Fun with a kit amp

My son loves to sit with me and work on anything electronic. My daughter likes this too – she’s soldered at least six or seven kits – but my son is absolutely driven, and asks me all the time if we can do some kind of project, whether it be tearing apart old hard drives, cracking open wall warts, or building kits.  He also loves mechanical projects, even riveting a shoe back together.  I keep my eye out for good projects to do together.

The Gobo Stereo Audio Amplifier Kit from Boxed Kit Amps looked the ticket. It is inexpensive at $100, which includes the laser-cut case.  There are three separate boards (power supply and linear amps for L and R) so we could do a little each day over the course of a week.  Here is our finished product:

The case is attractive, if a little delicate.  The flex hinges look great in transparent plastic, but although they look like they provide airflow, they don’t really.  There is, of course, no shielding that a metal case might provide.

The instructions are nicely done. (There was a minor mistake in the URL emailed for the PDF, linking to the wrong file, but squinting at it made the mistake clear.)  They emphasize safety, and being systematic to avoid mistakes.

It was amusing that the photographed examples were done on a Silpat silicone baking sheet exactly like the one we were using.

Here are my comments on assembly:

The transformer leads are a bit confusing. Throughout the instructions one has to make choices about how long to cut wires early, before well-informed choices about routing in the case are possible. It will eliminate a lot of anxiety if the next version of the instructions give a suggested length for each wire that needs to be cut.

The most aggravation came from getting stranded wires into their PCB holes.  The plated holes are exactly the width of the pristine wire if it is flawlessly stripped.  The smallest eccentricity, twist, or bend to the strand bundle will not allow it to fit through.  I usually ended up having to clip off a fraction of the strands, and then soldering both front and back of the board to try to recover some structural integrity from the lost strands.  Widening the holes slightly would go a long way to decrease the frustration.  EDIT: they’ve now fixed this by reducing the wire size, see the comments.

Assembly was otherwise straightforward and there are very few mistakes in the manual.  The “Options and Improvements” section needs work.  I put a neon bulb in the power supply board as we went, which was specified as optional on the schematics and appeared sensible at the time, but turned out to be in the way of wiring later.  I chose to install a pair of optional MKP1837 bypass caps in the power supply board, which was recommended in the manual and on their forum.  The bummer was paying overpriced shipping for the otherwise inexpensive caps from Newark.  I ordered two extra caps, so if you chose to do this kit, I’ll mail them to you.

The amp sounds fine but I didn’t carefully test it.  I did a fast comparison using Room EQ Wizard, a Dayton Audio UMM-6 mic, some cheapo TV speaker modules, and a comparable 50W AudioSource AMP-100 amplifier.  I simply swapped the amplifiers, keeping as much as possible the same, and measured the frequency response with each.  Volumes are not comparable, but the ratio ought to be the same across the range.

This graph shows the two measured responses averaged at 1/3 octaves, and their ratio:


The speakers can’t reproduce the low end so those wiggles are just noise, but that 5db lump indicates something is wrong.  I didn’t bother track it down yet.  I kind of doubt the amp is the problem – so many variables are poorly controlled here, and I lean towards blaming resonance in the speakers, since they were operated at different volumes.

In conclusion, this kit was well worth the time we spent on it, and I recommend it.  While you can buy an equivalent amp of superior construction for about the same price, you won’t feel your time wasted here.  My only regret was swearing in front of my son when dealing with stranded wires into small PCB holes.

2 responses to “Fun with a kit amp

  1. That’s so cool! We really love getting kids involved, it’s one of our primary motivators. I really apologize for the mess-up with the wire. We are now packaging a slightly smaller diameter (18ga as was originally specified). I know it is negative fun to have to struggle with that. The first shipments simply had the wrong wire included. But, thanks for your (otherwise) positive review. 🙂

    BTW I have seen this drop-off in bass response. It might not be your speakers. It’s more likely an impedance mismatch between the amp and the source. It is partly the amp’s fault because of its “passive” pre-amp. The impedance of the source forms a high-pass filter with the input decoupling cap. I have seen cutoffs as high as 100Hz(!) which matches your graph. What is your source? I might be able to recommend a fix. -Ken

    • Thanks for the fast response, Ken – it’s great to see a company so responsive to customer feedback.

      My measurement was really just a heatbeat check (do I hear any obvious artifacts, and does it get hot when turned up?) I have not yet put this amp through its paces in any generally useful sense. In this particular case, the source was routed through the cheap sub’s passive high pass, doing untold additional damage to the low end.

      The conventional 1uF cap at input should indeed affect the low end response. For usual listening I use phase matched active subs, equalized through a DEQ2496 (room correction alone demands this). You would be horrified to see the speakers I used to generate the graph. A better test will require much better speakers, or building a decent test setup using an oscilloscope and resistive loads, both of which I still aspire to.

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