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J.R. Buchanan

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Velleman MK190 mini-kit

A "5 watt" stereo amplifier

Using the TDA1517 audio amp IC

July 2015

amp
amp package

Introduction

I'm working on a project that requires a small low power stereo amplifier. While I was at Fry's I saw the Velleman MK190 kit. It claims to be a 5W (into 4 ohms) stereo amplifier, originally intended for use with an MP3 player. It was inexpensive, so I decided to try it.

Assembly

The kit comes as a small circuit board and a bag of components. No schematic or real assembly instructions are supplied. It was fairly easy to determine which parts went where due to the silk screening on the nicely made circuit board. Still, despite the kit being identified as "easy", I would only recommend this kit to someone who is confident with electronic construction.

Data sheet info and circuit

The amp sticks very close to the reference circuit in the data sheet, the only difference is that a 10K resistor and an indicator LED are connected between the standby pin (8) and ground and a rectifier diode is used to block possibly reversed power. This seems odd for two reasons, the data sheet claims that the device will not be damaged by reverse power, and the diode lowers the supply voltage by nearly a volt, lowering the power output of the amp.

Schematic

The data sheet specified a supply voltage between 6 and 18V. The Velleman kit suggests 6 to 14V. The nominal supply voltage in the data sheet is 14.4V rather than the 12 or 15 volts I would expect. The IC is claimed to have an output of 6W into 4 ohms with 10% distortion (10% ??!!) and 5W at a more reasonable .5% distortion. The Velleman amp produces a claimed 5W of output per channel into 4 ohms, presumably at the specified supply voltage of 14V. The data sheet claims a low frequency cutoff (at -3db) of 45 Hz, dependent on external circuitry. They're talking about the 1000uF output capacitors here. The high frequency cutoff is oddly specified at -1db. It is claimed to be 20kHz. The device is supposedly thermally protected, which is a good thing, because at full power in both channels with a 4 ohm resistive load, the package approaches, if not reaches, finger blistering temperatures. I don't have a good way to measure this, but I'm putting on a heat sink before assembling the final project.

Testing

View of amp being tested

After assembling the amp, I wanted to test it. First to see if it worked, second, to see if it lived up to its claimed specifications. The brief answer: It didn't manage to meet its claimed specifications. It is however fine for what I intend to use it for.

First I hooked up some resistors for a load, I used 8.2 ohms for part of the testing and 4.1 ohms for other parts of the test. Then I connected leads to attach a variable bench supply. These were connected to the terminal blocks on the amp. I didn't have the right adapter to connect the BNC output of my signal generator to the 3.5mm input jack, so I soldered some wires to the bottom of the board. This resulted in a problem. Whenever I connected the generator to the board, the generator was shorted out. An ohmmeter verified this. I found that I had to plug an otherwise unused cable into the jack, with no plug inserted, contacts in the jack short out the input when nothing is plugged in.

The first thing I wanted to test was power. I doubted it could put out 5 (or 6) Watts per channel. With a 14V supply, reduced to about 13V by the diode, the output can only swing about 13V peak to peak. That comes to 4.6 volts RMS, assuming a sine wave. That will indeed come to 5.3W per channel -but. Without a bootstrap circuit to pull up the base bias resistors of the output transistors in the IC well above the supply voltage (or a separate bias supply with a higher voltage than the power supply voltage, but who'd want to do that?), the output will *not* be able to swing all the way up to the full supply voltage. There just won't be enough base current available. For more details on this, see my article on a discrete power amp I built 20 years ago. Look at C4 and D2.

How much power did it put out? With no visible clipping an a 1kHz sine wave, I got this:

  12V 14V
4 ohms 2.5W 3.5W
8 ohms 1.3W 1.9W

A far cry from 5W per channel, but about what I'd have estimated. I'm going to use 8ohm speakers and 12V, so I'll get less than 1.5W per channel. It should be enough though. I am rather surprised that the IC maker didn't supply 2 more pins so that a bootstrapping capacitor and diode could be added to each channel. It would actually be able to meet its specifications then.

Next I checked the frequency response. The kit claims 30Hz to 100Khz, with no mention of the gain dropoff used to determine these points. So I used -3db in my tests. Here is what I got.

  Low High
4 ohms 45Hz 100KHz
8 ohms 25Hz 100Khz

The amp is a bit of an overachiever on the high frequency side, meeting Velleman's claims and far exceeding the claims of the data sheet. Not so good on the low side, at least with a 4 ohm load.

Conclusion

This amp will meet my requirements with no trouble, once I add a heat sink. I'd have to give it a less than perfect review when it comes to meeting its specifications, as well as the lack of a heat sink and lack of assembly instruction.