There is bit of an inside joke when one is building an audio system. Much time and effort are spent getting the best signal possible from the recording to the speaker only to have it all fall to pieces when it hits the cheesy speaker crossover. One of the most valuable things I learned from Roger was the benefit of multi-amplification with an active crossover and the control it gives me over the tonal balance of my system. I can compensate not just for the difference in the gain of the amplifiers, but for room and recording anomalies as well. If I sense the highs are excessive on a recording, I can easily turn them down with the "mid/tweeter” control. The same goes for the lower bass frequencies and surprisingly I find myself using that control more often.
However, rather than take this approach, audiophiles prefer to “find improvement” by swapping cables or using other tweaks. Let us assume for example that a high-priced speaker cable with its “uber pure wire” delivers the goods. What then when the signal hits the cheesy wire in the inductor(s) or resistor(s) of the speaker crossover? Rest assured that signal is going to navigate a lot of twists and turns before reaching the drivers and in many cases the signal will not pass transparently. Roger likened cable changes to filling up one’s swimming pool with expensive bottled water. I think that is a fair analogy. So, who is the inside joke on now?
Cable manufacturers would have you believe that their expensive cables are a more important component in a system than an amplifier. Why not instead invest the money on better amplification and an active crossover? Also, you do not even need expensive amplification for the bass frequencies. Roger felt a basic Class AB solid-state amplifier with adequate damping sufficed. I use Class D amplifiers as there are many available at very reasonable prices that can do the job more than adequately. My experience with a multi-amplification system is you come out ahead on the cost and the sonic improvement.
Separating the woofer and filtering the low frequencies from the input of the mid/treble amplifier will increase detail, improve driver control, and allow for higher dynamics and playing levels than any cable can possibly provide. While Intermodulation Distortion (IMD) is rarely mentioned, here it is more important than Total Harmonic Distortion (THD). Dedicated driver amplification decreases IMD resulting in reduced distortion at high levels. We are not just talking about distortion that produces odd or higher order harmonics. Contrary to popular belief, there is nothing benign about 2nd order harmonics. Think about that one for a bit.
Advantages of Line Level Crossovers Over Speaker Level Crossovers
Roger A. Modjeski
First, the problem and the bad news: Speaker level crossovers today often contain expensive boutique parts but are the weakest link in the high-end audio system. Would not the money spent to make such expensive crossovers be better spent on a second amplifier and electronic or passive line level crossover? These are the questions so few audiophiles are willing to ask.
Here is the insurmountable problem: At the speaker level all the components of the crossover must pass amperes of current. At 100 watts a 4-ohm speaker and its crossover experience peak currents of 10 amperes, or more if the impedance is lower at that frequency. No matter how good the capacitors are, they must handle these currents and let me assure you they do not like it a bit. Capacitors store charge, they are not designed to pass current, it is not in their nature.
In addition, inductors add hundreds of feet of 16, 18, or 20-gauge magnet wire. It seems odd to ignore this common, inexpensive wire, but it is inside the speaker where it is not seen. Given the aesthetic attention given to speaker cables it becomes obvious that most audiophiles are only concerned with what they can see. We must also not forget the resistors that are there to convert audio signals into heat. Besides that, 5 to 100-watt power resistors generally have internal brass wire crimps to connect the nasty resistor wire, which is a poor conductor, to the nasty leads. These crimps can oxidize over time making matters worse.
It is my opinion that any speaker manufacturer who is offering a high-end speaker offer it without a crossover at a discount that you can apply toward another amplifier to do it right. When hi-fi started, speakers were cheap in comparison to amplifiers. Now it is the other way around. There are many systems with inexpensive amplifiers that are a fraction of the cost of the speakers they drive. In some systems the cables coming in and out of the power amplifier cost more than the amplifier itself. This makes no sense at all.
Now for the good news. The simple alternative is both inexpensive and elegant. Crossovers are just filters with slopes of 6, 12, 18, or 24 dB per octave. They are implemented electronically with passive components alone or combined with tubes, transistors or integrated circuits (ICs). I have been making 6dB/octave crossovers for knowledgeable listeners who have already caught on to the improvements. Level controls offer added flexibility where one can easily find the setting that gives the mix of bass, treble, and midrange that the speaker manufacturer intended, or to the listeners liking, with a turn of the knob.
Here are the technical advantages:
- The audio currents in line level crossovers are in micro amperes, one million times smaller than the currents in a speaker level crossover.
- There are no inductors. No matter how large the wire used to wind them, they add resistance right where you do not want it, in series with the woofer. There goes your damping (woofer control).
- 1% sub-watt resistors are made with metal films that are much better than resistor wire.
- Capacitors are of small values around .01 microfarads vs. hundreds of microfarads in speakers. The QUAD ESL 63 has a 200-microfarad electrolytic capacitor that the signal must pass through along with series power resistors, ceramic capacitors, and miles of wire in the chokes that drive the concentric rings.
- It is easy to move the crossover point around and control the output levels so that any speaker and amplifier can be used.
- No power is wasted, and amplifiers can be of smaller size which generally sound better. While the woofer might require 100 watts from a solid-state amplifier with high damping, the tweeter can be driven by a little 5-watt single ended tube amplifier.
- Most speaker crossovers have over a dozen components. As examples: A 6 dB/octave 2-way crossover requires one capacitor and one inductor. For 3-way add one more inductor and one more capacitor. For a 12 dB slope add one more component per driver. For 18 dB add one more. Given that relationship, infinite slope would require an infinite number of components.
So, before you plunk down the money on your next speaker purchase ask the manufacturer if they are willing to build the speaker without the crossover and give you an appropriate discount. By the amount of that discount, you will quickly know how they value their crossover.
Postscript: A quick tutorial from Roger on calculating crossover component values for 6 dB/octave filters.
The formula is simple: Capacitance =1/6.3 x frequency x input impedance of the amplifier, or the component following the filter.
As an example: If you want a 600 Hz crossover at the input of an amplifier, and the input impedance of the amplifier is 150K ohms, then the value of the capacitor would be 1.76 nano-farads (1,760 pf). The closest standard value is .0018 uF, but anything from .0015 to .0022 would be fine since for gradual crossovers obtaining an accurate value is not critical. The only place 1% capacitors are needed is in an RIAA network or an active filter. I find it amusing when neurotic audiophiles want to use a 1% capacitor where the value is approximate anyway. That is just plain silly.
Now the input impedance of most power amplifiers is in the range of 15K to 150K ohms and you can usually check this by plugging an ohm meter into the amplifiers input jack. If you get OL or a reading of several meg ohms there is probably a capacitor in the input already and you must get to the other side of that capacitor, measure the resistance, and change that capacitor to the new value. I would use a polypropylene or mylar capacitor. That is all there is to it.