PA amp question: Velleman K4005

trwh

Member
2002-04-28 2:11 pm
UK
Hi

I'm looking to build a robust, small(ish) power amp for PA use. The power rating needs to be about 100W per channel into 8 ohms. I'd like a design that includes some sort of short circuit protection, and a project in kit form would be even nicer.

The Velleman kit K4005 caught my eye - it is very cheap and meets all my needs, having overload and short circuit protection built in, and delivers 2 x 75W into 8 ohms. It requires and external PSU, though. Has anyone had any experience with this unit? I've read negative comments about Velleman amps in these forums, especially regarding oscillation and sound quality. In this case, sound quality isn't a major consideration, but I need the amp to be stable.

Any feedback is much appreciated,
TRWH.
 

trwh

Member
2002-04-28 2:11 pm
UK
Velleman K4005 - buyer beware

A few weeks after making that post, I decided to buy and construct the kit. Unfortunately, I have had a very negative experience with it, and I have decided to make a post regarding this so that other people will be aware of its shortcomings.

In fact, I am very surprised (and disappointed!) that I have had such trouble with this amp - all of the Velleman kits I have built before have worked perfectly first time. Having said this, I have heard other negative comments regarding Velleman audio kits, such as the high-end K4010/K4020 300W MOSFET amplifier, which has some reported oscillation problems - not what would be expected of such a hyped (and costly!) unit.

After I had constructed the K4005 kit I double checked my work, and powered the amp up with the recommended +/- 45V supply (for 8 ohm operation). It drew a somewhat excessive 400mA from the supply rails, and the TDA7050 driver IC got very warm. Thinking the unit may be oscillating, I scoped the output, but it seemed to be perfectly stable. So I telephoned Velleman in Belgium, who informed me that this high current consumption was “quite within normal range”, and added that the kit would get “really quite warm”, but that it “would not be a problem”.

So I connected a set of speakers, and the amp seemed to work fine. However, it was still getting quite hot, and the main heatsink temperature continued to rise even with no input signal. With the input shorted, and a thermocouple on the heatsink, the temperature was observed to rise from room temperature to just over 50 degrees C within a few minutes. I did not disconnect power as I believed this temperature to be normal, as described by the technician at Velleman. However, this was too hot for the amp, and one channel stopped functioning, and developed a DC offset of around 2 volts.

Since I believed my construction was correct, I sent the amp off to my supplier in the UK, Maplin Electronics. They charged £15 (~$20) for an initial inspection of the kit (refundable should a manufacturing fault be found). Their engineer passed the amp “full working order”, despite a very clear description of the fault which I included with the return. Sadly, it seems I have no comeback to this, as I had mounted my own set of ¼” output jacks, and phono inputs on the chassis - thus modifying the kit. Whilst my implementation of it is electrically fine, this does invalidate any guarantee. I have therefore lost both the £70 (~$100) cost of the kit, and the inspection charge. (Heh, I shall make good use of some spares though...).

Please be careful to initially construct any complete kits such as these without any modifications, so you can verify their operation should you need to make a guarantee claim. Also, having entered into correspondence with another DIYer who has built amps based around the TDA7250 driver IC, it seems to be a quite unpredictable and difficult component to implement successfully.

I hope this is helpful to people,
TRWH.
 

trwh

Member
2002-04-28 2:11 pm
UK
Yeah, I've built loads of Velleman kits before with no problems: a Geiger Muller counter, metal detector, VU meters, small preamp, etc. It's a shame that this one didn't work out. I think its design is let down by an inherently picky driver IC, coupled with an overly compact layout - with everything crammed _inside_ the main heatsink.
 
I have not tried this kit, but I can say that the TDA 7250 driver chip is not a problem. If I remember correctly, I encountered some problems during the prototyping stage but it was eventually sorted out. I am quite certain I made some changes to ST schematics. PCBs were eventually made. Fairly compact for 2 channels using 2 pairs of TIP 142/147 power darlingtons. I have this amp running 24 hrs daily in a restaurant for about 2 years now in back-ground music application. No problems so far. Even when the fan failed recently(bearings wore-off), the amp continued working. This is with heatsinks rated for forced air cooling.
 

ivonj

Member
2007-04-30 7:54 pm
Hi.
I have tried to build one such amp based on ST recommendations, but found that excessive current was being drawn by the transistors. I also have channel 1 functioning properly, which I believe is due to the high current being drawn.

can you indicate what changes you made to it function normally, with a low quiescent current?