In another thread, Jørn post an excellent and exhaustive list of considerations when one looks at designing active speakers. What problems it solves. What difficulties it brings.
Here is his list from the other thread:
I thought I put up the list and then ask people to ask specific questions about the items above.
Here is his list from the other thread:
Well, since this thread is quite far off its initial question, I'll try to bring it back... for a while
I think many issues in active vs passive has to be investigated separately:
1: In active speakers, one can match the amplifier perfectly to the drivers. This can lead to less expensive amplification. One also has perfect control over clipping limits, impedance in driver vs load margin of the amplifier and other basic parameters.
2: In active speakers, nonlinear parameters like Kms and BxL nonlinearity can be corrected by using dynamic digital filters in a feed forward design.
3: Active speakers can be servo controlled.
4: Active speakers can be used with current feedback amplifiers. Such amplifiers will ignore voice coil heating, Le nonlinearities and so on.
5: Current feedback amplifiers will potentially have problems with change in resonance frequency over time and resonance jumping due to Kms nonlinearity.
6: When dealing with high Q phenomena like metal cone breakup phenomena, a passive crossover can be used as an electrical load that extracts energy from that resonance. In theory, this could be done with an amplifier directly on the driver, but in a real world, this does not work.
7: In theory, a measured impulse response can be corrected digitally for each driver in an active speaker. However, there are multiple phenomena that are not currently possible to solve correctly.
8: Different directivity effects can be achieved much more effectively with implementing individual DSP filters for each driver.
9: In some crossovers (passive) a lot of energy is being lost. (I always use thermographic measurements to map power loss when building crossovers).
10: Precision filtering close to a drivers resonance frequency is impossible with passive crossover.
11: Total energy from an amplifier will not change when splitting it into several limited frequency bands. The sum of power at a given moment in a multi amplifier setup is the same as for a single amplifier setup, given that the efficiency is the same.
12: In a passive system, the crossover is a contributor to lower efficiency. Especially where matching sensitivity leads to extensive use of resistors. Also shunt filters are very ineffective.
13: In an active system, the idle power of the multiple power amplifiers are in theory higher than the idle power of the single amplifier for a passive speaker.
14: The use of very steep filters do come with a price. If the filters are IIR type, they will have a lot of phase distortion.
15: The use of steep and phase linear FIR filters will cause a significant delay and pre ring effect. Some claim to have solved this but skeptics are not convinced yet.
16: A loudspeaker drivers nature of being a time variant load makes high resistance series components typically cause a significant raise in distortion.
17: In a passive speaker, a well designed SD system (copper sleeves covering critical parts in the magnet system) is key. In an active system, this is less important (and copper parts adds cost).
18: In an active speaker, one can compensate for theoretically improper use of drivers. For example, a woofer used in an enclosure smaller than it is designed for, can be EQ-ed to perform well. In a passive speaker, this EQ has to be added somewhere in the signal chain.
19: In an active speaker, one do not have to care that much about driver sensitivity. A 84dB bass section can be added to a 92dB midrange section and a 90dB tweeter section. This gives a wider selection of drivers to choose from as well as the possibility of even more powerful bass performance from a small enclosure.
20: Active speakers can more easily be equipped with dynamic compressor, temperature monitoring and so on to prevent the speaker from malfunction.
21: DSP based speakers makes upgrade very easy and cost effective.
22: Active speakers can more effectively be combined with room adapting tools (compensation for listening distance, room size, distance to near boundaries at low frequencies etc).
23: DSP based speakers can easily be equipped with EQ that enables the user to reduce the impact of room resonances.
I believe there are a lot of more issues, that I can not think of right now. An active speaker is not a completely obvious choice over a passive one and vice versa. It is often a question of budget.
I thought I put up the list and then ask people to ask specific questions about the items above.