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Loudspeaker Matching With Single-Ended Amplifiers
Article By Graham Tricker Of GT Audio
From Sound Practices Issue 9, Fall 1995

  I normally consider loudspeakers to fall into two categories. These are horn and direct radiator designs. From a commercial point of view 8 Watts is very limiting unless you have a market consisting of efficient loudspeaker designs like horns. I have a collection of vintage hi-fi which includes a matching pair of Voigt comer horns. In my opinion these are one of the finest sounding classic horns ever to have been made. On the whole, however, people using horns are few and far between in the UK and Europe. I guess this is due to the imposing physical dimensions needed to achieve realistic results.

Eight Watts does not sound much to the average audiophile, especially when most familiar well-reviewed valve amplifiers had power outputs greater than 60 Watts. Such high powered amplifiers are required to drive inefficient and power hungry loudspeakers like the panel, electrostatic and ribbon designs that have been popular these past decades. Surprisingly, eight Watts can be quite adequate with conventional loudspeakers providing the listener does not want to listen at loud levels; the chosen music is of the simple acoustic variety and not a full symphony or even worse rock music; the listening room is not too large; and lastly, the 8 Watt amplifier truly measures 8 Watts. Many so-called 8 Watt 300B designs which run in self-bias configuration in fact only deliver 5.5 to 6.5 Watts of power on the test bench.

Generally speaking, to meet all listening requirements with conventional loudspeakers, 12 to 25 Watts is a minimum requirement. To achieve this output level, a single-ended design has to connect more than one valve in parallel, i.e. parallel single-ended or (my preferred option) use a single high voltage triode like the 211 or 845. In order to permit wide application, our GT Audio "TRON" range of valve amplifiers focuses on single ended Class A designs in the 12 to 25 Watt range.

In choosing a loudspeaker to match an eight Watt amplifier there are two things to consider (this also holds true for matching all loudspeakers to amplifiers):

Rule # I is that there is no such thing as a perfect loudspeaker. Every design is a series of compromises.

Rule #2 is that one has to accept that there will be compromises and decide on a design which reduces these compromises the most for the size of loudspeaker and the type of music you will be listening to.

Horn Designs (Vintage and Current)
 The principles of a horn loudspeaker (i.e. high efficiency) dictate that it is most suitable for low-powered single-ended designs. There are a number of different interpretations of horn designs, like the famous Tractrix Horn from the legendary Paul Voigt and designs like Tannoy, Lowther and Klipschorn, etc. All of these loudspeakers have a broadly similar way of conveying a musical performance. Generally the sound is very realistic, very fast with excellent dynamics and it is often difficult to believe that a real instrument or voice rather than a recorded one is not actually in the room.

Horns were originally designed in the days of mono where a speaker had to throw as much sound as possible into a room or hall from a limited power output. This meant that they were generally placed in the comer of the room (hence the name "comer horn") and used the walls and floor as an extension of the horn. When two horn loudspeakers are connected in stereo a big wide sound stage is achieved. However, horns are very room dependent and poor specimens can sound nasal, colored and offer very little depth to the sound stage as well as producing large unrealistic images of voices and instruments. Some of the early Lowther and Tannoy designs can sound quite forward in the midrange often causing fatigue during listening sessions.

The best modem horn design which I have heard is the Great British Horn made by Nottingham Analogue. Based on the Klipschorn it uses very expensive drive units-see enclosed picture behind one of our TRON 300B prototypes.

Direct Radiator Designs
There exists a large variety of designs, e.g. bass reflex, sealed box, open baffle, etc. These designs generally give better bass extension, greater depth to the sound stage and better image placement but lack the dynamics and presence of horn loudspeakers. Loudspeakers that we have found work particularly well and can be used with low powered amps are the Snell designs from America, Audio Note designs from the UK, the Cadence ES from India and the Homing and Posselt Albatross (se above) from Denmark. The Cadence ES is an unusual design as it uses a bass reflex cabinet with an electrostatic mid range and treble unit mounted on top of the cabinet. Surprisingly this hybrid has an efficiency of 91dB/w/m and nominal 8 ohm impedance.

The Posselt Albatross designed by Jens Posselt from Denmark took seven years to develop. They are a 91dB six foot high floorstanding design, giving excellent focus and imaging. The size is very important as it determines the correct position of voices and instruments as if the performance was being performed in front of you. We chose the Albatross as a reference for developing our forthcoming range of amplifiers. They are probably the least compromised loudspeaker we have ever heard.

Questions:

1.What is the sensitivity of the speaker?

2. What is the nominal impedance of the speaker and does it have a wild impedance curve?

3. What type of music do you listen to, e.g. chamber, symphony, folk or rock?

4. How loud do you listen to music?

5. How big is your listening room?

From these questions one can establish in a few seconds whether an 8 Watt single ended amplifier will be suitable. Here is a useful chart (above) that can aid in the decision. Tick each question next to the appropriate column (either A or B) that suits your criteria. When complete count the ticks for each column. If there are more ticks in column A then you will need more power than 8 Watts. If on the other hand you have more ticks in column B then an 8 Watt amplifier may be suitable. In determining whether a horn loudspeaker will fit the bill or whether a conventional loudspeaker is more suitable to your requirements, physical size may well be the deciding factor.

However, the question should be asked what is the listening criteria? i.e. is it live in the room stuff or a much more relaxed sound with pinpoint imagery. If the latter criteria is more important then a reasonably efficient direct radiating design will give better results than a horn design. My own personal preference for SE amps is the high voltage triode design like the 211 or 845 as these drive conventional loudspeakers and horns very well. Eight Watts in certain domestic situations simply does not give enough power unless horns are used. Eight Watts can drive a conventional loudspeaker on simple music but a full symphony orchestra on full chat will run the amp dry.

GT Audio
5 Upper Road
Higher Denham, Bucks
UB9 5EJ   UK