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Old 16th September 2015, 06:48 PM   #1
Trileru is offline Trileru  Romania
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Default PA mono amplifier, want to remove output transformer

Hello all,

I can get a Bosch-Philips PA mono amplifier for cheap(~25$), but first I want to know a few things.
It seems it has an output transformer that offers 70V/100V/ and 8ohm outputs.
I can understand why they used an output transformer. But I would like to use this unit for a subwoofer that I have.
The unit has a rated output of 240W, giant toroidal transformer with a 34VDC output from the power supply. My subwoofer has 4 ohms impedance.
Now, I would like to remove the output transformer since I plan to use this for low frequency amplification. I don't think having the transformer at the output helps very much.
The output stage is in a push pull configuration and uses 4x2sc5200 per side.
I like the build quality of this thing, seems like a tank, proper power transformer (input power rated at 760VA) and good active cooling.
Any way I can transform this thing? Has anyone done this?
I could consider making another output stage using the same transistors and cooling system, for 200-300W system for the subwoofer.
I'm interested in 20-100Hz frequencies.
Any recommended schematics for 8 parallel 2sc5200?
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Old 16th September 2015, 07:12 PM   #2
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When you say 34V supply, do you mean +/-34V supply - in other words actually a 68V supply.

If so the amplifier, with the output transformer removed, will give you about 100W to a 4 ohm speaker.
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Old 16th September 2015, 07:27 PM   #3
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I'd pay $25 for that just for the heat sink. With a power transformer not smoked, $50.
Appears to use a JRC4580 as a input buffer, not a shabby part. Peavey uses a lot of those. Two driver transistors on little heat sinks, one maybe voltage regulator in the middle? Not bad.
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Old 16th September 2015, 07:48 PM   #4
Trileru is offline Trileru  Romania
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The power transformer outputs 26VAC RMS but it looks like it can deliver lots and lots of amps. Maybe a voltage multiplier?
The signal input PCB has another JRC 4558(same as the one on the main pcb). It's a preamp/gain stage. The one on the main PCB is a phase inverter/splitter.
If I remove the power supply pcb and the output transformer I have lots of room to make an adequate supply.
If the transformer outputs 26VAC RMS (under light load), and the input rating is 760VA, does that mean that the transformer can deliver 29A? If so, then it would be a shame not to use it for a sub amplifier

Zippo is for size reference.
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Old 16th September 2015, 07:57 PM   #5
wg_ski is offline wg_ski  United States
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Quote:
Originally Posted by Nigel Goodwin View Post
When you say 34V supply, do you mean +/-34V supply - in other words actually a 68V supply.

If so the amplifier, with the output transformer removed, will give you about 100W to a 4 ohm speaker.
Hopefully, it's not a single 34V supply, with the output stage operating push-pull into a center tapped output transformer. If so you are so screwed when it comes to removing it. PA amplifiers of yesteryear used to be built that way. I haven't seen a modern one built that way, but there is a first time for everything.
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Old 16th September 2015, 08:25 PM   #6
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Quote:
Originally Posted by Trileru View Post
The power transformer outputs 26VAC RMS but it looks like it can deliver lots and lots of amps. Maybe a voltage multiplier?
Again, is it a single 26V, or is it 26-0-26?

If it's only a single winding then you're pretty stuffed, as it's presumably a very low output impedance amp relying on the transformer to increase the impedance.

Without the output transformer power will be abysmally low.
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Old 16th September 2015, 08:39 PM   #7
Trileru is offline Trileru  Romania
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It's only 0-26VAC. Can't I make another power supply to get a higher voltage?
If I find a good design I would make another output stage with power supply. Already got the case, transformer, heatsinks and transistors.
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Old 16th September 2015, 09:33 PM   #8
sreten is offline sreten  United Kingdom
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Hi,

No-one is considering the power amplifier may be bridged.
I'm saying it must be, if the power supply is single rail.

rgds, sreten.
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Old 16th September 2015, 10:27 PM   #9
Trileru is offline Trileru  Romania
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I tried to reverse engineer the schematic, but I could only do the power output. It gets complicated really fast after that.
There's a 24V regulator used for the op-amps, there's a Samsung IC for the Vu-Meter on the front panel, and two kinds of thermal protection, one starts the Fan and the other throttles down the output I think.
Anyway, the two output pairs that deal with each half of the sine-wave are identical, with opposite phase.
The board has a connector for the output, with 5 pins, that all go into the transformer. from 1-5 it's like this:
1- output from one phase
2- V+
3- GND
4 - the other output
5 - I think this is some kind of feedback. If I disconnect only this wire, the whole signal becomes almost a square-wave. Don't know what this is.
I measure the resistance and I think the connection is like this:
O1~~~V+~~~O2
Feedback?~~~~GND
There's no reading between these two pairs of connections.
I did measure the signal after the op-amp and indeed it's the phase inverter.
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Old 16th September 2015, 10:35 PM   #10
Trileru is offline Trileru  Romania
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I drew the connections to the transformer as I think they are.
I don't know where that feedback link is coming from, I didn't have the patience to trace it, but I did see that among other components, it also went into the vu-meter IC, but for sure that is not it's only function.
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