Choosing loudspeakers for single-ended tube amplifiers: sensitivity, impedance and crossovers

A single-ended triode amplifier typically gives from a watt or two up to about 25 watts, runs out of them gently, and has an output stage that reacts to the loudspeaker more than most. Choosing the loudspeaker is therefore half of choosing the amplifier. The power of each output triode, and the decibel arithmetic, are covered in our guide to power triodes for single-ended amplifiers.

What makes a loudspeaker easy for a SET amplifier?

Four properties matter, roughly in this order:

• High sensitivity, so that a few watts produce realistic levels in your room.
• A high, flat impedance curve, so that the load does not reshape the amplifier’s frequency response.
• A simple crossover, or none, so that little power is lost before it reaches the drivers.
• A size that suits the room, so that the bass the loudspeaker produces is bass the amplifier can control.

Sensitivity: the first number to check

Sensitivity is the sound level a loudspeaker produces at 1 m from 1 W of input. For a single-ended amplifier it is the figure that decides most of the shortlist. Two cautions when reading specifications:

• 1 W or 2.83 V? Many makers quote sensitivity at 2.83 V rather than 1 W. Into 8 Ω the two are the same, but into 4 Ω 2.83 V is 2 W, so a 4 Ω loudspeaker’s figure reads 3 dB higher than its true 1 W sensitivity.
• Measurement conditions. Half-space or narrow-band figures can flatter a loudspeaker by a few decibels; treat one number as a starting point, and listen.

How many watts do I need, and is 2 W enough?

It depends on sensitivity, room size and listening level. As a rule of thumb, 98 dB and above suits a 2 W amplifier in a medium room, 93 dB and above a 300B amplifier, and loudspeakers from about 88 dB need the 15–25 W of the large transmitting triodes. The calculation is in our power triode guide.

Why the impedance curve matters as much as sensitivity

A loudspeaker’s impedance is not a single value: it peaks at the bass resonance, dips around the crossover and often rises in the treble. The “nominal” figure on the specification sheet is only a summary.

Most transistor amplifiers have an output impedance well below 1 Ω, so they deliver the same voltage whatever the load does. A single-ended triode amplifier without feedback typically has about one to a few ohms, and with the loudspeaker this forms a voltage divider: the amplifier’s frequency response follows the loudspeaker’s impedance curve. With an output impedance of 2 Ω, a loudspeaker that swings between 4 Ω and 20 Ω receives about 2.7 dB more level where its impedance is high than where it is low. A loudspeaker whose impedance stays at 8 Ω or above and changes little keeps its intended tonal balance.

The same effect shows in the bass. A low output impedance damps the woofer’s resonance; a higher one lets it ring a little more, so a bass-reflex box, whose impedance has two peaks around the port tuning, can sound fuller and looser on a SET amplifier. Sealed, aperiodic and open-baffle designs, and reflex designs voiced with tube amplifiers in mind, generally behave more predictably.

How to read an impedance chart

• Look at the lowest point and at how far the curve swings, not at the nominal value. A minimum well above 4 Ω and a small ratio between peaks and dips make an easy load; dips towards 3 Ω or below do not.
• Choose the output transformer tap that is close to the loudspeaker’s impedance across the midrange. Our amplifiers have an 8 Ω output as standard, with 4 Ω or 16 Ω windings to order.

Crossovers: simpler is easier

Every component in a passive crossover sits between the amplifier and the drivers. Steep filters need more parts, absorb more power and create deeper impedance dips. For a single-ended amplifier, fewer parts make an easier load: a first- or second-order filter, a crossover point where neither driver is strained, and drivers of similar sensitivity. A single full-range driver removes the crossover altogether.

Full-range drivers, horns and open baffles

Full-range drivers

A single driver covering the whole range is the classic partner for a SET amplifier: high sensitivity, no crossover, and a midrange from one point in space that owners commonly describe as coherent and immediate. The trade-offs are limited deep bass, since full-range loudspeakers rarely reach below 40 Hz on their own, and narrower dispersion in the top octave; a supertweeter or a subwoofer can fill either end. Perseus MkII uses a Supravox 215-EXC field-coil driver, described in our guide to field-coil loudspeakers.

Horns

A horn works as an acoustic transformer between a small diaphragm and the air, raising sensitivity and controlling dispersion; horn-loaded systems commonly exceed 100 dB. Owners describe the result as dynamic and lively; a poorly designed horn can add colouration, and bass horns are very large. A common compromise is a horn tweeter above a direct-radiating woofer, as in Cassandra MkII, where the SICA horn tweeter is time-aligned with the BEAG woofer.

Open baffle

An open baffle has no box, so there are no cabinet resonances; its sound is commonly described as open and free of box colouration. Below a frequency set by the baffle width the front and rear waves cancel, so an open baffle needs a wide panel, help from the room or a separate bass unit, and distance from the wall behind it. Orpheus and Perseus can both be built as open baffles; the enclosure type is chosen for your room before the build starts.

Room size and listening distance

The same loudspeaker needs more power in a larger room, because you sit further away and the sound fills more volume. In a small room, or in the near field at 2 m or less, a 2 W amplifier on a 98 dB loudspeaker has plenty in reserve; in a room of 40 m² or more, look for higher sensitivity or a larger triode. Room size also shapes the bass: a large woofer in a small room can boom, especially with the modest damping of a SET amplifier, while a small full-range loudspeaker in a large room can sound thin without a subwoofer. Placement changes the bass more than most component swaps; our article on getting the most from your loudspeakers covers it.

