KISS 101B
KISS 101B
Point source speakers
by Andre Jute
In KISS 101 I explained that a sensible audiophile chooses his speakers first, his sources next, and only then the amplifier to link them.
I expressed my preference for point source speakers. Beyond that, I expressed an ideal for sensitive speakers to reduce the costs (money, space, heat) of desirable tube amplification.
It is worth stressing that you really should not choose your amplifier first because you have heard single-ended is the choice of insiders this week, or that this tube or that is the preferred pleasure of the elite. If you make this novice audiophile mistake, you may have to choose speakers that you will not like or which for your tastes are somehow compromised. That way lies silly self-delusions to persuade yourself the money or other sacrifice was worth it, and eventual disappointment.
You will hear that speaker, not the amplifier. You will have to live with that speaker a long time if it is one of the more expensive choices below, or indeed of another kind altogether. Choose carefully.
Lowther Fidelio
The sensitive point source speaker I chose is a Lowther Fidelio bicor horn which has an acceptable footprint. My version is modified but starts with commercially available sawn wood, which is a boon for the amateur constructor who can build relatively cheaply a perfect copy of the same speaker the Lowther company sells for around three times as much as the DIY cost. It is to the Lowther company's credit that they make it so easy for DIYers by selling plans, permitting their dealers to sell sawn wood, and offering trade-ins of old drivers for new at reduced prices.
The 300B amplifier being designed in The KISS Amp is intended for this Lowther driven horn, my modified KISS 194 T91HWAF3.jpg. T91 is my audio design number, HWAF stands for High Wife Acceptance Factor to refer to the relatively modest size of the horn, and 3 is simply the series number to indicate that this one is based on two earlier developments in the series. If this is to be your first horn, build it as Lowther designed it, with presawn wood bought from a Lowther dealer like Audio Technik in Germany. Don't ask me for either the standard Lowther plans or the CAD files of my developments. Both are proprietary. You can download one page plans from the Lowther site and they will give you all their blueprints on CD free of charge when you buy drivers from them.
If you worry that the 3.8W 300B amp we are designing in The KISS Amp will not drive the horn properly, take a look at KISS 192 T68MZ417A.jpg which is the amp presently driving the HWAF horns to levels that can be clearly heard four floors down at my front door. The T68bis Minus Zero amplifier produces, flat out, a glorious one third of a watt.
The Impresario
I also promised to publish a design for a sensitive point source loudspeaker that can be built for a couple of hundred dollars.
KISS 195 The Impresario.jpg shows a plan for a sensitive point source loudspeaker to be built with a guitar speaker, the Eminence Beta 12LT. It can be driven by any 2W amp, preferably SE tube; my inexpensive SEntry single-ended EL34 triode-coupled amp was designed for this speaker. The loudspeaker is named Impresario because I first designed it for a music promoter. It is popular with music professionals without any audiophile pretentions who just want a speaker that sounds like the music in their favourite venue. Many use it without the tweeter. I suggest you try it with the tweeter first, especially if you are under 35 when one might reasonably expect your ears to be good to higher frequency levels than later in life.
Once more, I suggest you get the wood sawn, this time where you buy it.
Since this KISS series is about designing and building a 300B amplifier, it is not the place to go into great detail on speakers. If you want a variant design to my Impresario, Thorsten Loesch, a reliable DIYer, has published an article on the net on building with the Eminence driver. He also published a good deal of information and interesting speculation.
There is also an upmarket version of the Eminence driver available as a kit from Hammer Dynamics. It includes a custom version of the driver, other hardware and a plan but not the wood.
Other point source speakers
If the Lowther Fidelio frightens you by its cost or the complexity of its tapered construction from critical throat dimensions, you may want to consider Fostex horn drivers. The drivers are cheaper, so you can substitute a Fostex 8in driver in the Fidelio cabinet though the results will not be optimum. (A horn enclosure is designed for a specific driver, not just a generic size of driver. A horn is generally a far less forgiving design exercise than any other kind of speaker.) Fostex also publish stepped plans for their drivers, where all the joints are perpendicular and thus easier to glue up right. Fostex have horn drivers in a range of sizes and prices, starting very reasonably and giving good value for money throughout the range, whereas Lowther offer only two sizes and two prices (very expensive and outrageous) but do give superb long-term value and service.
If even a stepped horn is altogether beyond your woodworking skills and The Impresario just too big for your room (both the other contenders with the Eminence driver, mentioned above, have a similar footprint) but you must absolutely have a point source speaker, the solution is a Voigt Pipe which even with an 8in driver can have a footprint under 11in square and is easily disguised as sculpture in your living room. A Voigt Pipe can be built with a wide range of drivers but is usually built with inexpensive drivers because it is so suitable for newcomers to sensitive point source speakers. You will find a plethora of information on the net.
As a novelty, you may want to copy a point source speaker I built as a gimmick and liked so much that I used it on and off for months. This consisted of a cheap ali-coned 4in wide range driver good for about 115Hz-12+kHz fitted with brown parcel tape (it has to be brown or the sound will be too bright!) into the end of a cardboard tube. Suitable cardboard tube, used for rolling carpet around, is available free of charge at furniture stores. I tuned the assembly for bass boost by ear, simply sawing off bits of tube until it sounded right. I had four of these cheap drivers and made two sets of tubes. The longer ones with the enormous bass boost sounded fabulous on Gregorian Chant. Hi-fi? Never! Cheap? Definitely! Convincing? The ambience of the crypt at St Domingo de Silos from those six bucks each Christmas Pipes married me to point source speakers for the rest of my life. It is a cheap experiment which, carried forward with more sober restraint than I brought to it, may help you decide if you are really carried away by point source speakers.
There are further types of point source speakers but they are all more complicated or bigger or more expensive than proper horns. Here are two for the clever and the wealthy:
A point source may be mimicked by a Bessel Array of several loudspeakers but several amplifiers and/or complicated preamps are required and thus the KISS principle is broken. A Bessel Array is primarily useful for larger rooms or open spaces but it works well with cheap drivers because they are used at low power in their most linear region.
Electrostatic loudspeakers can be designed as point sources, as in the famous Quad ESL, but they are not a DIY option, the bipolar ones have the same space requirements as horns, the least expensive good ones (Quad) are more expensive than factory built Lowther Fidelio, and you want a really good clean amp capable of over 20W to match their splendid sound, which tends to load the expense up even more if you insist on an SE amp. (You should not, not with Quad ESL, but people do. A Class A PP amp is a better match. But that is another story I will tell another time.)
Back to our KISS 300B amp for sensitive point source speakers.
THE VOLTAGES IN THIS AMP WILL KILL YOU.
GET EXPERIENCED SUPERVISION IF IT IS YOUR FIRST TUBE AMP
and may not be reproduced except in the thread KISS xxx on rec.audio.tubes
