Crappy Desktop Speaker Mods
I bought some cheapo Genius PC speakers (SP-HF360A) hoping they would be good enough to plug into my audio interface and monitor guitar when I don’t want to use the headphones. Each speaker has a single full range 3″ driver and one of them has the stereo amplifier circuitry with volume and tone controls. Unfortunately, there’s no bass control and bass is cranked all the way up. I guess they wanted bass to impress and associate with warm qualities or wood or whatnot, but it’s terrible. I knew they were crappy beforehand, but I was happy to get them at least for the MDF boxes and power transformer. Making my own would cost much more in time and parts. As expected, they sounded so bad that the guitar could be barely heard (what was I expecting to get for 20 euros). They have way too much bass and no clarity at all. Just one massive fart of a sound. For stage one, I decided to gut the electronics out, use the existing power transformer and whip up some simple power amplifier with a neutral frequency response.
Being limited by the 9V@1A power transformer that was inside one of them, only stereo amps up to 2-3W were an option, so I got a LM1877 chip-amp. I started with the datasheet schematic for amplifier gain Av=200, which is way more than I need to drive line level signal from the interface. So I changed the feedback network from 100K+510R+10uF to 100K+4.7K+1uF for gain around 20 and low frequency response down to approx 34Hz, although that’s well below what drivers can produce. Also reduced input capacitors to 0.047uF (only had that value), which should be plenty anyways. Decided to keep it simple and not install any controls as my interface already has output volume control.
As far as the power supply goes, I just rectified AC and put a C-R-C network (820uF, 1ohm, 470uF+0.1uF film) to filter DC. After rectification and filtering, the power supply provides around 14V DC for V+. LM1877 works with a wide range of voltages and 14V falls right in the middle of the range. With that voltage, it can output around 1.5W per channel.
For the circuit, I used two of my favorite proto boards – compact 1×1″ 3-per-hole boards I bought from MeasureExplorer. The first board hosts the power supply and the second board contains most of the amplifier circuit – LM1877 chip, copper radiator fins, two 470uF capacitors, one 47uF capacitor, two large 1uF poly block capacitors, two Wima input caps and half a dozen of resistors (some on the other side of the board). It was a challenge laying out all the components in such a small space and it turned out there was no room for the two Zobel networks (2.7ohm+0.1uF) on the board, so I mounted those components on the terminal strip where I connect the output of the amp to speakers. Boards are held in place with a screw that goes in the MDF back cover.
You will notice three copper rings on the photos. These are supposed to act as radiator fins. Middle three pins of LM1877 chip are left for cooling, so I soldered three U shaped 1.5mm pieces of copper wire directly to the pins. They should help increase the surface and dissipate heat faster.
It fired up OK from the first try, which was a relief. Debugging that compact board would be a nightmare. It sounds much better than the stock amp. More neutral with no annoying bass boost. It was a good decision to set the gain to 20. My interface which has line level output has usable volume control up to maybe half. Over that point it starts overdriving the LM1877 in a rather unpleasant way. That means that we have room to hook up audio sources with weaker output. 2W is plenty loud for the application. I was a bit worried about that, but it’s even louder than I need. One of my concerns was heat dissipation, but those copper rings seem to be enough. At normal listening level (TV level, maybe slightly louder), it gets barely warm after an hour of continuous playback.
As far as the frequency response, Genius makes ridiculous claims 20Hz-20KHz which I can’t get with speakers that are 10 times the size and 30 times the price. For an empirical test of the frequency response, I hooked it up to my phone’s headphone jack and ran a (free) signal generator app, manually sweeping from 20Hz to 20KHz. It’s hard to tell how flat the frequency response is without a reference microphone, but it’s very easy to hear where it drops off. There’s a sharp bass roll off around 100Hz. Pretty much everything below 60-70Hz is inaudible. In the high frequency range, it’s good up to maybe 12-15KHz, but it starts declining as soon as 2KHz. To get them to sound semi-decent, I added an EQ at the end of my effect chain in Reaper that cuts some lows and boosts highs.
I’m happy with the project. The amp cost about 10$ and few evenings of work. It’s decent for what I need, these can act as reference monitors as they approximate the average speakers most people have on their computers, TVs, cheap stereos. I may consider upgrading the drivers in the future.
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