Active Crossover System Design  501

cause serious damage by maladjustment—if the highpass cutoff frequency for the HF loudspeakers is adjusted radically downwards, then expensive and gig-cancelling damage is virtually a certainty.

It is therefore common to cover up crossover controls with a panel to prevent tampering; this is sometimes called a “security cover”. This might be a substantial piece of clear plastic (the look-but- don’t-touch approach), or a solid piece of metal which is more robust against impact. It is usually fixed with so-called security screws, but since you can buy sets of drivers for those very easily, we’re not exactly talking Fort Knox here.

Features Usually Absent

There are also some features that, while appearing in many kinds of audio equipment, are rarely if ever found in active crossovers. These include:

Metering

Active crossovers are not usually fitted with comprehensive level metering, as the assumption is that they will be installed in some secluded spot where a visual display will not be seen. Peak-detect or clip-detect indicators can however be useful if there is a possibility of internal clipping. These are dealt with later in this chapter. Signal-present indicators that illuminate some way below the nominal signal level (often at −20 dB) can be useful for fault finding.

Relay Output Muting

Active crossovers are not normally expected to have relay output muting, the function of which is to avoid sending out unpleasant transients at power-up and power-down. Since there are likely to be six outputs, probably balanced, the extra cost of six good-quality two-changeover relays is significant. In sound-reinforcement work, putting mute relays on every piece of equipment is very

much not favoured, as they present one more place for things to go wrong and stop the signal. Thump suppression is normally considered to be the job of the power amplifier output muting relays, which have to be fitted, as one of their most important functions is protection of the loudspeakers from DC fault conditions.

Manual mute switches for each output are however often fitted to sound-reinforcement crossovers to simplify checking and fault finding.

Switchable Crossover Modes

Crossovers intended for sound reinforcement are often constructed so they can be used in several different modes. Figure 17.1 shows the block diagram of a stereo 2-way crossover that can be switched to act as a mono 3-way crossover or as a 2-way crossover with a single mono LF output for a subwoofer.Alternatively a crossover might be switchable between stereo 2-way and mono 3-way crossover, with a wholly separate mono subwoofer output always available.

502  Active Crossover System Design

Figure 17.1: Mode switching in an electronic crossover: it can be used as a stereo 2-way crossover,­ a mono 3-way crossover, or a 2-way crossover with a mono subwoofer output.

Figure 17.1 has the mode switches in the normal stereo 2-way crossover position, and 2-way outputs are obtained as shown in the left column of the text in Figure 17.1. If the 2-way/3-way switch is operated, then the right input is not used and the input to the right filter block now comes from the highpass output of the left filter block. If the left filter block is set to the LF/MID crossover frequency and the right filter block set to the MID/HF crossover frequency, 3-way outputs are obtained as shown in the middle column of the text in Figure 17.1. This kind of mode switching requires a very wide range of filter frequency variation, as the filter block must be able to cover both LF/MID and MID/HF crossover points.

For mono-sub operation the 2-way/3-way switch is left in its normal position, and the mono-sub switch operated instead. This causes the left LF output to be fed with the mono sum of the two LF outputs from the filter blocks. The right LF output is not used. In this case the left and right filter blocks are set to the same frequency—the crossover point between the two HF outputs and the single LF output. The summing function has to be implemented carefully if crosstalk between the two channels feeding it is to be avoided.

Each filter block is shown with a single frequency control to emphasise that the cutoff frequencies of the highpass and lowpass sections move together; this is usually implemented with a state-variable filter (SVF) that simultaneously gives highpass and lowpass outputs.

More complex mode switching schemes are possible.Astereo 3-way crossover could be switchable to work as a mono four-way or five-way crossover. This is all very ingenious, but it does require a lot of complicated switching and a very clear head when you are setting up all those crossover frequencies.

Manufacturers often warn that mode switches should not be operated while the whole system is active, stating that this can lead to damaging transients; presumably they are worried that you might get a level you don’t expect rather than concerned about minor DC clicks.

Active Crossover System Design  503

Noise, Headroom, and Internal Levels

The choice of the internal signal level in a piece of audio equipment is a serious matter, as it controls both the signal-to-noise ratio and the headroom available before clipping occurs. A vital step in any design is the determination of the optimal signal level at each point in the circuit; there is no reason why you have to stick to the same level in every section. Obviously a real signal, as opposed to a test sine wave, continuously varies in amplitude, and the signal level chosen is purely a nominal level. One must steer a course between two evils:

1.If the signal level is too low, it will be contaminated unduly by noise. The absolute level of noise in a circuit is not of great significance in itself—what counts is how much greater the signal is than the noise—in other words, the signal to noise ratio.

2.If the signal level is too high, there is a risk it will clip and introduce severe distortion.

You will note that the first evil is a certainty, in that there will always be some addition of noise, even if it is insignificant, while the second is a statistical risk.

The wider the gap between the noise level and the clipping level, the greater is the dynamic range. If the best possible signal-to-noise is required for hi-fi use, then the internal level should be high, and if there is an unexpected overload it’s not the end of the world. In sound-reinforcement applications it will often be preferable use a lower internal level, sacrificing some noise performance to reduce the risk of clipping. Heavy clipping, which in an active crossover system can be surprisingly hard to detect by ear, is likely to imperil HF speaker units, though not for the frequently quoted but quite untrue reason that excess harmonics are generated; the real problem is the general rise in level. [1] Later in this chapter we will look at some ways of detecting and indicating clipping.

