Matching Power Amps and Loudspeakers.
One of the things that always bothered me is how loudspeakers and power amplifiers are evaluated. My primary concern is the output impedance of the power amplifier and how the variation in loudspeaker system impedance varies with frequency, and how that variation in impedance affects the power amplifier.
It is impossible to tell from any data sheets that loudspeaker manufactures provide what the characteristics of the power amplifier was that was used to set the final adjustments in crossover networks to obtain a reasonably flat response at the listening position. All they usually give you is a ballpark figure of how many watts are required to drive them to reasonable listening levels.
If a amplifier was used during the final stages a a loudspeakers design, and that amplifier had a high fairly high output impedance (low damping factor) the output voltage of the amplifier may vary considerably with changes of loudspeaker impedance versus frequency, yet the loudspeaker provides a flat response on the SPL curve. Thus if the loudspeaker was designed to provide a flat frequency response when driven from such a amplifier it would not be able to provide a flat response from a amplifier with a lower output impedance (high damping factor). This is a fact because the drive voltages to the loudspeaker will vary much less with frequency and thus will cause fairly large peaks in SPL levels at certain frequencies. The lower and upper midrange frequencies would likely be affected most. The reverse is possible if the loudspeaker was designed using a amplifier with a very low output impedance.
It is quite expensive to design a speaker that has a relatively flat impedance curve across its entire frequency range, and in most cases is unobtainable. This is especially true of the low frequency end when ported enclosures are used. The high quality large value capacitors or inductors required eliminate the low frequency rises in impedance caused by the port and woofer resonance’s are very expensive. So most manufactures don’t even bother to attack this problem.
As I stated it can also be quite difficult to find a power amplifier loudspeaker combination that is optimum unless you buy or build a power amplifier that you like and build or modify loudspeaker to be happy with it. This is not always required but the possibility exists. To accomplish this matching can take a considerable amount of time and effort using some of the computer programs designed to do this type of work. Being a DIY does have its advantages if you have the resources to do this.
It nice to always have a power amplifier available that you can use as a reference standard (never get rid of it). Such a amplifier can be used as a fast A-B comparison for checking to see how well your loudspeakers like any new power amplifier that you obtain or build. You will need a pink noise source that can drive each amplifier to the same average output level along with a instant A-B switch to select which amplifier drives the loudspeakers. The one that sounds the smoothest provides the best combination. You may be suppressed at the difference you may notice even though all published specifications of the amplifier match very closely.
A additional factor also comes into play. As the frequency sensitive load provided by the loudspeaker loads the amplifier it changes in the amplifiers internal feedback levels versus frequency. In effect modulating the feedback loop. This causes the output impedance of the amplifier to be modulated as well due to the fact that the amplifiers actual output impedance is greatly affected by the amount of feedback applied. Even amplifiers claiming no feedback that use emitter or source follower output stages are affected.
Warning: If you do the noise test I mentioned with a tube amplifier you should provide a load for the amplifier that is not driving the loudspeakers. Most all solid state amplifiers are stable without a load and do not have to worry about generating high voltages that can arc across tube sockets or inside the output transformers. When a load is removed from a amplifier any negative feedback levels taken from the output will increase tremendously and this may cause some instability in poorly designed circuits. It is possible for a amplifier with such characteristic to self destruct its output devices or other parts..
Does anyone have any suggestions or comments on this issue?
John Fassotte
Alaskan Audio
http://www.audioamps.com
One of the things that always bothered me is how loudspeakers and power amplifiers are evaluated. My primary concern is the output impedance of the power amplifier and how the variation in loudspeaker system impedance varies with frequency, and how that variation in impedance affects the power amplifier.
It is impossible to tell from any data sheets that loudspeaker manufactures provide what the characteristics of the power amplifier was that was used to set the final adjustments in crossover networks to obtain a reasonably flat response at the listening position. All they usually give you is a ballpark figure of how many watts are required to drive them to reasonable listening levels.
If a amplifier was used during the final stages a a loudspeakers design, and that amplifier had a high fairly high output impedance (low damping factor) the output voltage of the amplifier may vary considerably with changes of loudspeaker impedance versus frequency, yet the loudspeaker provides a flat response on the SPL curve. Thus if the loudspeaker was designed to provide a flat frequency response when driven from such a amplifier it would not be able to provide a flat response from a amplifier with a lower output impedance (high damping factor). This is a fact because the drive voltages to the loudspeaker will vary much less with frequency and thus will cause fairly large peaks in SPL levels at certain frequencies. The lower and upper midrange frequencies would likely be affected most. The reverse is possible if the loudspeaker was designed using a amplifier with a very low output impedance.
It is quite expensive to design a speaker that has a relatively flat impedance curve across its entire frequency range, and in most cases is unobtainable. This is especially true of the low frequency end when ported enclosures are used. The high quality large value capacitors or inductors required eliminate the low frequency rises in impedance caused by the port and woofer resonance’s are very expensive. So most manufactures don’t even bother to attack this problem.
As I stated it can also be quite difficult to find a power amplifier loudspeaker combination that is optimum unless you buy or build a power amplifier that you like and build or modify loudspeaker to be happy with it. This is not always required but the possibility exists. To accomplish this matching can take a considerable amount of time and effort using some of the computer programs designed to do this type of work. Being a DIY does have its advantages if you have the resources to do this.
It nice to always have a power amplifier available that you can use as a reference standard (never get rid of it). Such a amplifier can be used as a fast A-B comparison for checking to see how well your loudspeakers like any new power amplifier that you obtain or build. You will need a pink noise source that can drive each amplifier to the same average output level along with a instant A-B switch to select which amplifier drives the loudspeakers. The one that sounds the smoothest provides the best combination. You may be suppressed at the difference you may notice even though all published specifications of the amplifier match very closely.
A additional factor also comes into play. As the frequency sensitive load provided by the loudspeaker loads the amplifier it changes in the amplifiers internal feedback levels versus frequency. In effect modulating the feedback loop. This causes the output impedance of the amplifier to be modulated as well due to the fact that the amplifiers actual output impedance is greatly affected by the amount of feedback applied. Even amplifiers claiming no feedback that use emitter or source follower output stages are affected.
Warning: If you do the noise test I mentioned with a tube amplifier you should provide a load for the amplifier that is not driving the loudspeakers. Most all solid state amplifiers are stable without a load and do not have to worry about generating high voltages that can arc across tube sockets or inside the output transformers. When a load is removed from a amplifier any negative feedback levels taken from the output will increase tremendously and this may cause some instability in poorly designed circuits. It is possible for a amplifier with such characteristic to self destruct its output devices or other parts..
Does anyone have any suggestions or comments on this issue?
John Fassotte
Alaskan Audio
http://www.audioamps.com