- DAMPING FACTOR
- The damping factor indicates
the ability of an amplifier to resist a change in
it's output signal. If an amplifier has a very
low damping factor, the speaker load (or any load
- like a resistive dummy load) can cause the
output to differ (possibly audibly) from what
it's intended to be. For virtually every
amplifier made, the damping factor is easily high
enough to prevent audible changes in the output
signal. Some people say a DF (Damping Factor) of
200 is the minimum acceptable DF. Others say it's
100. there are even those that say a DF of 50 is
OK. I've tried inserting a series resistor with
speakers and could hear no audible difference at
a DF of 50. Around a DF of ~25, I started to hear
a difference with 'some' speakers. At around a DF
of 10, the difference was significant enough to
be heard with virtually every speaker. You may
think it always sounded worse with a low DF.
Well, not always. Some speakers (mostly high
frequency speakers) actually started to sound a
little better (probably from a change in the
frequency response which was due to its impedance
curve). Of course, some speakers sounded worse
with the low DF. In virtually all cases, the
change in sound quality was a 'softer' sound. For
very low DFs, the punchy bass was significantly
reduced but the bass sounded smoother (which may
or may not be a good thing - depending on your
taste in music). Now, I'm not saying that the
'smoother' or 'softer' sound was better (because
the change in sound IS a type of distortion). I'm
just trying to let you know how the sound changed
with the low (extremely low) damping factor.
- Two different ways that the
damping factor (DF) can be observed:
-
- 1. As was mentioned above, the
DF describes how well the amplifier maintains the
desired output voltage under varying loads. An
amp with a HIGH DF will maintain virtually the
same output voltage whether it's driving a 4 ohm
load or has no load connected to its output
terminals. An amp with a LOWER DF will have a
GREATER voltage drop when switched from no load
to a 4 ohm load. Keep in mind that we are only
talking about a few millivolts difference between
the output voltage when loaded or not. Nothing to
worry about with either amp. This type of change
in output voltage would generally be observed on
the test bench with a test tone. Keep in mind
that a low damping factor for a 'real world'
amplifier would be around 50. Nothing nearly as
low as what I used in my testing (mentioned
earlier).
-
- Drag your mouse over the
picture below. You'll see how the voltage drops
slightly when the speaker is connected to the
amplifier.This amplifier would have a relatively
high damping factor. You may have to leave your
mouse over the diagram until it finishes loading.
Clicking on the picture will optimize it's
position.
- 2. Since speakers are reactive
(especially subwoofers), the amplifier has to be
able to absorb the out of phase voltage from the
speakers to prevent the signal at the amplifier's
output terminals from being distorted. If you
were driving a set of speakers that used passive
crossovers to split the signal between the high
and low frequency drivers (speakers), all of the
speakers would get their signal from the same
place (the input terminals of the crossover). If
the amplifier had a very low DF, the out of phase
voltage would cause the signal at the amplifier
to change slightly. Since the high frequency and
low frequency speakers are driven from the same
point, the HF drivers would recieve a slightly
distorted signal. If the amplifier had a high DF,
the woofers would have less of an effect on the
signal which would reduce the distortion at the
amplifier's outputs (and also at the input of the
crossover). This means that all of the other
drivers would get a 'cleaner' signal. Now you
should keep in mind that the amplifier's feedback
circuit, which for the most part determines the
amplifier's DF (in solid state class A and class
A/B amplifiers), is using the signal at the
amplifier's output teminals to determine the
level of error correction needed. This means that as soon as
you start to add anything between the amplifier
and the speakers, the amplifier can not
compensate for the added components. This means
that the DF starts to fall as soon as you connect
a length of speaker wire between the amplifier
and the speakers. Longer runs of speaker wire
will cause the DF to fall more than short runs.
If you have a long run of speaker wire, you can
compensate somewhat by using a larger gauge of
wire. In the calculator at the bottom of this
page, you can see how much the DF is effected by
the speaker wire. Again, keep in mind that as
long as the DF remains above a certain value, the
change in DF won't generally cause any audible
changes.
Damping
Factor Tests
- Standards:
- If you are going to test more
than one amplifier, you should adopt some
standards. You should pick a single frequency or
a defined set of frequencies. You should also
pick a load with a commonly used impedance (such
as 2 or 4 ohms). For this example let's say that
the test frequency is 50 hertz and the test
load's impedance is 4 ohms.
