Review - Velleman K4003

[JonM]

Paul Klipsch said it - What this country needs is a good 5 watt amplifier. Well, I think I've found a pretty good one, at a terrific price. Velleman, from Belgium, is known for their electronics kits (boy, who else makes electronics kits anymore?). Tube and solid state, some are fairly well regarded. They make little kits, too, ones with almost no parts, no cases, you provide the power supply - really, experimenters' kits.

The one I built is based around the Philips TDA1521 power amplifier module, really two high fidelity power amplifiers on a chip. Velleman takes this module and adds a little circuit board that carries a full wave rectified power supply (with 9400 mF capacitance - not bad), input blocking capacitors, an output load (I assume to keep the amp stable at RF), and a big heat sink. You provide the power transformer, the input and output connectors, and a case if you really feel like it. Velleman's part number for the kit is K4003.

Velleman makes some pretty bold claims for performance (2x10W at 8 ohms, 0.07% THD at 1 KHz/1 Watt, 98 dB S/N, FR 7 Hz to 60 kHz), so I downloaded the Philips datasheet, which is a bit more conservative (0.15% THD at 6 watts, 20-20 kHz power bandwidth) but which gives a full set of fairly impressive curves, especially when you consider it's all done, for both channels, in an IC not much bigger than a couple output transistors. (Oh, it even has a turn on/off pulse suppression, so there's no thump in your Khorns when you turn it on or off!)

So for $35 I bought one. (I could have paid less mail order, but immediate gratification was a priority :-) The kit comes with instructions in half a dozen languages, but assembly was a snap. It would be hard to mess this one up unless you really rush it and are simply careless. I mounted the amp on a scrap piece of pine - literally just held to the wood by a piece of copper wire across the heat sink. Inputs and outputs are via 22 GA copper bell wire. (See the attached picture - you can just make out the wood block, which was painted white, on top of a sheet of white cardboard.) Simple, expedient, fast and cheap. I wanted to hear what this thing could do!

I tried powering it with a $10 Radio Shack transformer (part #273-1512B - 25 VAC CT, giving two 12 VAC feeds, exactly what Velleman recommends and right at what Philips suggests), but that (cheap) transformer radiates such a hum field that I had to keep it several *feet* from the amp to keep the hum out of the music. Not practical. So I found a +/- 20 VDC supply that used to run some piece of office elctronics (a printer or router or something). I hooked it up to the same connections as the AC transformer. (So what if the DC gets "rectified" again in the amp's power supply? - that's no problem!). 20 VDC is near the 21 VDC upper limit of the supply voltage that Philips recommends in its data sheet, but it's within spec, so I think I'm okay. (And it's well within the K4003's filter capacitors' 25 VDC rating.)

Result? Ahh, quiet! In fact, putting my ear up to my Klipschorn tweeters, I hear virtually zero noise. I have to strain to hear anything! No hum, no hiss, no buzz - that's good. (Truthfully, when I moved that hummy transformer far enough away, the amp was equally quiet, so you don't need an outboard DC supply for a good S/N ratio, just a transformer with a low radiated hum field.) This is the quietest amp I have ever heard through my Khorns.

Power is plenty - only 60 ma of DC current was being consumed by the amp while playing at normal volumes (30 ma on both the + and - power feeds), so it looks like about 1.2 watts is going into the amp from its supply. Even if it's 100% efficient (which you know it isn't), I'm using less that a tenth of its output at what I consider to be full volume! And because of that, the heat sink never gets above room temperature. The heat sink Velleman provides is plenty - way over kill - when driving something as efficient as a Khorn. Well, better safe than sorry.

But what about the sound of music? Well, I've been using a stock Dyna ST-70, which gave me a sweet, slightly warm, and *very* listenable sound. The TDA1521 is clearer, more detailed, cooler without being dry or "transistory". Voices are very clear and smooth, instruments are well defined, front to back layering is the best I've heard to date on my Khorns. The lows are full and natural, less tubby and warm than with the Dyna; the highs are extended and clear. Purcussion instruments have a clarity and bite I never heard through the Dyna - they sound more "there", more real through the little Velleman. It's not perfect - there does seem some very slight roughness in the upper mids - the curves in the data sheet imply a class B output stage, and maybe I'm hearing a hint of grittiness from the crossover distortion often associated with no bias current in the output stage. But it's very subtle, and in reality, I may just be hearing imperfections in recordings (or my preamp, a 20 year old APT Holman) previously masked by my Dyna.

So am I happy? You betcha! It was inexpensive, easy to assemble, runs cool, seems completely reliable (the chip even thas two different thermal protection circuits and overload and short circuit protection!), incredibly quiet, and sounds really quite good. It goes to show that sometimes good things do come in small packages. Well done, Velleman! (Oh, and it will probably stay on that piece of wood - I get such a hoot out of seeing that tiny amp driving those huge speakers!)

PS - If you decide to build this, one thing Velleman left out that you'll need - transistor heat sink grease. I happened to have some, but not everyone does. And in my kit, the insulator between the IC and the heatsink had to be carefully trimmed to make slots for the heat sink's mounting bolts to pass through. These are both nits, but for a beginner, they may be confusing.

[Klipschguy]

Thanks for the post Jon. I've wondered how that kit would sound since I saw it in the Parts Express catalog.

Andy

[JonM]

I have no idea what commercial amps may use this part. It's low powered, so maybe boom boxes (or headphone circuits?). It's been out since '96 (the date on the spec sheet from Philips), and NTE makes a substitute, so it must have some users or they would have stopped production. It's probably lurking out there in more places than we realize :-) (I have seen places selling the part alone for about $6, for those who really want to roll their own.)

[JonM]

Oh, yeah, it was hum radiated by the transformer. When I moved it away from the amp by a few feet, the hum went away. I could change the hum level by reorienting the transformer. I couldn't mount the transformer on the same wooden block as the rest of the amp without the hum being unacceptable. That's why I say a transformer with better shielding and a lower radiated hum field is needed. If I'd known how bad the RS transformer was, I'd never have bought it.

The input impedance of the IC is 20 kohms, set by resistors internal to the chip.