Lo-Budget, Hi-Fidelity

8 Jul

I have a lot of tools in my workshop, but no music. I don’t really care, most of the time, as so often it would be drowned out by the hum/whine/screech/roar of machinery.  But still… It would be nice. Too bad I’m a cheap bastard and can’t see dropping cash on the problem.

Here’s what I finally came up with.  The centerpiece is a kit based on a TDA2005 car stereo amplifier chip and everything else is junk laying around the house:

Click for a close-up view
(click on the picture to see it in hi-res)

Now, before you read on, understand that some electronic devices were harmed in the creation of this thing, so if you’re squeamish about the interior of electronic gadgetry, it might be best if you stopped reading now. Also, since we’re on the subject of full disclosure, I have to admit that some innocent roly-polies were inconvenienced, bothered, and possibly squished.

The Genesis of the Solution… Or Maybe the Problem

I’ve been a cheap bastard for a long time, you understand.  I definitely was one when my beloved Nakamichi receiver that was plugged into my computer’s line-out jack finally failed.  That receiver wasn’t cheap, but it was purchased when I was young and felt that stereo equipment was a perfectly valid way to blow money.  Advancing years and aural damage have convinced me otherwise.

Seeing as the speakers were fine, I figured there had to be a way to get a cheap little amplifier. But no, it wasn’t so easy at all to find a decent amplifier for cheap. But then I found this kit for less than $20. Sold. Where’s my soldering iron?

So like I said, that was several years ago. I finished building the kit right around the time my first child was born. The finished amp was a glorious thing, dangling from a hacked up power line in my PC case.  But alas, I hardly used it, as opportunities to sit at my computer were usually concurrent with our baby’s sleep time, and, at such time, silence was golden, so I started wearing headphones. That habit stuck. I still wear headphones almost all the time I’m at the computer, and so that bodged up amplifier lay in my storage arrangement for all things electronic:

Highly organized stuff

As you can see, it’s all very neatly arrayed and carefully organized, and the old amplifier just happened to come into view as I was fetching a network cable out of there recently. A cunning plan emerged to use our family’s new laptop as a music source to this amplifier, deployed to the shop.

The Pot Problem

So when I said it was "working" when it was in my computer years ago, maybe I stretched it a little. It had a strange habit of cutting out the left speaker, which I was never quite able to figure out back then, but at some point the problem just seemed to go away and I left it at that.

After I dusted the amp off and cobbled together some connectors and a power source, that problem came back. Sigh. I set about debugging the problem, which was tough since my electronics toolkit basically consists of a soldering iron and a multi-meter. (In layman’s terms: a stone knife and some bearskin.)

Fortunately for me, the problem lay in something simple, the potentiometer that controlled the volume, shown below:

A bad Pot

So I know what you’re thinking, one of the leads is chipped off, but that’s just because I had to resort to violence to get it off the board because neither a stone knife or a bearskin is a solder-removing tool. I have one of these things. Somewhere. The only thing I know for pretty much certain is that it’s not in any of my usual highly organized containers. So, like I said, I had to resort to force.

A potentiometer is basically this circuit:

A Basic Potentiometer Circuit

Only in this case, it’s a stereo potentiometer, where you’ve got one knob controlling two circuits just like that, one for the left channel and one for the right. One channel wasn’t making proper contact – if the knob was twisted to max volume (where the full 10k resistance to ground was in effect), then it worked right, but anything less than all the way to full caused the resistance between R1 and ground to go to zero, effectively cutting the volume out of that speaker to zero. When I had it dangling in my computer, I guess the pot volume was in a sweet spot where it was working, and in any case, I was running the volume from the computer anyway.

But that wouldn’t do for this application, as the laptop gets plugged into a variety of places, and I wanted it such that when I plugged the laptop into this amplifier I wouldn’t have to fiddle with the volume.

So I suppose a rational individual would have gone off to find a replacement pot. Who knows, maybe radio shack has one. But it was sort of a point of pride at this point that I was going to just use what was cheap and at-hand.  In any case, I really didn’t need to be fine-tuning the volume; the computer could do a better job of that. I could get by with something simpler, like a switch. But not just any switch, no, I happened to have this masterpiece laying around:

The Big Switch

Is that awesome or what??  It throbs, nay pulsates with Steampunk-era cool, doesn’t it??  I mean, seriously, where could I have pulled a switch like that from?!!

