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The JBL "Bass Wave"
amplifier
The JBL "Bass Wave" amplifier is a small 100-watt
amplifier with built-in active filter with a single-pole high-pass at 10 Hz
combined with a single-pole lowpass at 85 Hz.. It costs an amazingly low $50
US. It also comes with line and speaker-level inputs and a volume control
for level-matching, and an "auto signal sensing power switch". It also
features reasonable build quality. However, it is not an "audiophile" or
even hifi-quality amplifier - a few corners have been cut in the design
in order to keep the price so low. However, it may be possible to improve it
a bit with a few tweaks.
So, what's "wrong"?
Ok, so what's wrong with the Basswave? Well, I've got
some of the "faults" listed below:
- The Bass Wave amplifier uses a "class-B" output
stage. Most audio amplifiers use a "class-AB" output stage because class-B
circuits are prone to crossover distortion, which audibly affects the
output at low volumes. The notch distorion may show as a fuzziness or
raspy character at very low levels. Note that this is of less consequence
in a subwoofer design, as the effect is less noticeable at low
frequencies, and how many times have you listened to a subwoofer at low
volumes?
- The output devices in the Bass Wave amplifier are used
in a common collector configuration. While this raises the voltage
gain of the stages (and is a cheap way to save parts), it also raises the
output impedance, which in turn reduces the current dumping and damping
factor of the amplifier. The voltage gain should come from the driver
stage of the amplifier, not the output devices. (Correction - Jason Cuadra says that it is a
composite 2 stage common emitter with local feedback set by R31 and R30,
setting gain to 10).
- There is no short-circuit protection in the output
stage of the Bass Wave amplifier.
- Power output seems a bit low for a subwoofer
amplifier.
The schematic for the Bass Wave amplifer is available at
the following location:
http://www.diysubwoofers.org/projects/other/basswave/BASSWAVE.pdf
Bill Wilson of
the DIY Loudspeakers List described the circuit design of
the Bass Wave amplifier as follows in message to the List (which I've edited
slightly here):
" A low pass input network feeds the main
amplifier which is a LF347 op amp driving a modified darlington current
amplifer which is wired to also give voltage gain. The LF347 is a nice op
amp with gain bandwidth product of 4 Mhz and FET inputs. There is no
quiescent current biasing of the output transistors. The bases of the 1st
stage output transistors (TIP31C and TIP32C) are copper bound to each other
giving 1.2 volts of crossover distortion. There are no emitter resistors
anywhere, not even in the 2nd stage TIP35C/TIP36C emitters. As far as
driving 4 ohms go, the TIP35C/TIP36C transistors have the current rating to
handle this but I don't know about the power supply. There are 4700uF
capacitors filtering the rectified DC and I would want to load test the
transformer before feeling safe about it. There is a thermistor on the heat
sink wired to an op-amp comparator which will shut the amp down at some
temperature, but note that there is no short circuit protection on the
output transistors. There is an audio signal detection circuit that drives
the front panel LED, but the power amp insided is not powered down, just the
LED. "
Modifications (proposed):
Listed below are some modifications to the amplifier
that have been PROPOSED by members of the DIY Loudspeakers list. These
modifications have not actually been tried out, but they do sound good
enough for consideration:
- Jason Cuadra:
" To increase output current capability, double up the filter caps, change
to beefier rectifiers. ( a la 5A 200V ) and double the output
transistors(in parallel), but with an emitter resistor for each one of
about 0.1 ohm, to make them share current. To bias it into class AB would
require re-designing the whole stage: - It would require an additional 2
power resistors, 2 smaller resistors, 2 constant current sources each made
of 2 transistors and a resistor, and a VBE multiplier stage made of a
transistors and 2 resistors; reducing the gain of the output stage (from
10 to 3 or 4), and re-checking the feedback loop stability. "
This is a circuit showing some of the some
mods I talked about, with the addition of current limiting / short cct
protection, and this is another circuit showing the actual modifications
to be made to the amplifier. Warning, the short circuit protection scheme
is untested. It may oscillate during over current conditions or may not be
set to a low enough current to protect the output devices. For short
circuit protection, emitter resistors ought to be more like 0.15 ohms
each. Current limit in each output device will be set to 0.7V / 0.15 ohms,
or about 5A each. Additional transsistors to do the current limiting can
be TIP31A and TIP32A like the driver transistors.
- Richard Hall: (see
diagram of modified circuit for reference)
" This is just a change of the design as I would start IF I owned one. The
resistor valuse are pure guesstimations. As I have no data books on hand
and do not have one of these amps, I can only pick values from other
similar designs. But the point being that this approach should fix the
lack of proper biasing on the amp. Also there is an additonal supression
network on the output to keep things from oscillating in the event that
the amp should show any sign of instability. R1003/R1005 give a little bit
of stability to the drivers by inducing a small amount of feedback into
the stage and help to stabilize the biasing. The diodes should be
insulated and mounted on the heatsink. I place heatshrink tube on the
leads, dip them in epoxy to cover the diode and the leads at the body and
them let it set until soft and moldable, then stick them to the heatsink
to form a flat spot on them for better heat transfer. When the epoxy has
setup hard, I put some Wakefield compound on the flatside and then clamp
them down with a small spring clip. The bias pot should be adjusted in the
center and should get about 20-30 ma. across R1008. The value of the pot
should work out to something in the 250-500 ohm range... maybe as low as
100 ohm. If the fine adjustment is not needed/wanted, then leave the pot
out, in which case the final current gain to the drivers is set by the
R1001/R1006 pair. As I do not know what type of current gain is needed in
these stages/transistors, these values may be off by a fair margin. I
would not play with this without a variac or adjustable power supply to
bring the amp up slowly to monitor the output current. If someone has
balls, (grin), they may want to just use some small value fuses... like 1
Amp. The circuit changes should be fine. Again the final value of the bias
pot and the resistors R1001/R1006 are total a guess. But with some
experimenting the correct values should be fairly easy to figure "
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