Sunday, 7 July 2013

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.

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