. But right now, I am in a better spot to move towards that end goal. So, instead of choosing an amp and speaker, I am going backwards, and choosing what I want in terms of sound, and then working back the audio chain. That is, desired sound, sound levels, speaker, amp and then source.

What I want is a system that is able to reproduce audio between 20 Hz to 40 kHz with high fidelity and ability to engage me beyond what can be 'objectively measured' - that is, it gives me enjoyment. What would I mean by high fidelity, I would assume low distortion and the frequency response to be generally smooth to within 3 to 6 dB in the frequency range, with a drop of say -6dB at 20Hz being acceptable.

Being honest, my listening room will never ever be larger than 7.0m by 5.3m by 2.7m high, so about 100 cubic meters. But at the moment, the listening room is 4.7m by 4.0m by 2.7m, about half the volume at 50 cubic meters. The system should be built for those two rooms.

The next question for me was how loud? I have been looking around the Internet, and measuring the values that I typically listen to. It seems that a value (for peak levels) of around 103 to 109 dB was about right, with most program levels being around 80 dB to 90 dB at the loudest normal levels.

In the larger room scenario, the listening position would be about 4m from the speaker, and the smaller room, it'd be 3m from the speaker. Which is not all that different. From this information, I would then determine the appropriate levels of speaker and amp performance to meet those dB levels. A listening position of 4m is about a 6 dB loss and at 3m, it'd be about 5 dB. In either case, the differences are marginal, so let's assume 6 dB loss due to speaker distance.

At this point, I will ignore room gain and other incidental losses, as they compound the complexity. For peak levels, let's assume for simplicity sake 104 dB. Therefore, the entire system of amp and speaker should be able to hit 110 dB. Therefore the first assumptions that are quantifiable are:

1. Frequency Response range of 20 Hz to 40 kHz. -6dB at 20 Hz and within 3 to 6 dB over the range (that is plus or minus 1.5 to 3.0 dB).
2. Combined maximum SPL levels of system should be 110 dB at 1 W at 1 m.

With the above in mind, we can now start to tinker with combinations of speakers and amps that meet the above requirements. The first for me is the philosophical combination of the two devices. So, if the speakers were to be 101 dB efficient, the amp would need to be at least 8 W in power. If the speakers were 92 dB, the amp would need to be at least 64 W in power. So, as the speaker efficient increases, the amp options open up for low powered tube amps. And as the speakers become less efficient, the ability to use tube amps rapidly decrease. The table below summarises the combinations.

Speaker Sensitivity // Amp Sensitivity
110 dB // 1 W
107 dB // 2 W (45 Amp)
104 dB // 4 W (2A3, AD1 Amp)
101 dB // 8 W (300B, KT88 SE, F2a Amp)
98 dB // 16 W (6C33, 211, 845, GM70)
95 dB // 32 W (211, 845, GM70)
92 dB // 64 W
89 dB // 128 W

Given my general interest in using Class A Tube amps, where generally SE tube amps limited to below 16 W or so, and PP tube amps around 32 W for Class A, it would appear that unless the speakers were to be at least 95 dB efficient, the use of tube amps would be not possible. Even with the rough limits mentioned above, the voltage and/or current requirements are quite large, limiting the ease of DIY. However, the choice in the number of amplifiers used has not yet been addressed.

Therefore, if I want to use tube amps (and I do) then I would have to find highly efficient speakers. This would generally rule out most of the speakers available. Focusing only on the mid-range (or pseudo full range) would limit the choice to high efficiency speakers such as 'traditional' full range family (e.g., Lowthers, Fostex, Supravox, etc.), horn loaded compression drivers, and other very efficient mid-range (generally) PA speakers.

Well, a longish entry, Part 2 of this will cover the issues dealing with speaker selection.

Relaxed session

Well, I had the first session in a long time with my EM4 in a somewhat high quality arrangement. I was using the O2 headphone amp and the D-zero DAC from my Macbook, using Fidelia as the source. Short commentary.

Source does matter. Some files were better, and some were worse. But I must say that hearing Bach's Cello suites through this setup was very intimate. In the stillness of the afternoon, and using IEM, I really felt like I was in the room with Janos as he plays the cello suites. On another thought, 1812 Overture was just flat, even on 176.2 kHz source file. I think that complex music just doesn't play well with the setup. Not sure why, but it was just not as inspiring as other kinds of music.

But, it was a refreshing and enjoyable session of one and a half hours.

EL84 PP Idea

Well, I was on the train home and was thinking about the Beyma. I was considering have it passive crossover. Using copper foil inductors and Jantzen caps, it turned out to be around US$350. I was then thinking what if I just built two simple EL84 PP amps instead, and then I can experiment with a cheapie PA active crossover. This was, I can tune them and then once the sweet spot found, I can do a hardwired active crossover. Would that be cheaper?

Anyway, more incoherent thoughts later, see below. 2.6W in SE mode, and thus is Class A PP, it'd be 5.2W. 2nd harmonic should cancel out (mostly) and the 3rd harmonic should sum, but the predicted 3rd harmonic is pretty low. The PSU below using a choke input. It could be a simple, 'cheaper' build. But it could also be quite good. The output transformer is 8k in SE operation.

