Active systems

From DRC

Jump to: navigation, search

It's not (currently) possible to build a speaker driver which can reproduce the entire audio spectrum, ie from around 20Hz up to around 21Khz. So most speakers use a compromise arrangement using several drivers each optimised to work over a specific frequency range. Often we refer to "bass" woofers, midrange drivers and tweeters (and sometimes subwoofers, super-tweeters and other jargon).


Contents

Passive speakers

So the problem in a traditional passive system becomes how to take the single channel signal out of the back of the single (stereo) amplifier and split it into different frequency bands and send each band to just the corresponding speaker driver.

How is this done with "passive" speakers? Well typically it's done "passively" using a network of resistors and capacitors to split the signal into multiple frequencies, pad down some of the frequencies (for example to match sensitivities of woofers and tweeters) and send it through to each driver: 

                        --> high freq crossover ---> tweeter
                       /
Pre-amp --> Power-amp ----> mid range crossover ---> mid-range driver
                       \
                        --> low freq crossover ---> woofer

Of course passing the signal through so many capacitors, resistors and inductors is not free:

  • Resistance is not constant with varying frequency. There are lumps and bumps which present a potentially difficult load to the amplifier
  • Component quality must be fairly high to avoid signal degradation
  • Drivers are connected to each other and hence back-EMF from one driver can potentially influence other drivers
  • Analog crossovers by necessity introduce phase distortion in the output signal (although one can mitigate this by designing the opposite filter to cancel out this effect)
  • Component accuracy is difficult to achieve at a reasonable price, hence freq ranges can be slightly different between crossovers due to parts tolerances (or simply parts availability, capacitors do not come in infinitely varying sizes)
  • Amplifier may be driven into clipping due to excessive bass signal, which by definition will be damaging to the treble components as well (there will be massive differences in power levels between bass signal and treble signals)
  • It's technically extremely difficult to build a good three-way crossover - it's not the same as two two-way crossovers (see reference in Vance Dickason book...)

So how does an "Active system" differ?

With an active system we use multiple amplifiers with each amplifier connected directly to a speaker driver. The crossovers are then placed in front of the amplifiers. It's possible to make these crossovers using analog or digital methods - analog implies some op-amps and clever circuitry, the digital method implies that the signal is somehow fed in digitally and then we can use DSP techniques to filter the audio into different frequency bands 


         --> high freq crossover ---> Power-amp ---> tweeter
        /
Input ----> mid range crossover ---> Power-amp ---> mid-range driver
        \
         --> low freq crossover ---> Power-amp ---> woofer

There are a number of advantages and a few disadvantages with this approach:

Disadvantages:

  • Cost!! You need more amplifiers (but potentially they can be of lower quality - the demands on an active system can be far lower than for a passive system)
  • Complexity. Tuning a complete multiway active system can require quite a lot of time, and even a lot of space. Consider for example a complete 5.1 system with 3 way main speakers, 2 way center channel and rear speakers and a sub.

Advantages:

  • Dynamic range gain. Even if it appears a bit counterintuitive, with wideband audio signals, like music, an active system with two 100 W amplifiers has a maximum SPL which is 3 dB higher than a passive system with a single 200 W amplifier. Usually the difference is even bigger because of the passive crossover losses, that get removed in the active system, and other minor factors, like bass range amp clipping being filtered by the intrinsic limited bandwidth of the woofers. Most of the times the perceived dynamic range gain is in the 6 dB range, i.e. a two way system with two 100 W amplifiers often sound almost as loud as a passive system with a single 800 W amplifer.
  • Better loudspeakers control. The series resistance and losses introduced by the passive crossover components reduces the amplifier damping factor seen by the drivers. This becomes audible, especially in the bass range, where big inductors with series resistances up to 1 Ohm are often used and where the speaker self damping is quite low. With the active approach the drivers are connected directly to the amplifiers, with no series resistance introduced by passive components.
  • Each amplifier sees a much more constant load
  • Amplifier operates in a narrow frequency band, hence lower IMD (intermodulation distortion, where frequencies combine and spurious distortions arise as a result)
  • Drivers are physical seperated so one cannot influence the other. Back-EMF from one driver goes straight back to the amp and cannot travel back into the sensitive tweeter.
  • It's easy to design the crossovers, you just build them for the freq you want and there is no influence from them on the other crossovers
  • Amplifiers may be operating at much lower levels for tweeters than woofers - cheaper amps, and amplifier now operating in much lower distortion range
  • Digital crossovers offer new and interesting ways to apply crossovers to the input signal, including some techniques which are simply not available with analog crossovers
  • Crossovers are operating with small signal levels (0-2v instead of 0-60V), consequently they can be built using much smaller parts with consequently higher tolerances and quality. The change in components also allows much more accurate crossovers to be built.
  • It's possible to "time-align" the drivers. i.e. delay the tweeter signal so that it arrives at the same time as the woofer signal
  • Physical relocation of drivers is possible! The most extreme form of the above is advocated by TacT where you physically locate the bass speaker (sub woofer) against a wall where it should work more efficiently, and then add a delay to the main speakers so that the sound from the woofer and main speakers arrives perfectly at the listening position!

Can I convert my current system to be Active?

Sure! But be aware that this really means removing the existing circuitry from within your current speakers and then wiring power amps to each driver. You also need to be technically proficient enough to re-implement the old crossovers in front of the new poweramps.

TacT have an interesting solution to this problem. Many of their amplifiers include a DSP processor which can implement crossovers at arbitrary frequencies. So the problem can be solved as simply as buying a pile of Tact digital amplifiers, connecting a laptop, telling the amps the freqs to use for each driver and squirting in a signal to all the amps. Voila each amp sends only the correct signal to each driver and they can even be time-aligned so the sound from each driver arrives perfectly in phase at the listening position!

A simpler solution

A simpler way to convert to an active system is to buy active speakers, which have already a properly tuned crossover and properly sized amps directly in the speaker box. This solution isn't much popular in the audiophile world, but it's the most common solution within professional studio monitors. Many studio monitors have specifications suited also for home audio use. Of course this way you miss all the fun in building your own active system from the ground up.

For some examples of good professional active studio monitors take a look at:

  • Genelec - Genelec have started moving into the home market and make some nice veneered systems for HT use
  • ATC
  • PMC - PMC make some speakers which can be bought either with crossovers or "activated" with electronic crossovers
  • Klein + Hummel
  • Dynaudio pro section
  • Tannoy pro section
  • KRK
  • ADAM
  • Quested

There are of course many others.


Also see:

Links

  • Add in some pointers here...
Retrieved from "http://www.duffroomcorrection.com/wiki/Active_systems"
Personal tools