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Old 22nd January 2013, 07:23 PM   #1
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Default single-ended Altec transformer for chip amp?

Hi all, I have a huge and beautiful Altec 1590C chassis with its original monster 25 Volt power transformer. Because the chassis is in great condition, and has huge heat sinks, I'd love to build a chip amp in it. 60 watts a channel would be fine. Is there a chip amp that uses a single-ended supply (I'm doubting it) or a way to make a two-rail power supply with a single 25 volt input? I don't have the specs but am guessing this transformer can supply a lot of current. It is huge. --- The original amp was no longer working, was made for voice PA in shopping malls (45 hz low end cutoff) and doesn't have 4,8 or 16 ohm outputs. --- Of course, I'm willing to buy a new transformer if that's what it takes, but I am a cheapskate and hate to waste that big iron.
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Old 23rd January 2013, 06:26 PM   #2
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25 V AC -> 35 V DC.

You can use any chip amp with single supply. The way to do it is usually shown somewhere in the corresponding datasheet.
With your transformer You can expect 12-13 W into 8 Ohm or about double as much into 4 Ohm. If you need more output power, there is the BTL configuration. Something around 50 W into 8 Ohm should be possible with that, check e.g. TDA7265 page 9.
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Old 28th January 2013, 09:39 PM   #3
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You can always replace those original chips if you like to get it working again. Just measure on each output (before any coupling capacitor) first from out to ground then from out to +Vcc. You will find one of the chips has a fried output stage that will show as zero resistance.
If you don't want that, you can do something rather easy with 4xTDA2050 for instance. That's 2 per channel in BTL mode for using 8ohm speaker. The schematic is the same for 2030, 2040, 2050 but i believe only 2050 will handle that voltage safely.
Click the image to open in full size.

OR you can do this even easier, and get no less than 40W:
Click the image to open in full size.
Pick the resistors carefully to be accurate so the same DC will be at both the outputs (for first schematic - meaning if there's 15V on each then no current passes in the speaker when there's no music) and you'll get a quite good sound. Of course the resistors in the NFB must be identical too.
Now the second, personally i always like better the sound of amps with capacitive coupling, but that's just me. Here you must match the DC gain of the transistors as well to get ridiculously low distortion.
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Old 29th January 2013, 12:56 AM   #4
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Thank you so much for your replies. I looked around in my parts bin and found a decent-sized power transformer, +/- 25 volts, that will fit in there, so I gutted everything that I won't be needing. So I'm going to go ahead and put the chip amp in there, probably an LM3886. I know it's kind of a come-down for such a beautiful (looking) Altec, but it will certainly be light enough for me to move now.
Thanks again,
Keith
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