Sunday, November 24, 2013

Improving the bass response of a (cheap) subwoofer amplifier system by *reducing* its bass response

Several weeks ago I happened to be at a local electronics type store (I'll admit it, it was Radio Shack!) and they had on clearance - for a pretty good price - a nice-looking set of computer speakers (40-288).  This set consists of a pair of small-ish speakers for the upper-bass, midrange and highs along with a single, larger-ish "subwoofer" for bass.  I didn't get these for a computer, but to reinforce the sound from a small, flat-screen TV that I have near my electronics workbench that has appallingly bad internal speakers.

Figure 1:
The inexpensive speaker/subwoofer system.
Click on the image for a larger version.
Upon connecting them to the TV I was immediately struck by the fact that they sounded OK - except that the bass sounds were frequently breaking up at moderate volumes during musical bass notes and the sound of explosions - which are very common on TV.  I flipped the subwoofer on end and noted that, as expected, it was a ported enclosure (see Figure 3) which is typical for low-frequency speakers, large and small.

Knowing the size of the speaker's enclosure - roughly a cube that is 11.5cm on a side internally - and also the diameter of the speaker itself - about 6cm - I also knew that it could not provide extremely low bass-frequency response.  Based on a guess, I figured that its usable frequency response would extend down to roughly 125 Hz or so:  You just can't get much lower than that with reasonable efficiency using a simple ported box and (inexpensive) bass driver that is that small!

The problem:

The problem with the design of this subwoofer is one that is commonly seen:  If the system is "asked" to amplify frequencies well below the range that may be reproduced by the bass driver and its enclosure, several things are likely to happen:
  • Power will be wasted with the speaker's cone flapping about and frequencies well below those in which it is likely to be able to move air efficiently.  What this means is that instead of working on frequencies that can be reproduced, much of the amplifier's power will be used up (e.g. wasted) on these other "useless" (to the speaker, anyway) frequencies!
  • The speaker itself may be damaged.  On a ported enclosure such as this, driving with too low a frequency, the speaker just can't transfer energy to the air mass efficiently and in so doing, its cone moves too "easily."  If this happens the speaker's excessive cone excursions can cause physical damage and heat can even build up in the voice coil assembly.  The latter is a bit less likely to happen with this small of a speaker and with the modest amplifier power level involved, but it is still possible.
  • It will sound terrible.  With the amplifier clipping, trying to amplify too much low-frequency range content that cannot be reproduced, and with the speaker itself flapping about trying to reproduce the low-frequency sound, you can end up with distortion, popping and buzzing.
Connecting it to an audio generator I swept from about 1 kHz down to 10 Hz or so and as expected, the crossover point from the smaller speakers to the subwoofer was in the general area of 200 Hz - not sharp, but definitely there.

I also saw that the speaker was still being fed power when I got down below 10 Hz, at which point uselessly flapping about as there was no way that it could reasonably be expected to efficiently transduce energy at that frequency - and that was the reason why it sounded like it was breaking up on low bass notes!  Careful observation revealed that the amplifier seemed to have a slight "peaking" effect in the area of 100-200 Hz - likely done to slightly emphasize the frequencies best conveyed by the subwoofer.  (I didn't reverse-engineer the circuit enough to determine if this was intentional or not.)

The "fix"

Figure 2:
 Bottom-side location of the capacitor to be changed along
with the added 10k resistor.
Click on the image for a larger version.
Popping apart the satellite speaker that contained the amplifier I started poking around with an oscilloscope while varying the frequency of the audio generator and quickly found where the wipers of the dual volume control went over to a pair of surface-mount 3.6k resistors and the audio from the left and right channels were combined ("3R1" and "3R2" in Figure 2).  I then followed the audio through a 2.2 uF coupling capacitor (mounted on the other side of the board) and then to the input of the audio amplifier for the subwoofer - a stereo chip configured to drive the subwoofer in bridge mode to achieve maximum power power output for the supply voltage.

Note:  I didn't remove the heat sinks, so I don't know which audio amplifier chips are used in this speaker system.

At this point something struck me:  In comparing with the oscilloscope, the audio level "before" and "after" the 3.6k resistors used to combine the left and right channel I could see that there was practically no difference, indicating that the amplifier itself minimally loaded the audio line beyond that point.  Having just followed this signal path I also knew that there was nothing that limited the low frequency response of the amplifier to something within a reasonable range of what the speaker itself was likely to be able to reproduce!

