Mark Rumreich
Introduction
Everybody wants a good sounding car
stereo nowadays, but what's the best way to do it on a budget?
For a lot of people, the answer is to
upgrade rather than start from scratch. For many systems, the
biggest improvement for the least amount of money involves replacing
poor quality speakers with better ones. After that, adding a
subwoofer tops the list.
A subwoofer provides two main advantages:
- it provides a dramatic improvement in bass, and
- it improves the rest of the sound by
letting you turn down the bass to the other speakers - this lowers
distortion and lets you play them louder.
After adding a subwoofer, many people
are astounded by the clarity that is added to the existing sound.
This is because the amplifier clipping and speaker excursion
"bottoming" that once occurred during loud passages
no longer distort the upper bass, midrange and treble since the
main amplifier no longer has to supply any high power bass to
the main speakers. The existing amplifier and main speakers are
simply coasting, operating well within their linear range of operation.
By doing it yourself, a complete high-quality
subwoofer system can be added for less than $150. This article
will explain how.
Subwoofer System Strategy
Our subwoofer system is composed of three basic components:
- power amp
- subwoofer (speaker & enclosure).
Figure 1 - Subwoofer System Strategy Diagram
To get the best performance for the
lowest cost, we will need to carefully choose our components,
and be willing to do some assembly work. This article will explain
how to construct a high performance subwoofer crossover and put
together a complete subwoofer system.
Subwoofer Crossover Requirements
A lot of subwoofer installations use
good subwoofers and power amps, but try to get away with using
a cheap or incorrect subwoofer crossover. The results are boomy
bass, poor imaging, high distortion and noise. The three most
important things to look for in a subwoofer crossover are:
- selectable cutoff frequency
- subsonic filter.
Steep cutoff slope is important to prevent
midrange and upper bass frequencies from going to the subwoofer.
Midrange and upper bass frequencies are "directional."
(This means you can hear what direction the sounds are coming
from.) Deep bass is "nondirectional" (which means you
can't hear what direction the sounds are coming from). Keeping
"directional" frequencies out of the subwoofer is necessary
to let you install it anywhere in the vehicle without degrading
the stereo image. It also lets you use a single mono subwoofer
rather than separate left and right subwoofers.
A second reason to keep midrange and
upper bass out of the subwoofer is to prevent "boomy"
sounding bass. Only a steep cutoff slope is able to effectively
filter out the "boomy" sounding upper bass without reducing
the "solid" sounding deep bass frequencies you want
to keep.
Selectable cutoff frequency lets you
match the subwoofer to the rest of the system. This prevents
having a "peak" or "hole" in the combined
frequency response. The ultimate approach to selectable cutoff
frequency is the continuously variable type of filter. This lets
you "fine tune" the subwoofer response to the best sounding
value and allows you to compensate for such things as vehicle
interior acoustics and the low frequency masking effects of road
noise.
A subsonic filter prevents subaudible
energy from reducing dynamic headroom and distorting the bass.
It also eliminates annoying turn-on thumps. To be effective
without sacrificing deep bass response, it should have a slope
of at least 12 dB/octave.
The next sections describe a cost effective
subwoofer crossover you can build which meets these requirements
and more.
Subwoofer Crossover - Description
The figure shows a block diagram of
our subwoofer crossover.
Figure 2 - Subwoofer Crossover Block Diagram
It features:
- 24 dB/octave cutoff slope
- continuously variable cutoff frequency
- 18 dB/octave subsonic filter
- polarity switch
- 40 Hz boost switch
- speaker level inputs with active ground loop isolation
This design incorporates a tunable 24
dB/octave Butterworth filter using switched capacitor technology.
The cutoff frequency is continuously variable from 50 to 150
Hz.
The subsonic filter is 18 dB/octave
and has a cutoff frequency of 20 Hz. In addition, the frequency
response of the subsonic filter is switchable between a "flat"
characteristic and a "boost" characteristic (which provides
a 5 dB peak at 40 Hz). This boost can be used to extend low-end
response and provides some deep bass punch without torturing your
woofer.
The subwoofer crossover is engineered
to prevent system noise problems. It uses a differential amplifier
at the front end to provide ground loop isolation and a precision
linear regulator to provide clean power for clean sound. This
combination eliminates such problems as "alternator whine,"
blower motor pickup and ignition noise.
Subwoofer Crossover - Schematic
The figure shows the schematic of our
subwoofer crossover.
Figure 3 - Subwoofer Crossover Schematic Diagram
Basic operation of this circuit is as
follows.
The inputs through Q1 form the differential
summing amplifier. SW1 is the polarity inverter. U1 is a 24
dB/octave switched capacitor filter IC and is the heart of the
continuously variable subwoofer filter. R13 controls the cutoff
frequency of this IC by controlling its sampling frequency. Because
of the inherent sampling action of switched capacitor filters,
it is necessary to have an anti-alias filter at the input of U1.
