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Old 27th July 2005, 03:45 AM   #1
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Question need some help

I found a scrapped realistic amplifier, from possibly the 70s or 80s.


Its a nice amp, has the dual analogue VU meters on the front, silver face, has all the knobs, and all kinds of stuff, the only thing wrong, it is missing the amp board. but the other boards are there.

Im assuming, that someone didnt finish the repair job, and left it for the destind trash.

Anyway, I was thinking about building a 4 - chip lm3886 amplifier module in it.

the system will power on, the meter piolet lights still work, and the power supply has a 4amp fuse, and the unit spits out +/-44v rails with no ripple as seen on my scope.

So, is it possible to build a lm3886 amp in this thing? I would love to, as I like the look of it.

I was thinking about a 2-chip parallel per channel, totalling about 4 chips.

Anyone got a schematic?

i read somewhere that +-42 volts already pushes it on the edge, which tells me I might need regulation, but the datasheet clearly says that it will work with a span of 96 volts I think?

well this only has a span of 88.6volts, according to the measurements of my meter.

Any thoughts on this?
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Old 27th July 2005, 06:40 AM   #2
Stocker is offline Stocker  United States
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You can always waste a few volts with regulation or else unwind them right off the transformer if possible. If you like the look of the unit, go for it.
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Old 27th July 2005, 07:30 AM   #3
boholm is offline boholm  Denmark
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The data sheet clearly states, that the LM3886 max. can handle +/- 42 Volts. So no worries there. The 96 volts (=+/- 48 volts) can be applied, if it is not playing music. Normally - when the music starts - you will also start to draw current from the PSU, and then the voltage will drop *if* it is an unregulated one.

If you have some power resistors you can try this test: Connect them to the PSU and see, if the voltage drops.

As for parallel coupling schematics: Look into the other threads in this "chip amp" forum.
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Old 27th July 2005, 07:31 AM   #4
Mad_K is offline Mad_K  Norway
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You cannot go beyond 84V supply on the LM3886. I wouldn't even think about 84V because of the thermal issues. Atleast you would have to parallell them and use massive heatsinks or a fan. How about regulating it down to +/-30/35V? (depending on transformer regulation).

see http://www.national.com/an/AN/AN-1192.pdf#page=8

for parallel schematic
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Old 27th July 2005, 03:01 PM   #5
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its an unregulated power supply.

If you are familiar with a realistic SA=2000, thats what it is.

its a sweet looking amp.


its a big huge transformer, that goes into a couple of good sized filters, and unloaded, it puts out 44.3 volts per rail.
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Old 27th July 2005, 03:04 PM   #6
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that schematic is lacking some stuff. such as the muting, etc...


the problem is, I dont have the parts to regulate it down. I can only use what I have, and this is what I have.

im not going to be able to go to a parts supplier any time soon. (lack of funds).
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Old 27th July 2005, 03:42 PM   #7
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Quote:
Originally posted by mbates14
that schematic is lacking some stuff. such as the muting, etc...


the problem is, I dont have the parts to regulate it down. I can only use what I have, and this is what I have.

im not going to be able to go to a parts supplier any time soon. (lack of funds).
If you lack funds there's little you can do. You will need them to buy the parts for an LM3886 project, particularly if it's in parallel.

With that kind of voltages, I would try to find the original Realistic circuit and see if the amp can be repaired. They are too high for the LM3886 or LM3875. You can only use them if you regulate them.

Using two chips in parallel is good if you have low impedance speakers. But for that you would have to go lower in the PS voltage too.

So the choices are not that many.

Perhaps a different chip, like an STK type, would fit your bill better.


Carlos
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Old 27th July 2005, 03:48 PM   #8
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thing is, I already have the 3886 chips.

I have a +/-35 supply, but its an SMPS, and I dont think it would handle the currents required for parallel. but im not sure.

Also, I have a stereo circuit bult and functioning already, but from a much smaller power supply. I wanted to go parallel, because I want 100W per channel instead of 50W per channel.

my question is this:

I have a stereo circuit that is operating. it contains two circuits. if I build two more circuits (like two stereo amps in a way). Can I put the output resistors between the first two circuits, and that makes one complete circuit in parallel? or no.

they are inverting designs.
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Old 27th July 2005, 03:54 PM   #9
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oh god, dont even get me started about STK chips.

I HATE them with a pasion. they always die in premature failure, even in commercial units. They suck.


sorry, you got me started.
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Old 27th July 2005, 04:13 PM   #10
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I have a good chip for you: TDA7293 It takes a total of 120 volts, 60 volts per rail and they are 100 watts per chip.

About stk modules, they are very nice and user friendly to use. I have never had one fail even tho i ran a commersial amp using one at the max volume the power supply could withstand for atleast 30 minutes with no problem. The heatsink was too hot to touch when i turned it off and it still runs fine, so you have done something wrong if you fry them.
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