A tube amplifier for classical music: watts, loudspeakers and the orchestra

Classical music is the hardest test of a system’s honesty. A string quartet asks for the grain of gut strings and the air of a small hall; a Mahler symphony asks for an orchestra that can rise from a single held note to a full tutti in seconds; a piano recital asks for both at once. A tube amplifier can serve this music very well, but only when its power, its output stage and the loudspeakers it drives are chosen together. This guide explains what classical music asks of an amplifier, how to work out how many watts you need, when a 2 W single-ended amplifier is enough, and how single-ended and push-pull designs suit orchestral, chamber, vocal and piano music.

What classical music asks of an amplifier

Dynamic range and orchestral peaks

Classical recordings are usually mastered with much less compression than pop recordings, so the gap between the average level and the loudest moments is large. Peaks of 15–20 dB or more above the average level are common in orchestral and piano recordings. Every 10 dB of extra level needs ten times the amplifier power. An amplifier that sounds relaxed during a quiet movement can therefore run out of power on the first fortissimo, and the result is heard as compression, a hardening of the strings and a crowded climax.

Timbre

Much of the pleasure of classical music lies in timbre: the difference between a violin and a viola, the reed of an oboe, the wood of a cello, the overtones of a piano’s bass strings. An amplifier that keeps these colours apart lets you follow the inner voices of an ensemble. This is where single-ended triode (SET) amplifiers are most often praised: owners commonly describe a dense, natural midrange and voices and acoustic instruments that sound like themselves.

Quiet passages

A great deal of classical music is quiet. In a pianissimo the amplifier is delivering a small fraction of a watt, and how it behaves in that first watt matters more than its maximum power. Hum and hiss also become audible in the pauses, especially with sensitive loudspeakers, so a low noise floor is as important for classical listening as power.

The hall in the recording

Most classical recordings are made in real acoustic spaces, and the hall is part of the music: the reverberation that follows a chord, the depth between the strings and the brass, the sense of the room around a soloist. These cues are low in level and easy to lose. A system that resolves them gives a stable image of the orchestra and the space it plays in.

How many watts do I need? Power and loudspeaker sensitivity

The number of watts you need depends on three things: the sensitivity of your loudspeakers, the size of your room and your listening distance, and how loud you listen. Sensitivity is the sound level a loudspeaker produces at 1 m from 1 W, and it is the starting point.

A simple calculation

The peak level one loudspeaker can reach at 1 m is its sensitivity plus ten times the base-10 logarithm of the amplifier power in watts. In practice:

• 2 W adds 3 dB to the sensitivity figure.
• 5 W adds 7 dB.
• 15 W adds about 12 dB.
• 25 W adds 14 dB.

Doubling the power gives only 3 dB more; ten times the power gives 10 dB. At your seat the level is lower than at 1 m. At about 3 m from a pair of loudspeakers, a rough allowance is 6–7 dB less than the figure for one loudspeaker at 1 m: the distance takes away about 9.5 dB and the second loudspeaker gives back about 3 dB. A lively room gives back a little more.

A worked example

Suppose you listen to an orchestra at an average of about 75 dB at your seat and want 20 dB of headroom for the peaks. You need clean peaks of about 95 dB at the seat, or about 101–102 dB at 1 m from each loudspeaker.

The table gives the peak level at 1 m from one loudspeaker. With 103 dB loudspeakers, 2 W reaches 106 dB and leaves several decibels in reserve. With 95 dB loudspeakers, 5 W just reaches the target and 15 W leaves about 5 dB to spare. With 88 dB loudspeakers, 2 W falls about 11 dB short, and even 25 W only just reaches the target. These figures are estimates: sensitivity is measured in different ways (some makers quote it at 2.83 V rather than 1 W), and rooms differ. They are still a reliable guide to which combinations will work.

Is 2 watts enough for speakers?

It is, with the right loudspeakers. A 2 W amplifier on 45 or 2A3 triodes, such as our Solaria or Eurybia, suits loudspeakers of about 98 dB and above in a small to medium room. On full orchestral music with less sensitive loudspeakers, 2 W is not enough, however good the amplifier. A single-ended amplifier’s distortion rises gradually as it nears its limit, so brief overloads are commonly heard as a thickening rather than a hard edge; but a sustained climax will still be compressed.