OB Design

Here is my idea for my convertible open baffle. The overall size is 1,100mm high and either 550 or 600mm in width. It will have a 90mm hardwood return frame that may have bracing and sound damping applied. The back will allow the addition of a boot or shroud that can make it a U frame, closed or ported box. The main issues will be how to make it rigid enough for the closed and ported box.

The lower driver will be an Eminence Alpha 15A. Simply because I have a pair. They could be changed in the future, but for the moment, that at it. The main speaker panels will be removable to allow me to change different drivers. I am not sure whether to have a separate panel for a super tweeter,or to have that sitting on the top.

But if this does work, I will have a very modifiable and tenable OB speaker that will fit my Fostex F207e, Lowther DX3, Sabe Green cones and potentially also Visaton B200, Supravox 6" or 8" Alnico, PHY or anything else I can get my hands on.

This, my Beyma Coax ported box and the Jordan TL will form my main speakers.

Oarta output transformer

It looks like the Lundahl LL1623 rated at 90mA would be the best choice for this amp. The amorphous core price is less than half of the Tamura, but more importantly, it is available. The interstage transformer would be the next choice. This would probably be the most important choice, given the different voltage swings that needs to be accommodated by the different tubes.

I think at this point, it would be important to at least try to limit the range of tubes to be used. This has to be correlated with my current stash of tubes. In this, I would say that there are four families that I would like to run:

1. 45, 145, 245 and 345. I have a number of pairs and have been universally hailed as an excellent audio tube.
2. F2a. I have three pairs of these long life tubes.
3. KT88/EL34 family. I have two quartets of the KT88 and EL 34.
4. 300B. I have one pair. 

Looking at the above, it would make most sense that I provide more effort towards the first three families given the volume of tubes on hand. With the 300B narrowed out, the voltage swings required are now limited to 30 Vpp to 60 Vpp, a little more sensible. This would mean that a tube with a gain of 20 would be plenty. At this point the 5687 and C3g seems the right direction. If I use a higher gain tube, I can use a step down interstage, which should be a good think overall. The other option, is to allow a different tap on the interstage to be used to connect to the grid of the output tube, which can select between say 1:1 and 2:1. But this may affect the performance and bandwidth of the interstage.

More questions... 

Oarta Output Transformer

It seems that the Tamura F5003 is in limited supply, and may not be available. So I may have to revert to another brand of amorphous core. Maybe Lundahl?

Revised Beyma Box

I have been playing around with the box model for the Beyma co-ax, and since then, I have made the following changes. 

1. 300 l vented box - a nominally 1.2 H x 0.55 W x 0.6 D box.
2. Vent tuned at 30 Hz.
3. HP at 20 Hz, 2nd order.
4. LP at 700 Hz, 4th order.
5. No Linkwitz transformer.
6. Source voltage at 14 Vrms, which is around 25 W amp.
7. Disabled baffle step correction.

At this condition, I was able to get the following predicted response. Note that the driver excursion is limited to below 4mm. At 25W power, it is giving around 100 dB, with f-3 at around 27 Hz. Fairly decent result, with less tinkering with EQs. Bigger box, but that may have to be the sacrifice. 

Beyma Ported Cab

To design a simple box for my Beyma 15XA38Nd, I have employed Basta. Using a 200l ported box with two 75mm diameter 147mm length ports (tuned to 25 Hz). The box is also fairly standard at around 1m in height, 0.5m in width and 0.4m in depth. Something easy to construct.

My speaker box model also includes a Linkwitz Transform at 20 Hz along with a 4th order low pass at 1,800 Hz in tandem with a 2nd order high pass at 15 Hz produces a near flat frequency as model between 30 Hz to 700 Hz. With -3dB points around 20 Hz and 1 kHz. See below.

Crossover at 1.8 kHz and maximised bass response

There is quite a lot of filtering in there to achieve this. The 1.8 kHz crossover was set from the standard Beyma passive crossover (2nd order) tuned to that frequency. I think that 1.8 kHz seems a bit high for a 15" driver, so if I set the model to 1 kHz instead, it gives a much more 'flat' predicted response. Note also that the Xmax of this driver is 4.0mm and according to the model we hit 3mm at around 20 Hz with 1W of input. I may have to tune the high pass from 15 Hz to something higher to limit the response. The below shows using a 4th order HP at 20 Hz, which limits the Xmax to under 2mm at 20 Hz. 

Crossover at 1 kHz and slightly reduce bass response to limit Xmax.

Given my listening room is only 5x4m in size, I think that a flat response down to 30 Hz may well be plenty enough. My question now is how to experiment with the various cross-over points? The easiest solution would be to employ a PA style crossover, something cheap from Behringer or similar. Which would allow me to test and tweak to my heart's content. 

Another option would be to make the box closed, and with the same settings as the second scenario above. The response is still pretty decent. 

Closed box with same parameters as above.

To be honest, if the speaker performs anywhere near what the model shows, it'd be great. Even if I were to use a closed box or vented with the standard passive 2nd order filter, the response is still pretty decent. See the two simulations below.

Passive crossover ported box, no EQ.

Passive crossover closed box, no EQ.