|
Some Insight into Proper Speaker Selection
Above we have provided you with a list of possible speaker candidates for use with low powered SE
amplifiers and down below we also provide the DIY'er with a choice of drivers that are also SE friendly. Many of you aren't ready to take the leap
towards a horn-loaded speaker system and therefore it is critical that you make the right choice when it comes to matching a low powered SE amplifier
with a pair of less than efficient speakers. It can be done with a little thought and maybe some helpful guidance on our part.
Generally my minimum recommendations for speaker sensitivity are as follows:
3 watts or less - 94dB minimum sensitivity
8 watts - 90dB minimum sensitivity
15 watts - 88dB minimum sensitivity
25 watts - 86dB minimum sensitivity
These are general guidelines and can vary by plus or minus 2dB depending upon the impedance of the
speaker, the type of music listened to, the size of the listening room and how loud you like to listen to your music.
Let's walk through an example. This example uses an 8 watt SE amplifier mated to a pair of 90dB
sensitivity speakers. Speaker sensitivity is based upon a measurement of the speaker's sound pressure level (SPL) measured at a distance of 1
meter from the speaker with an output voltage of 2.83 volts (or one watt of output power into an 8 ohm load). So 1 watt pumped into our speaker
will provide 90dB of output measured at 1 meter. Now in order to achieve a 3dB increase in output we have to double the power, to 2 watts.
So at 2 watts input our speaker now produces 93dB of sound pressure measured at 1 meter. To increase the output another 3dB we will have to double
the power again so 4 watts will be required to obtain 96dB. For another 3dB (or 99dB total) we'll need to double the power again to 8 watts.
Note: these measurements are taken at 1 meter and you most likely will be listening at a distance of 2 meters or more so there will be some loss
associated with the extra distance and absorption by carpet, furniture, etc. Let's say 1 to 2 dB is a typical in room loss.
Before we proceed we should discuss and define "loud".
How loud is loud and what is a typical in-home decibel level for the majority of individuals?
Well, the majority of you, when critically listening to music, will listen at a level of 88 to 94dB
with instantaneous peaks possibly approaching 98-105dB. 100dBs is a concert sound level experienced in a big auditorium and typically this level
cannot be listen to for extended periods of time in a smaller area (your living room for example). Some of you never listen to music louder than
88dB and there are a few of you that listen in excess of 96dB all of the time (you'll be sorry when you're 50 years old and are intimately familiar with
hearing aides!
So based upon these averages you can see from the above example that while an 8 watt amp is OK for use
with 90dB speakers you aren't left with much headroom. Go through the same example using a 93dB speaker instead and you'll end up with a little
more headroom. Here are the numbers: 1Watt - 93dB, 2Watts - 96dB, 4Watts - 99dB, 8Watts - 102dB. With the more efficient speaker you'll have
better dynamics and lower distortion. You see, since most SE amps do not use feedback there are no circuit tricks available to control the
distortion at higher power levels. Therefore the best sounding watt of a typical low power SE amplifier is it's first watt! This is
completely opposite of most push pull amplifiers that don't really "open up" and sound clear until you pump about 5 to 10 watts through them. This
is a very important point for you you to remember, so to reiterate, with low power (<10watts) SE amplifiers you're going to want to match them up to
speakers that only require a few watts for normal listening levels because this will provide you with the best sound. For higher power SE amps
(20-30 watts) their best sound, with lowest distortion, will be provided with the first 1-8 watts so mate them with the appropriate speakers for normal
listening levels.
"But Sam Tellig (Stereophile) and some other reviewers say you can use a 2A3 amplifier with 90dB
speakers" you say. Well yes, you can provided you don't listen to your music very loud.
But do the math and you'll quickly see that you won't have any headroom and the amplifier will be operating at its' higher distortion levels.
1Watt - 90dB, 2Watts - 93dB, 4Watts - MAJOR DISTORTION.!!!
As mentioned previously there are some exceptions and your mileage may vary depending upon your
specific circumstances. For example, if your listening room is small (10' x 10') or you live in a small apartment you're less likely to listen at
levels greater than 90dB so yes you can get by with less efficient speakers. Or maybe you are the type that listens to nothing but acoustic music,
light jazz and vocals, then yes you don't need super efficient speakers.
Another important parameter is speaker impedance. A speaker with a very flat impedance is very
beneficial. "What constitutes a flat impedance?" you ask. Speakers are typically rated at 4 or 8 ohms but these values are rarely constant
and due to the reactive elements within the actual driver and crossover components this value will vary with frequency. So for example a certain
speaker might be rated at 8 ohms nominal and 6 ohms minimum. Check the specifications for your speaker or call the manufacturer for more details.
For SE amps you want to avoid speakers that possess impedance dips down in the low frequency range. i.e. an 8 ohm speaker that dips to 4 ohms at
50Hz or a 4 ohm speaker that dips to 1 ohms at 60Hz because these dips have a tendency to suck the life out of your amp resulting in a soft bass
response and loss of dynamics at these specific frequencies. So a speaker with a really flat impedance helps and generally one that fluctuates 2 ohms or
less will be a good choice.
"I've been told that you must avoid 4 ohm speakers with SE amps",
you say. This is a complete fallacy! What do you think the 4 ohm tap on your amplifier is for? 4 ohm speakers are fine provided they
possess a relatively flat impedance.
Follow my recommended guidelines for speaker selection and you'll have a speaker/amp combination that
plays plenty loud and even possesses some dynamics. Be forewarned that if you've chosen a speaker right on the minimum of my recommended
requirements, because you say you normally listen at low levels, that occasional evening when you like to crank it up a bit you'll most likely be
disappointed with the results. So choose accordingly for all your expected listening conditions.
I hope this helps!
Ron Welborne |
