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Old 2nd December 2007, 02:18 AM   #1
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Default My first project

After building speakers for a while i thought i would have a go at a chip amp as my first project.
Being a natural bower-bird and skin-flint type i have a heap of old stuff floating around here, i have a good supply of heatsinks and cabinets won't be a problem( at least if I use old computer cases ) but using the Jaycar chip-amp kit using the cheapest possible parts
http://www.jaycar.com.au/productView...=&SUBCATID=557
How far can you push the rail voltage?? I ask this question because I have on the bench a 28-0-28 CT transformer, but when I multiply by 1.4142 I get 39.6V, 5 volts more than the recomended 35V.
Is there any cheap, easy and reliable way of using this Transformer?? I have lots of transformers here but this is the smallest (amps die but trannies live for-ever)
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Old 2nd December 2007, 03:20 AM   #2
BWRX is offline BWRX  United States
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That kit appears to use the LM3876 chip (based on the picture), so the max rail voltages you can safely use are +/-42V. With mains fluctuations it would be safer to find a transformer that will give you +/-35V rails after rectification (one with ~25VAC secondaries). There's nothing barring you from using a lower voltage transformer either. I'm personally using trasformers with 18VAC secondaries on my LM3875 amps and they work great and the chips stay quite cool.
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Old 2nd December 2007, 04:42 AM   #3
ttan98 is offline ttan98  Australia
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Moondog,

I am from Melb., I think LM3886 is a newer chipamp and slightly better, I bought a 6 units from

AVS electronics, at Bayswater, the last time they are out of stock, ask, the product maybe back in stock.

try them.

LM3886=AUD$6 each
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Old 2nd December 2007, 05:43 AM   #4
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This one is a fairly old unit, I just checked it; it is rated 28-0-28 at 240V input our nominal voltage has changed to 230 would this drop in nominal voltage make much difference to the rated output??
Thanx ttan98, I did look for those at Jaycar when I bought the kit but Jaycar are becoming like Tandy/DickSmith more gadgets than gear
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Old 2nd December 2007, 07:28 AM   #5
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hi
Look i'm using 2 LM3886 with +/-40V and works realy good . . .
i don't know if the LM3875 and LM3886 are exactly the same, but i think that the LM3875 sould work.
to be shure put a big heatsink
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Old 2nd December 2007, 08:45 AM   #6
Nuuk is offline Nuuk  United Kingdom
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Quote:
I just checked it; it is rated 28-0-28 at 240V input our nominal voltage has changed to 230 would this drop in nominal voltage make much difference to the rated output??
Nominal mains voltage here is 230 volts but I have measured as low as 218 and as high as 252! In other words, don't assume your voltage stays the same. I have run LM3386 amps at +/-38 volts with no problem but I wouldn't go any higher.

It also depends on your speakers. My main speaker sounds much better with +/-27 volt rails

To sum up, you could try it but you are taking a risk, and it probably won't sound its best!
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Old 2nd December 2007, 09:15 AM   #7
AndrewT is offline AndrewT  Scotland
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Hi Moon,
connect your transformer and measure the secondary voltage.
Phone your supply company and ask what their range of supply voltages are.
Calculate the worst case voltage from your transformer.
If the offload DC voltage is over +-42Vdc (>30.2Vac) then you are sailing close to the wind, particularly if you adopt my philosophy of +-20mF per 8ohm speaker.
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Old 2nd December 2007, 10:50 PM   #8
ttan98 is offline ttan98  Australia
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Quote:
Originally posted by ttan98
Moondog,

I am from Melb., I think LM3886 is a newer chipamp and slightly better, I bought a 6 units from

AVS electronics, at Bayswater, the last time they are out of stock, ask, the product maybe back in stock.

try them.

LM3886=AUD$6 each

sorry the correct name is

AVNET, tel:958 55511

about $6 including GST.
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Old 4th December 2007, 12:43 AM   #9
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Wonderful bunch in this site, thanx all for helping.
I am goint to go with the tranny I have here and if it blows the chip I have only lost $AU 20-, while I know i should follow the advice and get a slightly smaller transformer the temtation to save a few dollers ( well $ 42.95 actually) is too strong.
I checked with my power company ans fluctuations of 12% are apparently quite common, I assume this wil cause the chip to run very hot so no mica or silicon pad, silver heat paste and isolating the heatsink seems to be the option.
I have a big slab of acrylic ( Perspex) here for mounting purposes and if I use a chimney arrangement the heat sink should be around 0.6 C/W.
Iam making the assumption that the reservoir caps (10,000uF 100V ) are still OK, this is too big but they are fairly new 3yrs old and came out of an old amp that died a little while ago as did the heat sinks and this amp was rated 200W OVERKILL?
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Old 4th December 2007, 12:54 AM   #10
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Default Re: My first project

Quote:
Originally posted by Moondog55

How far can you push the rail voltage?? I ask this question because I have on the bench a 28-0-28 CT transformer, but when I multiply by 1.4142 I get 39.6V, 5 volts more than the recomended 35V.

Hi,
Personally, I wouldn't worry too much about that voltage. You are still under the +/-42V and you're not going to run this amp full bore all the time, right? Big heatsink, volume not at 100% and higher impedance (8 ohms or more) speakers = good times.
Good luck.
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