The internal level chosen depends on the purpose of the equipment. For example, suppose you are designing a mixing console. If it is intended for studio recording, you only have to get the performance right once, but you do have get it exactly right, i.e. with the best possible signal-to-noise ratio, so the internal level is relatively high, very often −2 dBu (615 mVrms), which gives a headroom of about 24 dB. If it is broadcast work intended to air you only have one chance to get it right, and a mildly impaired signal-to-noise ratio is much more acceptable than a crunching overload, so the internal levels need to be significantly lower. The Neve 51 Series broadcast consoles used −16 dBu (123 mVrms), which gives a much increased headroom of 38 dB.Apart from this specialised application, general audio equipment might be expected to have a nominal internal level in the range −6 dBu (388 mVrms) to 0 dBu (775 mVrms), with −2 dBu probably being the most popular choice.

If you have a given dynamic range and you’re not happy with it, you can either increase the maximum signal level or lower the noise floor. The maximum signal levels in opamp-based equipment are set by the voltage capabilities of the opamps used, and this usually means a maximum signal level of about

10 Vrms or +22 dBu. Discrete transistor technology removes this absolute limit on supply voltage and allows the voltage swing to be at least doubled before the supply rail voltages get inconveniently high. For example, ±40 V rails are quite practical for small-signal transistors and permit a theoretical voltage swing of 28 Vrms or +31 dBu. However, in view of the complications of designing your own discrete circuitry and the greater space and power it requires, the extra 9 dB of headroom is bought at a high price. You will need a lot more PCB area, and of course the specialised knowledge of how to design discrete transistor stages. My book Small Signal Audio Design will be of considerable help

Active Crossover System Design  505

intermediate level control, it can be guaranteed that the output of the power amplifier will clip long before the crossover outputs, as even the most insensitive amplifiers are unlikely to need more than +8 dBu (2 Vrms) to drive them fully. It therefore occurred to me that it would be quite safe to raise the crossover internal level to 2 Vrms, which would theoretically give a 10 dB better signal-to-noise than the often-used −2 dBu level. That is a significant improvement.

As an example, look at Figure 17.2a, where the crossover is operating with an internal level of 0 dBu throughout, which is also the input required by the power amp. Only one of the paths through the crossover is shown. There is a unity-gain input amplifierA1 which has a noise level of −100 dBu (the input is assumed to be completely noise-free; keeping down the noise level from the preamplifier is someone else’s problem). The filters, etc of the crossover have a noise level of−85 dBu, and when this is summed with the −100 dBu fromA1 we get −84.9 dBu at the crossover output; the noise from A1 makes only a tiny contribution. The signal is then passed directly to the power amplifier, and our signal-to-noise ratio is 84.9 dB.

An elevated internal level +8 dBu (2 Vrms) is used in Figure 17.2b, input amplifierA1 having a gain of +8 dB. The noise out fromA1 is now −92 dBu, and with the −85 dBu of noise from the crossover filters added, the total is −84.2 dBu.Apassive 8 dB output attenuator R1, R2 then reduces the signal level back to the 0 dBu required by the power amplifier, and the noise is also reduced by 8 dB, giving us −92.5 dBu at the amplifier input. The signal-to-noise ratio has increased from 84.9 dB to 92.5 dBu, an improvement of 7.6 dB. We do not get the whole 8 dB because of the increased noise from input amplifierA1.

An elevated internal level not only makes the signal more proof against noise as it passes through the signal chain but also against hum and other interference, though a good design should have negligible levels of these last two anyway.

You will note that I specified a passive output attenuator, so that the very low noise output is not compromised by an opamp stage after it. The attenuation can be made variable to give an output level trim control, working over perhaps a ±3 dB range. Given opamps with good load-driving capability, it is possible to make the passive attenuator with low resistance values so the output impedance is still acceptably low for driving long cables. The attenuator values shown in Figure 17.2b give an output impedance of 166 Ω, which is not perhaps to the highest professional standards but quite good enough for a domestic installation with limited cable runs. The load on the last opamp in the crossover is 700

Ω, which is high enough to prevent significantly increased distortion from a 5532 stage. There is more on such output networks, and how they can be combined with balanced outputs, later in this chapter.

There is an assumption here that the crossover is mostly composed of unity-gain circuitry such as the standard Sallen & Key filters. This is not always true—if you are using equal-C Sallen & Key Butterworth filters, then each 2nd-order stage has a gain of +4.0 dB, and this will have to be dealt with somehow. Equalisation circuitry may have gains greater than this at some frequencies, with a

corresponding reduction in headroom; this applies particularly to equalisers intended to extend the LF speaker unit response, which may have gains of +6 dB or more.

However, let’s take it a little further. If our power amplifier clips with an input of 0 dBu, which corresponds to a crossover internal level of +8 dBu (2 Vrms), then as far as the crossover is concerned there is a range of signal levels from 2 Vrms to 10 Vrms (opamp maximum output) that is unusable.

That is a 14 dB range. The effective signal-to-noise ratio could be further improved if the crossover ran