- Testing:
- For this test, you want to
drive the amplifier to a sufficiently high level
to produce large numbers. Since your test
equipment has a limited number of decimal places,
the larger numbers will help increase the
accuracy of the tests. You will drive the
amplifier to a level which will be below clipping
when loaded with the test load. You must be
absolutely sure that the level of the input
signal is not changed throughout the test
procedure. Measure and make note of the output
voltage of the amplifier with no load.
Connect the 4 ohm load to the amplifier's output
terminals and measure the output voltage again.
Make note of it. Please note that the voltage
measurements should be taken as close to the
amplifier as possible. OK, enough of that.
- Example:
- For this example, we will use
the numbers from the 'output impedance' page. If
we know the output impedance of an amplifier and
the load that it is going to be driving, we can
find the damping factor by dividing the load
impedance by the output impedance of the
amplifier. By using the output impedance of .01
ohms (that we calculated on the output impedance
page) and a 4 ohm speaker load, we get 4/.01
which gives us a damping factor of 400. If we
were going to drive a 2 ohm speaker, you can see
that the DF would fall to 200.
- Since the manufacturer usually
doesn't usually include the amplifier's output
impedance and may not supply the damping factor,
you can find it with minimal test equipment.
- We will use the formula: DF = Eno
load/(Eno load-Eloaded)
- DF
= Eno load/(Eno
load-Eloaded)
If Eno load = 20 volts
and
Eloaded =19.95 volts
DF=20/.05
DF=400
- The Damping Factor at 50 hertz
is 400 when driving a 4 ohm load. The same DF as
when we used the output impedance and a 4ohm
load.
-
- Speaker Wire and DF:
- Amplifiers with a HIGH damping
factor have a LOW output impedance. As soon as
you start adding speaker wire between the amp and
the speakers, the damping factor at the speakers
starts to fall. An amplifier with a DF of 400
into a 4 ohm load, has an output impedance of .01
ohms. If you use 5ft of 14g speaker wire, the
total resistance of the wire is .016 ohms. If you
add this to the output impedance of the
amplifier, you have a total output impedance of
.026 ohms. This reduces the effective damping
factor to 154. Longer runs of speaker wire will
reduce the effective damping factor even more.
- NOTE:
- Many people test the damping
factor at 50 hertz into a 4 ohm load. You can
make the test at any frequency that you like and
with any load. When comparing the damping factor
of one amplifier to another, you must know the
frequency at which the test was performed and the
test load's impedance. Otherwise, you may be
comparing apples to oranges.
- Transistors vs Tubes:
- In general, 'solid state'
amplifiers (transistor amplifiers) tend to have a
higher damping factor than tube amplifiers. Transistor amplifiers drive the speakers
directly and may have a damping factor greater
than 200. Most tube
amplifiers drive the speakers through a large
transformer which tends to lower the damping
factor of the amplifier. The damping factor of
top notch tube amplifiers may be as low as 20.
Please understand that the lower DF is NOT an
indication of the quality of the amplifier or the
sound quality that you will get from it. Low DF
amplifiers can sound really good, easily as good
as amps with higher DFs.
- External Servo Feedback
Compensation:
- There are a few amplifiers
which claim VERY high
damping factors (over 2000). These usually employ
a second pair of wires which connect to the
actual speaker terminals. These feedback
terminals are not the speaker output terminals.
These are the servo feedback wires. Virtually all
transistor amplifiers use lots of feedback. The
large amounts of feedback are responsible for the
high damping factors. As I said before, the
speaker wire causes the DF to diminish. The extra
pair of wires going out to the speaker allows the
amplifier to 'see' and correct for any loss in
the speaker wires. This causes the damping factor
to be maintained. As to the amplifiers that claim
a DF in the thousands, well... I'm skeptical to
say the least but I do believe that it is
possible to maintain a high DF at the speaker
(even through long runs of speaker wire) when
using the external feedback circuit. If you
missed the 'servo'
page, go back and read more on compensation
circuits.
- Switching Amplifiers:
- Switching amplifiers like Class
D amplifiers generally have a lower DF than their
Class A/B counterparts because the output of the
amplifier has to pass through an inductor. Since
the inductor is wound with copper wire which has
resistance (albeit a very low resistance), the DF
is reduced. Generally the lower DF is completely
inaudible.
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