Q2, Q3 and their associated components form this anti-aliasfilter (which is a second order low-pass).
The subsonic filter with boost stage
is after pin 5 of U1. (When SW2 is closed, the boost characteristic
is selected.) Additional subsonic filtering action is provided
by C1 and C2 at the inputs. R18 and C10 form the "reconstruction
filter" which eliminates sampling artifacts at the output
of U1.
The voltage regulator circuit (D1 through
Q5) provides both an 8.6 volt main supply and a 4.8 volt bias
supply. D1 protects against negative voltage spikes and incorrect
hookup. D2 biases the 78L08 regulator reference pin to .6 volts
in order to provide 8.6 volts (rather than 8 volts) out.
Subwoofer Crossover Construction
The parts list itemizes the components
you will need. Be sure all electrolytic capacitors in "value
sensitive" applications (mainly C10 and C11) are rated 105
C. Electrolytics rated 85 C will deteriorate to a lower capacitance
value in a few years under the high temperature conditions found
in automotive environments.
| Parts List | ||
Resistors (1/8 W, 5%) | ||
| R1, R2, R3, R4 | 47k | |
| R5, R6, R19, R21 | 10k | |
| R7 | 20k | |
| R8 | 16k | |
| R9 | 3.3k | |
| R10, R11 | 9.1k | |
| R12, R14, R16, R23 | 2.2k | |
| R13 | 10k linear taper pot | |
| R15 | 100k | |
| R17 | 5.1k | |
| R18 | 1k | |
| R20 | 47 | |
| R22 | 18k | |
|
Capacitors | ||
| C1, C2 | .22uF | |
| C3 | .047uF | |
| C4, C6, C7 | .1uF | |
| C5 | .01uF | |
| C8, C9 | .22uF | |
| C10 | 1uF, 25V, radial electrolytic, 105 C | |
| C11, C13 | 10uF, 25V, radial electrolytic, 105 C | |
| C12 | 220uF, 25V, axial electrolytic | |
|
Semiconductors | ||
| Q1, Q3, Q4, Q5 | 2N3904 NPN | |
| Q2 | 2N3906 PNP | |
| D1, D2 | 1N914 | |
| U1 | MF4CN-100 switched capacitor filter | |
| U2 | 78L08 voltage regulator | |
|
Other Components | ||
| SW1 | DPDT | |
| SW2 | SPST | |
| J1, J2 | RCA jack, PC mount | |
| Miscellaneous | ||
| PC board, project case, hardware, 18 AWG stranded color-coded wire | ||
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Certified or cashiers checks, money orders, personal checks, PayPal, VISA/MC/Discover accepted. (Personal checks will be held until they have cleared prior to shipping.) |
The photo below shows the component side of a stuffed PC board. All component locations are clearly
marked except for C12 (big lytic) which should be mounted as shown.
Components not listed in the parts list are not used. Follow
all polarity and orientation markings on the PCB to save time
and avoid errors. Be sure to include jumper wires in these locations:
- next to C2
- next to R5
- next to C7
- across output jacks (2 jumper wires)
- (do not use jumper wires next to
R3 or R4)
Note that the output jacks are shown
mounted on the copper side of the board (jack pins are soldered
to the long jumper wires). This allows the PC board to fit properly
into its case.
Figure 4 - Component Side PC Board Picture
The figure below shows the locations of the input and power connection points. All have big pads to allow wires to be soldered on the copper side of the PC board. Use wires of the color shown in the table to conform to the de facto industry standard.
Figure 5 - Copper Side PC Board Picture
An assembled subwoofer crossover is shown below:
Figure 6 - Assembled Subwoofer Crossover Picture
Subwoofer Crossover Test Procedure (Optional)
The following test procedure can be
used to thoroughly check the operation of the subwoofer crossover
before installation. It can also be used to debug a crossover
suspected of being defective.
Test points are shown on the schematic.
Initial Setup:
- Apply 3.0 VRMS, 130 Hz to L & R inputs. (+: TP1 & TP2, -: TP3 & TP4)
- Set pot fully CW, Eq switch to flat position, polarity switch to down position.
- Monitor output signal with scope at TP8.
Test1: Supply Current
- Measure power supply current.
Test2: Output Level
- Verify that output is sinusoidal.
- Measure output level.
Test3: Polarity Switch
- Change polarity switch to up position.
- Measure output level change.
Test4: Frequency Response
- Change signal freq to find lower -3 dB freq.
Test5: Equalization Frequency
- Set Eq switch to boost position.
- Change signal frequency to find peak freq.
Test6: Equalization Boost
- Set Eq switch to flat position.
- Measure output level decrease.
Test7: Filter Range
- Set signal frequency to 50 Hz.
