Poorly designed kits - Page 1
Forum section: electronics / beginners

author: Zero999
author_rank: Super Contributor
timestamp: October 01, 2017, 02:25:25 pm
content: The classic is the 0 to 30V PSU. It's a classic, cheap Chinese kit. It won't meet its specifications and the op-amps are powered from too higher voltage. At worst it'll smoke, at best it'll just be unreliable. Here are some threads where this has been discussed previously.https://www.eevblog.com/forum/beginners/bangood-psu-enhancements/msg897523/#msg897523https://www.eevblog.com/forum/beginners/poorly-designed-kits/msg1314404/#msg1314404I've recently found another crappy kit, a stereo amplifier based around the TDA2009A.I bought one of these for an intercom project at work and discovered it's got some design flaws, so I thought I'd give any potential buyers the heads up. I decided to post in the beginners section, as that's the target audience for the kit and an expert will easily be able to see the flaws. Overall it's pretty good value for money (I'll might buy another one) but it needs to be modified.https://www.rapidonline.com/Catalogue/Product/70-0189http://www.rkonlinestore.co.uk/rkamp4-2-x-10watt-stereo-amplifier-project-kit-580-p.asphttps://www.rapidonline.com/pdf/70-0188.pdfThe TDA2009A is only specified to work down to 8V, yet the kit is supplied with a battery holder for four AA cells! There's also a reverse polarity protection diode, which makes it worse by dropping another 0.8V.http://pdf.datasheetcatalog.com/datasheets/208/378193_DS.pdfThere won't be much treble response. R2, R4 & R6 form a potential divider, with an output impedance of 10k|10k|3k3 (3k4 is an error on the schematic) = 2k, giving a cut off frequency with C4 = 10nF 8kHz.Replace the C4 and C5 with 2.2nF capacitors and power it from a 12V mains power supply or 12V lead acid battery of eight alkaline cells.The gain of the amplifier is 73.22, but R2, R4 & R6 form a potential divider with a gain of 0.2, giving an over all gain of just over 14.5. There's nothing wrong with that, but wouldn't it make more sense to just lower the gain of the amplifier? It will give less noise, distortion and a higher power supply rejection, with a lower gain. Unfortunately the TDA2009A isn't stable at again below 20, so the potential divider is still needed but it can be minimised. The gain could be set to about 29 by changing R10 & R8 to 3k3 and R9 & R11 to 120R. R6 and R7 can be omitted and C4 and C5 changed to 1nF for a total gain of 14.5 and cut-off frequency of 32kHz.I wanted a slightly higher gain so I omitted R4 to R4 and changed R10 & R8 to 8k2 and R9 and R11 to 430R for a gain of 20 and used 470pF for C4 and C5 for a cut-off frequency of 34kHz. My power supply is a 24V regulated SMPS and the maximum voltage rating of the TDA2009A is 28V, which is fine but I upgraded C3 from 25V to 35V, just to be safe.

author: exe
author_rank: Unknown
timestamp: October 01, 2017, 10:49:35 pm
content: Quote from: Hero999 on October 01, 2017, 02:25:25 pmWhat's wrong with it? May be there is a thread about its flaws?

author: MK14
author_rank: Super Contributor
timestamp: October 02, 2017, 12:10:08 am
content: Elsewhere, it seems to be supplied with around 12 Volts, by those AA batteries.If you look carefully at the picture, it seems to be a x8 AA Battery holder.So presumably there has been a mix up, somewhere along the line.It specifically seems to say12 Volts, at (presumably up to) 500mA.Source(s):http://www.ebay.ie/itm/TDA2009a-RKAmp4-10W-10W-Stereo-Amplifier-Project-Kit-Various-Options-UK-Seller-/182307853884QuoteEDIT:On further investigation, that same seller also seems to sell x6 AA and x4 AA battery holder sets. So the jury is still out I guess.In my experience (attempting to be polite and politically correct, in case the author reads this), slightly weaker Electronics Engineers, can be happy enough, if their one-off unit, works down to (say) 4.5 Volts (even though the datasheets says Min=8V, also there is the diode drop to take into account. So it should be a lot more than 6V, such as >= 9 Volts).Although technically speaking that is slightly (politically correct) poor design. In practice it does happen.There is a tiny to small chance, that the manufacturer (TDA2009A) has agreed to the lower voltage and/or extensive testing has confirmed it and/or the parts are individually tested (when putting the kits components together) that they work at the lower voltage and/or they are slightly different parts (e.g. Chinese Clones), which have a more flexible voltage range, due to a different manufacturing technique.

author: MK14
author_rank: Super Contributor
timestamp: October 02, 2017, 12:46:27 am
content: A lot of the electronics assembly kits come from China, these days.I'd suggest taking the specification with a huge pinch of salt (i.e. it may be very bogus).E.g. 18650 batteries with 10 Amp/Hour capacities.50 Volt max, 10 Amp max power supplies. Which probably can't even achieve half that output current, and the heatsinks are so small, it would probably let the magic smoke out, sooner rather than later.

