Hacking Amplifiers
Side note: A number of friends have commented that my blogging completely stopped a couple of months ago. I’ve been busy with a new role at work and haven’t had as much online time recently. I have a lot to blog about, but less time to do it in. Expect a few entries rapid fire style over the next few days.
I’ve been wanting to make it easier to use the speakers in our kitchen and outside and at the same time reduce our power consumption for doing so. I had been using a single amplifier for either set of speakers and manually switching between speakers depending on which pair I wanted to use. Turning them on involved using the “second room” mode of our receiver even though I always had it playing the same sound as what was going on in the living room. This made it more complicated than necessary.
Getting away from manually switching speaker wires meant getting another amplifier. I had another use for this one, so getting a single amplifier that could handle 4 channels (stereo in the kitchen, stereo in the backyard) was the best solution. Commercial 4+ channel amplifiers are expensive, even when purchased used. However I did find a small kit made by a company called 41hz which promissed to drive 4 channels at about 50W each (way more than enough for background music speakers). The kit uses a Tripath chip, and I know from prior experience with the Teac A700LP amplifier that they sound very good and are efficient (so they use less power and run cooler). The kit also had a small number of parts which seemed good considering how long it had been since I soldered anything. At about $75 shipped for the kit, plus a transformer, case, and some connectors it was much cheaper than buying a commercial amplifier. So I ordered the kit and it showed up at my door a few days later. I also ordered a temperature controlled soldering iron, a cheap case, a transformer, and all of the connectors that I thought I’d need. At this point the total cost was at about $200… a little scary since I didn’t have anything that worked,but still cheaper than commercial amplifiers.
I spent a quite Saturday building it. It went together quite quickly,the most painful parts were surface mount soldering 16 diodes and winding 8 toroids by hand. By the end of the day I had this:
Iwas happily surprised when it worked brilliantlythe first time that Ipowered it up.I used an old power supply from a long dead set of computer speakersfor testing because my transformer had not yet arrived. It sounded great with my simple basement test.
About a week later the transformer showed up and I got the whole thing installed into it’s case. I forgot to account for connector sizes and the size of the transformer when laying out the case and it turned into a very tight fit, but it did all fit. I also changed from using banana plugs for speaker connectors to Neutrik Speakon jacks, leaving me with some extra “ventilation” holes in the case. Overall I could have done a much better job of laying out the components in the case. The next one will look prettier and luckily no one sees this one because it lives in the basement.
Here is what it all looked like when assembled and connected to a single set of speakers:
and on the inside:
As you can see the toriodal transformer takes up the vast majority of the space. The board is hidden under the wires on the foreground. The heatsink was salvaged from an old Pentium MMX processor. That case is 10″ wide, 6″ deep, and 4″ high so the whole thing is pretty compact. Since it is living in the basement I put the switches, connectors, and volumepotentiometerson the same side, making it easier to setup and adjust.
When finally assembled I measured the power consumption at 7 watts when there is no input signal. That is a massive change from 80W on the old amplifier that I was using. That’ll save us about $5/mo in power bills.
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While building all of this stuff I remembered reading that the $100 3-channel Teac A700LP amplifier that we use for the bathroom speakers is actually a 4 channel amplifier with only 3 channels connected. I decided to explore the insides and found that there are two 2-channel circuit boards for amplifiers and a mainboard that connects them to the chassis and input/output connectors. The mainboard was very well labelled and this made it easy add the 4th channel. I just needed to add an output and wire it all up. I didn’t even need an input because I wanted to use 2 inputs on this amplifier for 4 channels (two L outputs and 2 R outputs). I just tapped off of the existing L channel input. A couple of cheap parts and a 30 minutes with a soldering iron and I had this:
I just made a couple of modifications to the case to expose the new potentiometer for the 4th channel and the speaker outputs for the 4th channel. This time I was much more careful with my drilling and the end results look pretty good:
The power supplyin the Teac A700LP is a little underbuiltand wouldbe taxed if I ran all fourchannels at full power.For backgroundmusic use this isn’t much of a concern, it is rarely going to be used at anything other than low loads.
So there we have it.A few evenings of soldering and hacking and I got 2 4-channel 50W amplifiers for just over $300 total (including the $100 that I paid last year for the A700LP). They power the speakers in our bathroom, kitchen, upstairs deck, and backyard. The soldering projects were fun enough that I’m now considering making a small amplifier for work built around the 41hz AMP6 kit.