Perseus MkII and Cassandra MkII: one driver or two ways

Every loudspeaker has to cover about ten octaves, from deep bass to the top of the audible range. One school gives the whole job to a single full-range driver; the other divides it between two or more drivers with a crossover. Our Perseus MkII in its single-driver form and Cassandra MkII stand on either side of that line. This guide explains how each approach works, what it gains and what it gives up, and how each pairs with a single-ended triode (SET) amplifier.

What is a single-driver loudspeaker?

A single-driver, or full-range, loudspeaker uses one driver per channel to cover the main band. There is no crossover in the signal path: the amplifier reaches the driver’s voice coil through nothing but wire. To reach the treble with one cone, a full-range driver needs a light, stiff cone and a strong magnet, and many designs shape the cone or add a small secondary cone to help the top octaves. The result is usually a driver of high sensitivity, which is why single-driver loudspeakers have long been paired with low-power tube amplifiers.

Some full-range designs add a supertweeter for the top octave, or a subwoofer for the bottom ones. The single driver still carries the whole voice range on its own, so its character remains the core of the sound.

What is a multi-way loudspeaker?

A multi-way loudspeaker splits the band. A two-way design uses a woofer, or a low/mid driver, for the bass and midrange and a tweeter for the treble; a three-way adds a separate midrange or bass driver. A crossover, which in a passive loudspeaker is a network of capacitors, inductors and often resistors, sends each driver only the frequencies it handles well. The frequency at which the two drivers share the signal is the crossover point, and the order of the filter describes how steeply each side rolls off: a second-order filter falls at 12 dB per octave.

Full range vs multi-way: the main trade-offs

Coherence and the point source

With one driver, every frequency in the voice range comes from the same point and the same moving surface, and there is no crossover between drivers in the midrange. Owners commonly describe the result as coherent and immediate, with voices and acoustic instruments that sound of a piece and an image that is easy to place. This is the main argument for the single driver, and it is strongest in the midrange, where hearing is most acute.

The frequency extremes and beaming

The same cone must move a lot of air for the bass and carry very little mass for the treble, and no cone does both equally well. In the bass, a full-range driver of moderate size cannot move enough air to play very deep at high levels; the enclosure helps, which is why labyrinths, open baffles and aperiodic boxes are traditional companions. In the treble, a cone radiates more and more narrowly once the wavelength becomes comparable to its diameter, an effect called beaming: the sound on axis stays strong while the sound to the side falls away. Listening position and toe-in therefore matter more with a single driver, and the top octave is where full-range drivers differ most from one another.

Power handling and excursion

Because one cone reproduces everything, large bass movements and fine treble detail share the same diaphragm. At high levels, heavy bass excursion can affect the midrange and treble carried on that cone, which is one reason single-driver loudspeakers are usually voiced for moderate levels and paired with low-power amplifiers rather than driven hard. A multi-way design shares the load: the woofer takes the large excursions and leaves the tweeter free for the treble.

Crossovers, phase and time alignment

A crossover is a filter, and every filter shifts phase as well as level. Around the crossover point both drivers contribute, and how their outputs add depends on the filter and on where each driver’s sound originates. If the tweeter’s sound starts earlier or later than the woofer’s, the two arrive at the listener out of step. A time-aligned design positions the drivers so that their sound arrives in step at the listening position. Simple, low-order crossovers keep the parts count and the phase shift down, and they are also easier loads for a tube amplifier.

Single-driver loudspeakers and SET amplifiers: why sensitivity decides

A single-ended triode amplifier gives from a couple of watts to a few tens of watts, so the loudspeaker’s sensitivity sets how loud and how relaxed the system can play. Sensitivity is the sound level at 1 m from 1 W. Every 3 dB of sensitivity halves the power needed for the same level, and 10 dB divides it by ten. A 103 dB loudspeaker therefore needs roughly one sixth of the power that a 95 dB loudspeaker needs for the same level. From 3 W, a 103 dB loudspeaker reaches about 108 dB at 1 m, before listening distance and the room are taken into account.