author: Zero999
author_rank: Super Contributor
timestamp: October 02, 2017, 07:56:42 am
content: Quote from: exe on October 01, 2017, 10:49:35 pmIt's a classic, cheap Chinese kit. It won't meet its specifications and the op-amps are powered from too higher voltage. At worst it'll smoke, at best it'll just be unreliable. Here are some threads where this has been discussed previously. I'll add them to my original post, to clarify it.https://www.eevblog.com/forum/beginners/bangood-psu-enhancements/msg897523/#msg897523https://www.eevblog.com/forum/beginners/poorly-designed-kits/msg1314404/#msg1314404Quote from: MK14 on October 02, 2017, 12:10:08 amThe one I bought from Rapid Electronics came with a four cell AA holder. I haven't tested it at lower voltages than 12V, to see what it would work down to. It will probably work at lower voltages, than the IC is specified to, but clipping will cut in at progressively lower power levels, until it ceases to work.I don't give a stuff about political correctness. I'd rather warn people of the pitfalls of buying cheap kits. If there's no guarantee it'll work properly from lower voltages than 8V, then it should not be supplied with a an alkaline battery holder for any less than eight cells.Quote from: MK14 on October 02, 2017, 12:46:27 amThat's certainly true in this case. 12V 500mA, with two 10W outputs. BSOhm's law states that the RMS maximum power of a sine wave, into an 8 Ohm load, with a single ended amplifier, powered from 12V, is just 2.25W.

author: IanMacdonald
author_rank: Frequent Contributor
timestamp: October 02, 2017, 08:17:15 am
content: Exactly why they have this input filter/attenuator is uncertain. Maybe they had stability problems due to board layout, and that was they way they solved it.Though you think they'd have tried reducing the amp gain first.

author: bd139
author_rank: Super Contributor
timestamp: October 02, 2017, 08:30:39 am
content: I think every kit I've built over the years has had one or more serious problems with it.I've never walked away with one not feeling dirty. It's one reason I tend to design stuff from scratch these days.

author: Kleinstein
author_rank: Super Contributor
timestamp: October 02, 2017, 08:42:55 am
content: Some filtering at the input is a good idea. However the capacitor value does not look right. The resistors for reducing the amplitude might be optional to choose the level you need. Due to the low supply voltage they might need the divider at the input, since there usually is a limit on how low one can set the amplifiers gain. Too little gain for the amplifier itself might make it oscillate.One problem with such kits is that they are sometimes copied / changed without much thought, mainly to make them even cheaper. This could be the reason to go from 8 AA to 4 AA. Such a copy / paste error could also cause capacitors to change from 100 pF to 10 nF. They look very similar in SMT.

author: Zero999
author_rank: Super Contributor
timestamp: October 02, 2017, 08:51:34 am
content: Quote from: bd139 on October 02, 2017, 08:30:39 amI design my own things too but often it's cheaper to get a kit/module and modify it. I never intended to build this kit as is. I knew about the power supply issue beforehand but that didn't affect me, as the application is for 24V. The too lower cut-off frequency was something I discovered later, after studying the schematic properly.Quote from: IanMacdonald on October 02, 2017, 08:17:15 amQuote from: Kleinstein on October 02, 2017, 08:42:55 amMy guess is they just copied the TDA2009 circuit from the data sheet, without reading it properly and understanding how it works. Then they realised the gain was too high for use with a mobile phone, MP3 player etc. and it picked up GSM, causing a buzzing sound to be emitted, when the phone transmitted a signal, so added an attenuator and filter but got the cut-off frequency too low.An input filter is a good idea with an audio amplifier because it helps to block RF interference. I included one in the final design, not only to attenuate RFI but because the input and output conductors shared an unscreened cable. I previously built an amplifier of my own design and used the now obsolete TDA2030. Without a filter it would oscillate at RF, due to positive feedback from capacitive coupling between the input and output. Not only did this give excessive power consumption but the zobel network got very hot. Adding a filter with a cut-off frequency above the audio band fixed this.

author: dcbrown73
author_rank: Regular Contributor
timestamp: October 02, 2017, 05:04:29 pm
content: Who are the quality kit manufacturers?

author: schmitt trigger
author_rank: Super Contributor
timestamp: October 02, 2017, 05:13:39 pm
content: Quote from: dcbrown73 on October 02, 2017, 05:04:29 pmWhowerethe quality kit manufacturers?Heatkit, for sure. Dynaco was other. I've heard good things about Velleman, but have not personally built one myself.

author: MK14
author_rank: Super Contributor
timestamp: October 02, 2017, 05:24:41 pm
content: Quote from: Hero999 on October 02, 2017, 07:56:42 amI agree. I also wonder if the original kit design, was for 12V. But another Chinese kit maker, copied it badly. Putting in 6V (x4 AA) battery holders, instead of the 12V ones. Anyway, there is some explanation, we just don't know what the right explanation is.There is mention (somewhere, one of the kits instructions I vaguely remember), that it needs heatsinking above 6V, so maybe it was being run at 6V, to avoid the slight extra expense of adding a heatsink.Quote from: Hero999 on October 02, 2017, 07:56:42 amSadly, even in the classic old days (e.g. 1970's), the claims of amplifier Hi-Fi systems, output power was sometimes exaggerated.E.g. (very rough example) a genuinely 2.5 watt (MAX) per channel stereo amplifier.Then gets called 5W + 5W, because of the PEAK power.Then the 5W + 5W gets added together, so it's a 10W of music power.Hence they market/claim it is a 20 Watt amplifier, saying 20W ( = Peak Music Power).I.e. A 2.5 Watt amp (x2), turns into a 20 Watt amplifier, by messing with the specifications, as shown above.In those (old) days, you had to make sure you were seeing the "True RMS power output". Anything less, could just be big exaggerations.

author: Zero999
author_rank: Super Contributor
timestamp: October 02, 2017, 11:20:25 pm
content: Here's is the circuit I built. Everything not shown remains the same. The gain will be higher, than the original. If less gain is required, then an attenuator is required.