- Rotate pot fully CCW.
- Measure output level change.
Test8: Regulator Dropout
- Monitor DC at 8.6V regulator output, TP7.
- Reduce power supply until DC decreases .1V.
- Measure power supply voltage.
Amplifier
I chose to use a "2x18W" amp
because this provides a good match for "high-powered"
factory or aftermarket stereos. (For those systems using higher
power separate components, a larger power amp and subwoofer should
be considered.)
Another important consideration is "power
per dollar." The best power per dollar in automotive power
amps is usually in the "2x18W" (RMS) category. Power
per dollar is actually worse for higher power amplifiers because
costly DC to DC converters are required to generate the higher
voltages needed to provide the higher power output.
Speaker & Enclosure
To get the best value here, I recommend:
- building your own box, and
- using a Dual Voice Coil speaker.
If you prefer, premade speaker boxes
can be purchased from Crutchfield, Parts Express and MCM Electronics.
Installing your own speaker in a premade box can still be considerably
cheaper than buying a comparable preassembled unit.
A Dual Voice Coil speaker is a single
speaker with two sets of speaker terminals. It allows one speaker
to be used in place of two. This cuts the cost essentially in
half, and works fine if the total power rating of the dual voice
coil subwoofer is not exceeded. DVC speakers provide an excellent
way to reduce the cost of a subwoofer system. A single 8"-12"
DVC woofer is more than adequate for systems not trying to entertain
the neighborhood.
Choosing the right model of speaker
should also be based on specifications and value - not sex appeal.
I prefer Polypropylene cones because they offer good moisture
resistance. The speaker I chose was the Madisound 81524DVC.
I have used a number of Madisound woofers and have found their
specifications to be accurate, and their quality and prices excellent.
The 81524DVC is an 8" diameter polypropylene woofer with
dual 4 ohm voice coils. It is available directly from Madisound
and costs $34.
I chose a 1.25 ft3 ported box as the
best tradeoff between box size and bass response. This box volume
provides a cutoff frequency of 37 Hz. The box was constructed
of 3/4" thick particle board and the port was a 7 1/2"
long section of 3" diameter PVC pipe.
The computer-generated frequency response
of the Madisound 81524DVC in this enclosure is shown below:
Figure 7 - Frequency Response Diagram
Free Air Subwoofers
"Free Air" (or "enclosureless")
subwoofers are intended to be mounted on the rear deck of a vehicle
(and therefore use the trunk as an enclosure). This would seem
like a great way to save space and avoid having to build or buy
a box. The problem is, they don't work very well unless the trunk
is "sonically isolated" from the passenger compartment.
The sound produced by the back of the subwoofer is just as loud
as the sound from the front, but has the opposite polarity. If
the sound from the back of the speaker is not isolated from the
passenger compartment, deep bass will be "cancelled out."
Most trunks I have come across are not
very well isolated. You can check this by having someone with
a deep voice start talking to himself inside the car, then stick
your head inside the open trunk and listen.
A trunk with poor isolation can actually
be a blessing in disguise - it's the perfect place to put a subwoofer
box! I have used this technique successfully a number of times.
If you put a subwoofer box in your trunk, secure it with adjustable
straps or use Velcro to keep it from sliding around. This will
also allow you to easily take the subwoofer out when you temporarily
need the extra trunk room.
Installation - Hooking It Up
The figure below shows the wiring diagram
for a typical installation:
The subwoofer crossover should be connected
to the left and right speaker outputs of the stereo. If the stereo
has front & rear outputs, use the outputs which will be turned
up the loudest.
Since the subwoofer crossover has high input impedance, it does not load the receiver and full power is still provided to the main speakers. Heavy wire is not needed for this connection because of the high impedance inputs. The subwoofer crossover sums the left and right channels, so the left and right inputs are interchangeable. Since the inputs of the subwoofer crossover are isolated from ground, the plus and minus inputs are also interchangeable - just make sure the same polarity is observed for the left and right channels. This feature is particularly useful when speaker polarity is unclear. The phasing switch allows reversing the input polarity to provide the correct phase.
Use a short cable between the subwoofer
crossover and the power amplifier to prevent ignition noise pickup.
The subwoofer crossover sums left and right inputs, so both outputs
are identical. Avoid installing the crossover and power amp where
they would be subject to high temperatures (such as in direct
sunlight) or exposed to moisture or dirt.
Adding bass-blocking capacitors in line
with each of the main speakers is recommended (see the figure).
This reduces the burden of deep bass on the main speakers (and
the stereo too!), allowing higher volumes with less distortion.
The table shows the capacitor value to use (with 4 ohm speakers)
for popular bass-blocking frequencies:
Bass blocking capacitors should be of
the nonpolarized (bipolar) type. Use 135 Hz bass-blockers for
most applications. Choose a higher frequency when the main speakers
are weak in bass or unusually small.