The second factor is the load. Without a crossover there are no filter-made impedance dips; the amplifier sees the driver’s own impedance curve, with its peak at the bass resonance and a gentle rise in the treble. A multi-way design with a simple crossover and a high, even impedance can be just as kind to a SET amplifier. Our guides to loudspeakers for single-ended amplifiers and power triodes for single-ended amplifiers cover impedance and the power of each output tube in detail.

Perseus MkII: one field-coil driver per channel

Perseus MkII is built on Supravox 215-EXC mk2 field-coil full-range drivers, in which the magnetic field comes from an electromagnet with its own supply, so the whole range comes from one diaphragm without a crossover. Its sensitivity ranges from 93 to 102 dB depending on the configuration, at 6 Ω / 8 Ω, and it handles 70 W. Passive by default, it can also be built with active amplification inside. The enclosure, whether aperiodic, open baffle, sealed, transmission line or bass reflex, is chosen for the listening room before the build starts. Our guide to field-coil loudspeakers explains the principle.

Cassandra MkII: two ways, aligned in time

Cassandra MkII takes the multi-way route with as little as possible between the amplifier and the drivers. The low/mid driver is a BEAG HX-302 with a paper cone and a cobalt magnet; the treble comes from a SICA high-efficiency horn-loaded tweeter through a simple second-order crossover at 3.5 kHz. The tweeter and the woofer are aligned in time, which keeps the image precise and three-dimensional. A rear port takes the bass down to 28 Hz, and with 95 dB sensitivity and an impedance of 10–14 Ω it works from 3 W tube amplifiers. It suits listeners who want deep bass without a subwoofer and a monitor’s precision, while staying within reach of a low-power triode.

Orpheus and Zeus: three sections and the lowest octaves

Orpheus takes the multi-way route with three sections: low-, mid- and high-frequency AER drivers joined by a crossover. In its top configuration the low-frequency driver has an iron-core inductor, the mid-frequency driver a large capacitor and the high-frequency driver a small one. Its sensitivity is 103 dB (1 W / 1 m) at 8 Ω, so it, too, comes alive on a few watts and is usually paired with single-ended triode amplifiers. For larger rooms Perseus MkII can also be built as a three-way system with Fostex horn tweeters and a 10–18 inch woofer, so it can stand in either camp.

If you like the single driver but miss the bottom octaves, Zeus adds them. It is a subwoofer with a 15-inch field-coil woofer, covering 10–300 Hz, built either passive or active with its own Class A or Class D amplifier, so the lowest octaves need not come from the triode.

One driver or several: how to choose

• Choose a single driver if you listen mostly to voices, acoustic and chamber music, at moderate levels, in a small or medium room, from a low-power SET amplifier, and value coherence above the last octave at each end.
• Choose a multi-way design if you want full bass extension and wider treble dispersion from one pair of cabinets, play large-scale music at higher levels, or sit off axis as often as in the centre.
• Combine the two with a single driver and a subwoofer, or a single driver with a supertweeter, if you want the midrange of one and some of the reach of the other.

Which Demograf Audio piece fits

• A low-power single-ended amplifier and mostly vocal and acoustic music: Perseus MkII.
• Deep bass and a monitor’s precision from a few watts: Cassandra MkII.
• A three-way system that still works from a few watts: Orpheus.
• The lowest octaves under a single-driver loudspeaker: add Zeus.

For the amplifier, start with the 3 W Tantal on 3C24 triodes or the 5 W Atlas on 300B triodes; for more headroom, the 15 W Helios MkII on G-811 triodes. Our amplifiers for loudspeakers page lists the full range. Every loudspeaker and amplifier is made to order and measured by Dmitriy Gubchenko in Kazakhstan, with its own measurement report. Tell us about your room and your music, and we will suggest the pairing.