Why a 2–8 W SET amplifier needs high-sensitivity loudspeakers

The small triodes, the 45, the 2A3 and the 300B, give roughly 2–8 W in single-ended Class A. That power is enough only when the loudspeaker turns it into sound efficiently. Every 3 dB of sensitivity is worth a doubling of amplifier power: a 98 dB loudspeaker on 2 W plays as loud as a 95 dB loudspeaker on 4 W or an 89 dB loudspeaker on 16 W.

Sensitivity is not the whole story. A single-ended amplifier usually runs with little or no negative feedback and has a relatively high output impedance, so its response follows the loudspeaker’s impedance curve. A loudspeaker with a fairly even impedance of 8 Ω or more and a simple crossover suits it far better than one whose impedance dips low in the bass. Our guide to loudspeakers for single-ended amplifiers explains how to read these figures.

Single-ended or push-pull for classical music

Our guide to single-ended and push-pull amplifiers explains the circuits. For classical music, the choice follows the scale of the music you play most.

• Single-ended is commonly described as the more intimate and colourful presentation, with fine detail at low volume. It suits chamber music, voices and small ensembles, and it can play symphonic music well when the loudspeakers are sensitive enough to give it headroom.
• Push-pull gives more power from the same tubes and a lower output impedance. It is commonly described as having firmer bass and larger dynamic scale. It is the safer choice for less sensitive loudspeakers, large rooms and large-scale orchestral music played at realistic levels.

Room size and listening distance

A larger room and a longer listening distance both cost level at the seat. Each doubling of distance takes away up to 6 dB in free space; a room’s reflections give some of it back, the more so in a lively room. A small room with the seat at 2 m asks much less of an amplifier than a large room with the seat at 4–5 m. If your room is large, choose more sensitive loudspeakers, a more powerful amplifier, or both. Bass is the other room question: an organ pedal note or a bass drum excites the room’s resonances, and placement matters as much as watts.

How to choose a tube amplifier for classical music

• Start with the loudspeakers. Note their sensitivity (and how it is measured), their impedance and the lowest point of their impedance curve.
• Estimate the level you need. Take your usual listening level, add about 20 dB for peaks, and work back to the power you need with the table above.
• Match the output stage to the music. Chamber and vocal music reward a small-triode SET; symphonic music and opera at realistic levels need large triodes, more sensitive loudspeakers or push-pull.
• Check the quiet end. With sensitive loudspeakers, listen for hum and hiss with no music playing.
• Allow for the room. A large room or a distant seat moves you up a row in the table.

How Demograf Audio builds amplifiers

Every tube amplifier from Demograf Audio is built single-ended, in pure Class A, as standard. On most models a push-pull build is available to order; it doubles the output power, for less sensitive loudspeakers or larger rooms. The output transformer winding is 8 Ω as standard, with 4 Ω or 16 Ω available; transformers and chokes are custom-wound for each circuit. The amplifiers run from an external power supply with kenotron (tube) rectification. On several models the input tube can be chosen at order time; the EF14 is offered for a more natural midrange in opera and symphonic music.

Each amplifier is made to order in Kazakhstan by Dmitriy Gubchenko and his team, measured before it ships, and delivered with its own measurement report. The steps are on how to order.

Which Demograf Audio piece fits

• Chamber music, voice and early music with very sensitive loudspeakers: Eurybia (2A3, 2 W) or Solaria (45, 2 W), with loudspeakers such as our three-way Orpheus at 103 dB. Tantal (3C24, 3 W) also suits chamber music and adds an electrostatic headphone output.
• A broad classical library with loudspeakers of about 93–97 dB: Atlas (300B, 5 W), or Helios MkII (G-811, 15 W) for more headroom. Our Cassandra MkII is rated at 95 dB and reaches down to 28 Hz.
• Symphonic music and opera at realistic levels: Endymion (845, 211 or 805, 15–25 W), Ether (GM-70 Triode / GK-71 Pentode, 25 W) or Tantal Senior (4E27, 30 W), or a push-pull build to order.
• Organ and the lowest octave of the orchestra: the Zeus subwoofer, 10–300 Hz, with a 15-inch field-coil driver.

Browse our amplifiers for loudspeakers, or read our guide to power triodes for single-ended amplifiers. Tell us your loudspeakers, your room and the music you play most, and we will suggest the amplifier and the tube to build it around.