Adding a choke coil in series with each
channel of the dual voice coil subwoofer (see figure) will prevent
power amp oscillation problems. The inter-winding capacitance
or mutual inductance of DVC subwoofers can cause some power amplifiers
to oscillate. The oscillation can be at almost any frequency,
from very low ("motorboating") to very high - even ultrasonic.
Ultrasonic oscillation is not directly audible, but results in
reduced headroom and increased distortion.
You can purchase choke coils inexpensively,
or wind your own. Twenty turns of #22 wire around a bolt will
provide about 20 uH of inductance - a good value. Make sure the
DC resistance is less than 0.1 ohms to prevent signal loss to
the subwoofer and insure adequate power handling of the choke.
The subwoofer crossover may be connected
to either the ignition +12V or the remote turn on/power antenna
+12V for power. The power amp should be connected to the ignition
+12V (or the battery +12V if a power amp remote turn-on wire is
provided and used).
Installation - Turning it on the First Time
Before turning the system on for thefirst time, turn the volume controls of the subwoofer amp and
the receiver to minimum.
Set the subwoofer crossover cutoff frequency
control to max (fully clockwise). After turn on, slightly increase
the receiver volume to check that sound is coming from each of
the factory speakers. If not, check for shorted or open connections.
If sound comes from each of the main
speakers, increase the subwoofer power amp level control. Listen
for bass from the subwoofer. If little or nothing is heard, try
setting the receiver's balance control all the way to one side.
If this produces bass, one of the subwoofer crossover's inputs
has been connected to the system backwards. If bass is still
not heard, check for +12V at the subwoofer crossover power wire
and power amp supply and remote turn on wires. Inspect for shorted,
open or reversed speaker connections.
Installation - System Adjustment
Proper system adjustment is just as
important as choosing the right components and hooking them up
correctly. In fact, a system with inferior components probably
sounds better than a system adjusted improperly.
The most important and most overlooked step in adjusting a subwoofer system is: set (and keep) the stereo's bass control at or below the flat (or normal) position.
This provides two important benefits.
First, it reduces the burden of bass on the main speakers and
stereo. (The subwoofer will pick up the slack!) This lets you
play the stereo louder with less distortion. Secondly, it provides
tighter, less boomy bass. Vehicle interior acoustics are notorious
for overemphasizing upper bass frequencies. By turning the bass
control down, this boomy sounding upper bass is reduced. By adjusting
the subwoofer crossover cutoff frequency for optimum sound, the
subwoofer will fill in with a deep, solid sounding bottom end.
Perform the adjustments in this order for best results. Use your favorite cassette, CD or radio station. Use several sources to get a good average.
CROSSOVER POLARITY SWITCH
Set to whichever position gives the
most bass. The effect is usually small.
CROSSOVER EQ SWITCH
The BOOST position boosts deep bass
frequencies to help extend the subwoofer's response. Use the FLAT
position if you ever hear distortion from the subwoofer.
CROSSOVER CUTOFF FREQUENCY and POWER
AMP LEVEL
Adjust the cutoff frequency control
of the subwoofer crossover and the level control of the subwoofer
power amplifier to provide the most natural sounding bass. The
bass control of the receiver should be set below the flat position
as explained earlier.
Conclusion
My completed "trunk box" subwoofer
system is pictured below. The subwoofer crossover and power amp
are mounted to the subwoofer enclosure. This makes the whole
arrangement portable for when I need the extra trunk space (or
park at the airport for a week). This approach could also be
useful if you have multiple vehicles or plan to get a new car
in the near future and want to reduce the hassle of transferring
the subwoofer system to it.
Figure 9 - The completed "trunk box" Subwoofer System Picture
In the introduction, I claimed a complete
subwoofer system could be added for less than $150. Here's the
grand total for my system:
speaker $34
box materials $10
amp $49
crossover kit $39
wire, bass-blockers $5
-------------------
total $137
Remember, to get the best performance
for the lowest cost, choose the right components and be willing
to do some work assembling and properly adjusting your system.
Don't underestimate the importance of a good subwoofer crossover
- use one with a steep cutoff slope and selectable cutoff frequency.
Most importantly, enjoy your music!
Sources
Crutchfield (car stereo)
1 Crutchfield Park
Charlottsville VA 22906
(800) 955-3000
Madisound Speaker Components (speakers
and speaker building supplies)
8608 University Green
P.O. Box 44283
Madison WI 53744-4283
(608) 831-3433
MCM Electronics (speakers, boxes, car
stereo, electronic components)
650 Congress Park Drive
Dayton OH 45459-9955
(800) 543-4330
Parts Express (speakers, boxes, car stereo, electronic components)
340 East First Street
Dayton OH 45402-1257
(800) 338-0531