A worked example: the level at your seat

Sensitivity, amplifier power and listening distance give a first estimate of how loud a pair can play. The level of the direct sound at the seat, leaving out the room’s reflections, is about

SPL ≈ sensitivity + 10·log10(P) − 20·log10(r)

where sensitivity is in dB at 1 W and 1 m, P is the amplifier’s rated power in watts and r is the distance in metres. Each doubling of power adds 3 dB; each doubling of distance takes 6 dB away. The table gives the result at full power at 3 m, in dB.

At 2 m add about 3.5 dB; at 4 m take away about 2.5 dB.

Four pairs from our catalogue, at full power and 3 m: Orpheus (103 dB) with Solaria (2 W), about 96 dB; Orpheus with Atlas (5 W), about 100 dB; Cassandra MkII (95 dB) with Atlas, about 92 dB; Cassandra MkII with Helios MkII (15 W), about 97 dB.

Read these numbers as an estimate, not a promise of loudness. The formula counts only the direct sound, as if there were no walls: in a real room, reflections usually add a little level, while the impedance curve, the amplifier’s behaviour as it nears full power and the loudspeaker’s own limits are not in it. Music also has peaks well above its average level, so the average level you listen at sits below the figure in the table. Use the table to compare pairs on the same footing; the final choice is made together with us in a consultation, from your room, your seat and the music you play.

Which loudspeakers make life hard for a SET amplifier?

• Low sensitivity. Many compact sealed loudspeakers and modern multi-way designs sit between about 84 and 88 dB, where even a 25 W single-ended amplifier has little headroom.
• Impedance dips. A nominal 4 Ω loudspeaker that falls to 2–3 Ω in the bass or lower midrange asks for current a SET amplifier cannot supply, and its tonal balance shifts with the curve.
• Complex crossovers. Steep fourth-order filters, many drivers and compensation networks absorb power and multiply the dips.
• Electrostatic loudspeakers. A largely capacitive load whose impedance falls as frequency rises: our two electrostatic models are rated at 2–4 Ω, falling to about 1 Ω at 20 kHz, with a sensitivity of 85–86 dB.

Can a SET amplifier drive electrostatic loudspeakers?

Usually not well: into a load that falls towards 1 Ω, an output impedance of a few ohms rolls off the treble and the amplifier runs short of current. That is why both models are also offered as active systems with four dedicated amplifier modules; our guide to electrostatic loudspeakers explains the load.

Matching examples from the Demograf Audio catalogue

Orpheus has a high sensitivity: 103 dB at 8 Ω, with a three-way crossover. Its specification says single-ended amplifiers of 3–10 W drive it fully, which points to Atlas and Tantal; the 2 W Solaria and Eurybia suit it only in small rooms at moderate levels. Cassandra MkII is gentle for a different reason: its 10–14 Ω impedance is high and varies little, so it works from 3 W. Perseus MkII ranges from 93 to 102 dB depending on the configuration, so the amplifier is chosen together with the build. For a large room, ask us about the three-way Perseus with a 10–18 inch woofer, driven by Endymion, Ether or Tantal Senior. The active Zeus subwoofer adds the lowest octaves with its own Class A or Class D amplifier, so they do not have to come from the triode.

Which loudspeakers suit a 300B amplifier?

Atlas, our 300B single-ended amplifier, gives 5 W. In a medium room, look for 93 dB or more with an impedance that stays near or above 8 Ω: Cassandra MkII at 95 dB and 10–14 Ω, Perseus MkII, or Orpheus for more headroom still.

Which loudspeakers can an 845 amplifier drive?

A single-ended 845 amplifier gives about 15–22 W; Endymion gives 20 W with 845 tubes, 15 W with 211 and 25 W with 805. That opens the choice to loudspeakers from about 88 dB in a medium room, or, in a large room, to sensitive loudspeakers; ask us whether the three-way Perseus suits yours. The output impedance argument still applies: an 845 amplifier prefers a high, even impedance curve and is not a match for low-impedance electrostatic panels.

Which amplifier for high-sensitivity loudspeakers?

Low-power single-ended amplifiers are the natural choice, though not the only one: Perseus MkII also suits transistor amplifiers. At 100 dB and above, noise matters as much as power: hum or hiss inaudible on an 88 dB loudspeaker can be heard from the seat on a 103 dB one, so listen to an amplifier at idle with your ear near the loudspeaker. Our tube amplifiers, apart from Tantal Senior, have a ground-lift switch for ground loops.

Which Demograf Audio piece fits

For Orpheus, start with Atlas or the 3 W Tantal; the 2 W Solaria and Eurybia suit it in small rooms at moderate levels. With Atlas or Tantal, also look at Cassandra MkII and Perseus MkII. For more headroom, pair Perseus or Cassandra with Helios MkII; for a large room, ask us about the three-way Perseus with Endymion, Ether or Tantal Senior. Add Zeus for the lowest octaves. If you are drawn to electrostatic panels, choose Menelaus or Atreus in their active versions rather than a SET amplifier.

Every Demograf Audio loudspeaker and amplifier is made to order by Dmitriy Gubchenko. Tell us your room and what you already own, and we will suggest the other half of the match.