To satisfy my hunch I replaced the 2.2uF capacitor with a 0.022uF capacitor and noted that it only just started rolling off the frequency response below 100 Hz, indicating that the amplifier's input impedance was likely in the range of 50-100k, so with the 2.2uF coupling capacitor, the amplifier was going to amplify signals down to less than 1 Hz with minimal rolloff! What I needed to do was to limit the frequency range of the amplifier to something more reasonable in terms of what the speaker was likely to be able to reproduce!

To do this, there are two reasonable options:
  • Build a nice, multi-pole high pass filter that will sharply cut off the audio below a certain frequency - say, 100 Hz.  This would require either a transistor or two or an op amp along with a handful of other components and would be built upon a small circuit board that was added into the enclosure and connected inline with the subwoofer amplifier's audio path.
    Figure 3:
    The bottom side of the subwoofer cabinet showing
    the driver and port.  If you throw too-low a frequency at this sort
    of speaker it just thrashes around and doesn't really
    produce much sound.  It's always  best to send only those frequencies
    to the sub's amplifier that the speaker will be able to reproduce!
    Click on the image for a larger version.
  • Just kludge it and make a simple R/C high pass filter.  This wouldn't as sharply cut off the low frequencies, but it would likely do the job of preventing ridiculously low frequencies from getting to the amplifier and cause it to waste effort!
I chose the latter.

In poking around on the audio input pin of the amplifier I saw that there was no DC offset, so I temporarily connected a 10k resistor between it and what appeared to be a nearby ground - at least, it was where there was a capacitor connected across the input to roll off the high frequencies above which the subwoofer was not supposed to amplify.  Temporarily tacking the 2.2uF back into place, I saw with the oscilloscope that the 10k resistor made almost no difference the subwoofer's output level and that its output remained clean.  I then made the 10k resistor a permanent part of the circuit, soldering it on the bottom side of the board as can be seen in Figure 2.

Grabbing a calculator I crunched a few numbers and decided that a 0.1uF capacitor (a nice, round value) in place of the original 2.2uF capacitor would be worth trying as it, in conjunction with the 10k resistor, would provide a -6dB roll off frequency of about 159 Hz - a fact confirmed using the oscilloscope and audio generator.  With the amplifier's slight "peaking" noted above, the power wasn't down by 6dB at 160 Hz, but closer to 100 Hz at the -6dB point.  (I used a plastic capacitor rather than a ceramic capacitor because the latter would have had terrible temperature stability.)

Feeding some music with a lot of low bass into the speaker system, it seemed to sound just fine:  Reasonable low-frequency response and no obvious distortion - even at fairly high audio levels.  Tacking the 2.2uF capacitor back into place the perceived bass response improved slightly, but now the audio amplifier was breaking up badly with obvious clipping and distorting as before.  Taking the 2.2uF capacitor off again I tacked another 0.1uF across the first (for a total of 0.2uF) to set the hypothetical -6dB frequency to about 80 Hz and could hear a very slight increase in amplitude of the bass notes and a bit of occasional clipping at fairly high volume, but it didn't seem to be worth it to have the extra capacitor on there so I left it at just 0.1uF.

Figure 4:
The completed modifications - along with the added
heat sink material.  This is just a scrap of copper - probably from a
piece of water pipe - that I cut down the side, cut tabs to match the chip's
"heat sink" and ground pins on the foil side of the circuit board, and then,
using a very hot soldering iron to get the job done quickly, attached
it to the circuit board.  The foil side of the circuit board faces up
within the box so the heat actually radiates better this way!
Click on the image for a larger version.
While having the unit apart I noticed that the heat sink of the subwoofer's amplifier chip was far too hot to touch after a few minutes of abuse from the signal generator.  Typically, these sorts of chips have built-in thermal protection, so it was not too likely to be damaged by getting hot, but this thermal protection often works by causing the amplifier to cut its power back - usually causing distortion.

To be sure, my reducing the frequency response of the amplifier greatly reduced that amplifier chip's thermal load, but I decided to solder a bit of scrap copper to the heat sink fins on the "bottom" side of the board (which actually faces up when the board is installed) to increase its heat dissipation ability.

Now, with the very low frequencies eliminated from the amplifier it could put more power into reproducing those low frequencies that were well within its capabilities without wasting it on frequencies that were too low.

Putting the entire thing back together, it now works fine in its intended role:  As a half decent sounding speaker system for the small TV!


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