16  Crossover Basics

The Next Step in Hi-Fi

As we have just seen, active crossovers have a long and convincing list of technical advantages. The score is 22 very real advantages and five not-too-convincing advantages, as opposed to nine advantages for passive crossovers. It is generally accepted that active crossover hi-fi systems sound obviously better than their passive crossover counterparts. Any sort of consensus is rare in the wide field of audio, so this is highly significant. I strongly suspect that the widespread adoption of active

crossovers, suitably matched to their loudspeakers, would be The Next Big Step in Hi-Fi, and possibly even The Last Big Step possible with current technology.

Nonetheless, it is undeniable that active crossovers, despite their compelling advantages, have made very little headway in the domestic market so far, though they are used in all but the smallest soundreinforcement systems, and extensively in automotive audio. The big question is how to make active crossover technology more acceptable in the marketplace. The first thing we shall do is look at ways of solving the “too many boxes and wires” problem.

Active Crossover Systems

We will consider the various ways in which an active crossover system can be configured, with an especially hard look at making it acceptable in a domestic environment. Sound-reinforcement systems are a separate issue. One of the significant disincentives to the active crossover approach is the sheer amount of hardware required, in terms of electronic boxes and cables. This can be hard to fit into a minimalist decoration scheme—and indeed often hard to fit into any sort of decoration scheme at all. It is desirable to package the technical functionality as neatly as possible. I am assuming here that a highquality system is intended, and so 3-way active crossovers will be used.

In terms of cabling and equipment the tidiest setup is undoubtedly achieved with a mono active crossover and its three power amplifiers built into each loudspeaker enclosure, but it is vital to realise that this is not an acceptable approach for people who take their power amplifiers at all seriously.

Figure 1.3: Active crossover system using six monobloc power amplifiers.

Crossover Basics  17

Figure 1.3 shows an active crossover system using six separate monobloc power amplifiers. This could be configured with all six amplifiers in one location, in which case putting them all near one of the loudspeakers reduces the length of at least one set of loudspeaker cables to a minimum. The amplifiers may be all of the same type, but all that is really required of them is that they have the same gain. Even if we have six nominally identical amplifiers made at the same time by the same manufacturer, somewhere in each amplifier will be at least two gain-setting resistors, each with a tolerance; nonetheless, in a competent design the variation should be comfortably less than variations in the drive units.

Alternatively, three of the power amplifiers could be placed adjacent to each loudspeaker, considerably shortening the total length of what might be expensive loudspeaker cable, as three of the connections are now very short.Three of the line-level cables to the power amplifiers naturally become correspondingly longer, but since their resistance is of much less importance than that of the loudspeaker cables, this is overall a good thing. The increased resistance of long line cables makes the link more susceptible to voltages induced by currents flowing through the ground connection, but Ithink it is fair to assume that a hi-fi system with active crossovers would use balanced connections to cancel such noise. Ultimately the placing of the various parts of the system is going to be influenced by furniture arrangement and the availability of handy (and hopefully well-ventilated) cupboards in which to stash the boxes.

Adding it up, a system configured in this way consists of seven electronic boxes, (not including the preamplifier) eight line-level cables and six loudspeaker cables. If the connection from the preamp to the crossover is a 2-way cable (i.e. two parallel cables joined together along their length), that is reduced to seven boxes, seven line cables, and six loudspeaker cables, which is hardly a great improvement.

However, it must be said that there are excellent technical reasons for using 2-way cables when you can. Their construction keeps the grounds for the two links physically close together and prevents them forming a loop that could pick up magnetic fields which would induce current flow; this current would cause voltage drops in the ground resistance and degrade the signal. The use of balanced connections greatly reduces the effects of ground currents, but it is of course much sounder to prevent the currents arising in the first place.

In Figure 1.4, the system is configured with three stereo power amplifiers. This has the advantage that stereo amplifiers are the most common sort and give the greatest choice. The ones used here

Figure 1.4: Active crossover system using three stereo power amplifiers.

18  Crossover Basics

are assumed to be identical; if a lower-power stereo amplifier is chosen to drive the tweeters, then Figure 1.4 would need to have its connections rearranged. Figure 1.4 uses five electronic boxes, eight line cables, and six loudspeaker cables. Using 2-way cables between the preamplifier and crossover and also to the power amplifiers simplifies this to five boxes, four line cables, and six loudspeaker cables. There are always going to be six loudspeaker cables.

Figure 1.5 shows a variation on this approach which puts one of the stereo amplifiers adjacent to the right loudspeaker instead of piling them all up on the left side. This cuts down the total length of loudspeaker cable required, but there is still a long run from one amplifier on the left to the right loudspeaker on the other side of the room.

The opinion is held in some quarters that very high degrees of isolation between left and right channels is essential to obtain an optimal stereo image. This is wholly untrue, but audio is not a field in which rational argument can be relied upon to convince everybody. The configuration of Figure 1.5 could be criticised on the grounds that left and right channels pass through one stereo power amplifier, and this might compromise the crosstalk figures. It is not actually harder to get a good crosstalk performance from a stereo power amplifier than from a stereo preamplifier; in fact it is usually easier because the preamplifier has more complex signal routing for source selection and the like. Nonetheless it is only fair to point out that there might be objections to the 3x stereo amplifier arrangement because however it is configured, at least one amplifier will have to handle both right and left signals.

We will now take a radical step and assume the ready availability of three-channel power amplifiers.

Multi-channel power amplifiers at a reasonable cost have been available for surround-sound systems for many years, to deal with 5:1 formats and so on. The last multi-channel power amplifier I designed (The TAG 100x5R:10) could be configured for ten channels of 80 W/8 Ω each.Athree-channel power amplifier of high quality presents absolutely no new technical challenges at all.

As you can see from Figure 1.6, using two three-channel power amplifiers simplifies things considerably. There are now four electronic boxes, eight line cables, and six loudspeaker cables. Using a 2-way cable from preamp to crossover and 3-way cables between the crossover and amplifiers is well worthwhile and reduces the parts count to four boxes, three line cables, and six loudspeaker

Figure 1.5: Alternative setup of three stereo power amplifiers, with one placed on right speaker side.