ebay Amp

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Some guy selling these on ebay for £160. I hope is actually units are not up to the quality of the example pics he's got.

I don't know what you're trying to say. I looked at those 'amps' and all I can say is UGLY. He says the chip amp is capable of 56 Watts - but not with a 48 Volt supply. My calculator is very optimistic at 35 Watts. I certainly wouldn't show it opened up. From the outside it's not so bad but inside? Yuk. He says it takes 3 days to build one but I don't see 24 hours worth of work there.

 
:scared: Would be better if a lot of those wires and component leads were made as short as possible rather than as long as possible! The design of the PCB could be a lot better. Either mount all the components on a PCB or wire the chip up pin-to-pin with its components directly, not both (leads everywhere, long signal paths)!

Using the case as the heatsink isn't ideal - the metal isn't thick (2-3mm?) - fine if you're not pushing it above 10W I suppose but it's all guesswork - there's no C/W rating for a start.

Personally, I think the plastic toroid case squatting uncertainly on top with exposed wires looks awful! Plastic can rattle and hum too and it looks like there's a gap between the plastic case and the main case.

80% of the cost is in the toroid - why use such a large unit when the heatsink is so dodgy? The LM3875 works with plenty of headroom with 160VA per channel and those are half the height of the transformers shown (I can't quite read the label on them). Toroid cost goes up a lot with VA rating, so you're paying a lot for PSU power the chips can't use.
 
Just to add to the current criticisms No anti chaffing relief around the cable though the case. No internal fuse.

You're right, he's just poking the mains cable through a hole in the case. Huh?!!?! Not good. I can't even see the traditional dodgy knot or cable tie to stop the whole thing being yanked out by an accidental tug or gradual increasing tension.

No fuse? Argh!

I wouldn't begin to think about selling a hand made mains powered device unless I knew for certain it could take all reasonable abuse without becoming deadly! I currently like the idea of a fused IEC socket and separate power cable - what I'm using in my fist chip-amp build at the moment.

Further, I don't like the idea of selling a device that follows the absolute minimal design of the original Gain-Card device (over-rated IMHO - points for the nice case and joyously optimistic attempted profit margin :eek: , but what's with the awful power supply design?). It can all go wrong when you don't know if the target load is too capacitive to drive without a zobel network (it's happened with moderate to long lengths of esoteric cross-woven speaker cable). And it only takes another two components to help filter out light switch, motor and florescent light EMI pulses.

For general sale, these extra components are, IMHO, important. They're all detailed (more or less) in the LM38xx datasheet notes (my thanks to people who've recently helped me with what some of them like the negative feedback capacitor actually do! :) ).

By all means use a minimal design if you have explicit control over your environment and know what you're driving, but customers with challenging environments will have issues - as a seller, you never know exactly how people will be using your device.

No fuse?! Nooooo...

[end rant]
 
is 3days any different from 24hours of work?

No. Quite. It's a rudimentary mistake to assume the period of time specified is entirely occupied by one task. Sleep, if nothing else, puts a dent in the hours worked.

Current wisdom is that the most productive work pattern is the following:

Estimate how long you can concentrate on a single task.

Reduce the time by 20%. Most adults have an attention span about 70-80% of what they think they should have. On average people can spend about 40 minutes working on a single intellectual task before their minds wander. Very logical and diligent people can spent more. People with short attention spans can spend less. It can take some experimentation.

Work for that amount with a timer or alarm clock.

When the alarm goes off, drop what you're doing (no pressure, if you need to spend time finishing the thing you're doing, do that) then spend 10-15 minutes doing something else. Toilet break. Cup on tea. Snack. Walk. Preferably something enjoyable and different to the work to be done.

When the 10-15 minute period is up (again, use an alarm clock or similar), start another work period.

Repeat.

This appears to be much (by a surprising amount) more productive than trying to get into a job for hours and hours at a time.

Just thought I'd throw that in there!
 

sippy

Member
2008-06-17 11:58 pm
My oh My, i can cope with fugly hifi components but these things are truely attrocious - 300 each or a pair, either way you could build something just as fugly and make it safe/use pcb's and maybe even build 2 pair of stereo for that kind of money.
I'd even say its worth sending this person an email detailing his errors and maybe something to ebay (though what would they care, as long as they get their commision)
 
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