The Gainclone – why a chipamp?

From Gaincard to Gainclone

Experimenter’s Corner

The Gaincard amplifier was not the kickoff of IC audio amplification but more the hobbyist’s acceptance of IC audio amplifiers.  I was brought into the much debated “greatness” of these amplifiers when I actually took the time and trouble to build one, using the 3886.

On Semi STK412-000 Shift PSU ChipAmp
No Gain Clone but a great On Semi STK412-000 Shift PSU ChipAmp

First of all, my own non-unique experience was based on the simplicity of the design compared to the rather surprising output quality, which was rather good.  But let’s kick off to the very beginning…

Cape Town’s electronics shops of yesteryear!

At Analog Ian we like to reminisce about the good old days, especially electronic component shops in Cape Town, one being the old Hamrads’ shop in Buitengracht Street, the starter shop for Ham radio enthusiasts,  a good selection of electronic components, test equipment and kits. Then we had Electronic Research Labs, ERS on the Foreshore, Electronics Supermarket in Burg Street, Cape Electronics, to name a few.  Where are they now?

And now!

Modern times calls for same measures and we have some of the bigger bulk resellers/wholesalers, Mantech and Communica being two of the bigger stores catering for small to large. As a DIYer we have some good online stores now and at Analog Ian we prefer to push potential clients towards these smaller stores hence our recognition of DIY Electronics, Pro-Tec (under the same roof in Durban), Yebo Electronics in Cape Town and Lantis in Pretoria. I list these companies, I try to buy from them time to time and their service is all exceptionally good. Readers are welcome to drop me a line however and tell me where their “go to store” is located.

Pricing of the LM3886

Back to Chipamps and Gainclones:  None of these stores offer the LM3886, you will need to go to Mantech or Communica. Pricing is around R175.00 inclusive.  Bear in mind for a mono amplifier now, output of 50W you need to shell out about R200, which brings us to the question, why do it?

A cheaper solution – the STK series

It’s simple, it’s easy and in 99% of times it works. However, there is another angle, a cheaper angle:  Purchasing two of these including the power supply is going to set you back R600-R700 easily.  Somewhere on the website we wrote this highly intelligent article on the JVC 5042 which can be had for the measly sum of R500.00 through Gumtree, Cash Crusaders or most 2nd hand dealers.  It’s a better buy than the LM3886 because here you get two audio chips for the price of one. Ah-Ha! The smaller JVC5042 boasts two Sanyo STK412-430 ICs designed to put 100W into 8 Ohms.  Because each chip drives two channels you now have one spare. Not only do you have one spare, you have a power supply which can deliver 100W + 100W easily. In fact I have driven the STK412-430 into a Verboten load of 4 Ohms and ploughed Pink Floyd Pulse through the better part of a case of Amstels without anything going awry.

No, I don’t have anything against the LM3886, I just think pricing in South Africa is off the charts.  The STK412-430 (or any in the series you can get your hands on) makes a very good quality, reliable amplifier.  Don’t forget the PSU though, it is a device which requires dual rails.

Specs here.

Chipamp STK412-000
STK412-000 Pin Compatible 50+50 to 180+180 (Watts per channel).
STK412-000 Supply Rail requirement
STK412-000 Supply Rail requirement
STK412-430 JVC5042
STK412-430 JVC5042

An application diagram for the STK chipamp with shift rail PSUs is above or on the link, that of the PSU etc can be obtained from the manuals (easily downloaded on the net).  As  mentioned the JVC 5042 amp is often advertised on the net and if the amplifier is shot it would have been caused by chip failure or thermal link in power supply. Don’t offer more than R100.00 for such a unit.

It is not advisable to bridge these amplifiers because the JVC 5042 transformer will not handle the 200W+200W load. Yes, you will crank it.  Also, the linearity of the two amplifiers whilst rail shifting may give unpredictable results. The transformer secondary should be something like 40-20-0-20-40.  Two 42-21-0 transformers will also work in series. You do need the V(high) and V(low) rails. The voltages shown above was for the 5042.

ICs and their short-comings

Obviously one cannot tinker around inside the IC.  This may good for some and bad for others. You may not like the cross-over distortion.  Cannot tune the bias? This may be a good thing. These amplifiers use quite a hefty supply for the +/- V(H) so be warned about fiddling with the supply on.  It will give you a nasty kick,  and can be lethal.  You have been warned.

An IC has no user serviceable parts and will short out if reverse polarised at the supply rails. Do not attempt to play into a load it is not designed for. The output overload protection circuit works very well (don’t ask how I know).

This amplifier will easily dissipate more than 100W per channel when fully driven. Use a heatsink for the purpose, either the one supplied in the cabinet or ask technical counter for a spec’cd unit, possibly the popular 19488 offered by Mantech. 200mm run.

 

Further Reading:

https://analogian.co.za/tag/100w-stereo-amplifier/

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