Frankenstein Switch

Oh yeah… That…

Resistance is Futile

So here’s the switch, all wired up:

Finished Switch

Delightful, isn’t it? 

Basically, it’s got three volume settings. With the switch up at the top, as shone above, it runs from the input, through a 1k resistor, up through the middle posts, and then back to the amplifier where a very small resistor is waiting to take it down to ground. If you flip it to the bottom, the signal passes through the medium-sized resistor shown in the picture, then to the little resistor, and then to ground. That’s the setting you want to use if you don’t mind annoying the neighbors a bit when the computer is set to max volume. If you don’t close the switch at all, then there’s no path to ground and, well, that’s your punk rock setting right there.

Of course finding all these resistors was a simple matter, seeing as how I’m so organized:

Okay, so really, this mess isn’t my fault. Radio Shack did it. A long, long time ago, I know this because I got it from my Grandmother’s late second husband. Bless his soul, he wanted to be a geek, he really did, but, well, his multi-tester is still in the shrink-wrap:

And he saved the receipt. Do ya think Radio Shack would take it back?

So anyway, I found some resistors with values that worked out somewhere in there.

Scottie, I Need Warp Drive in 3 Minutes or We’re All Dead

So there’s a lot to like about the amplifier kit I bought, but its hook-ups aren’t one of them. Input jacks are these awful 3-post screwdriver affairs and the power consists of a pair of leads soldered to the board. Now, as you can see from the previous pictures of my box full of crucial stuff, finding a wall-wart power supply that could deliver the requisite power was no problem at all… I’d just have to cut off the power cord and wire-nut the leads on there. But cutting the power cord on something as valuable and reusable as a wall wart power supply? Sacrilege.

Now it just so happens that we recently had to replace our phone… And it turns out that the power supply for that phone used the same jack as the one that I wanted to use for the stereo. So. I had to augment the toolset with a hacksaw to produce this result:

Yup, it was a bad ending for that phone.  It had it coming; trust me.  Oh, and I saved some screws and a few other fiddly bits for, well, I’m sure it’ll come in handy…  Anyway, today it was all about the power plug:

Destroyed Phone

And hey, seeing as how I’m already this far in, there’s no sense turning back now. How about I apply this RCA Jack farm to the problem.  I imagine some pre-amp from ages past had to die for that thing.  I don’t know where it came from but it’s a good fit:

The amp kit just has these awful 3-stud screw on ports that are very fiddly and, again require you to tear up a perfectly good wire to use. Why they couldn’t put on a female 1/8" jack, at least for the input, is beyond me. Anyway, I’ve got lots of 1/8" to RCA jack adapters.

Really, lots. I found like 6 of them in that box. Who knows why I have that many. And happily a one of them was one I soldered myself back in the wayback.  I imagine that somehow I couldn’t find the other 5.  Must have been before I got organized.  Anyway, I un-soldered it and took the RCA plugs out of that and soldered them into speaker wire, so now I have a much nicer plugbox.

About those Roly Polies..

So you noticed that high-quality mounting board I chose? Yup, only the finest for this project. It was hand-selected plywood from a pile of various stock left by my home’s previous owner. Digging around in this pile dislodged a considerable number of invertebrates, much to the delight of my children.

Somehow it became the plan to use the bulk of the sheet, plus some leftover window screening to construct this fine habitat for all the roly polies and whatnot that my children capture:

Roly Poly Hacienda

As you can see, they contributed the paint job and screen trimming.

They’re available for hire, by the way, if you, say, need them to give that treatment to your living room…


One Response to “Lo-Budget, Hi-Fidelity”


  1. Shop Stereo 2.0 | Indisputable Facts - April 24, 2016

    […] music in my shop comes from a laptop running Pandora pumped through a kit-build amplifier.  I definitely think no woodshop is complete without a SketchUp-capable laptop, but there are […]

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