First, there seems to be a lot of noise about torodial transformers. What are the advantages of them that make them so much worth the extra cost? And, for what I'm doing (live sound) should I worry about it, or just go with a pedestrian transformer?
I pulled the transformer out of my dead reciever and I'm looking at it, wondering... would this transformer do what I need done for my LM3875 project? I don't know much about it, but I'm pretty sure it's 250 VA. There are a bunch of markings on it, as well as some little PCBs soldered to its leads (!) so I'm wondering if it's worth worrying about or not... I think it may be just too weird to use in my project.
The leads I think I'd end up using are obvious; the two marked "primary" which are blue and brown, respectively, and then, on the opposite side of it, two red, a black, and two white, in that order.
The thing that throws me is that on the same side of the transformer as the primary leads, there's this little PCB with "littlefuses" on it (they look like unmarked resistors but it says they're fuses) and 7 leads coming out of the transformer and then 7 wires going out into the rest of the circuitry for the beast... is this the set of rails for the control circuitry, perhaps?
How do I find out what's coming off the secondaries? Should I worry about the odd PCB with the 7 leads? Is this even worth the money I could save, or should I go buy an off-the-shelf transformer?
I pulled the transformer out of my dead reciever and I'm looking at it, wondering... would this transformer do what I need done for my LM3875 project? I don't know much about it, but I'm pretty sure it's 250 VA. There are a bunch of markings on it, as well as some little PCBs soldered to its leads (!) so I'm wondering if it's worth worrying about or not... I think it may be just too weird to use in my project.
The leads I think I'd end up using are obvious; the two marked "primary" which are blue and brown, respectively, and then, on the opposite side of it, two red, a black, and two white, in that order.
The thing that throws me is that on the same side of the transformer as the primary leads, there's this little PCB with "littlefuses" on it (they look like unmarked resistors but it says they're fuses) and 7 leads coming out of the transformer and then 7 wires going out into the rest of the circuitry for the beast... is this the set of rails for the control circuitry, perhaps?
How do I find out what's coming off the secondaries? Should I worry about the odd PCB with the 7 leads? Is this even worth the money I could save, or should I go buy an off-the-shelf transformer?
Well, the obvious answer is first do Ohms measurements to find out the windings, then put power on the primaries and measure the sec voltages. I assume those 7 wires are thin? That would be for support circuitry. The red and whites, if thicker, could be the main secondaries. There may be a single wire apparently connected to nothing, that may be the screen (black?). That would also be thin.
Jan Didden
Jan Didden
The fuses are fuses.Nappylady said:
I'm not sure what you mean by the 7 wires. Are there beside the
blue and brown (primaries) 7 other wires on the primary side?
You'll need to power up the trafo and measure with an AC voltmeter across the different secondary.
Picture perhaps?
Does it come out of a Sony receiver?
/Hugo
It was an RCA reciever, and it was a very nice one, with all the optical and digital and component inputs and outputs and switching and it wouldn't even turn on after I blew it and WAAAH!
I took the thing and wired it up and measured the voltages. The wires look like this:
white white black red red
The inside red and white both showed 27 VAC against the black; the outside showed 47 VAC.
Once I get the filter caps in there, it becomes 27 volts DC, right? My meter measures the RMS voltage, which is the same value as the filtered DC, right?
If I power my LM3875's with +/- 27v, is that enough? Will I kill something? If the chips don't swing to within 3v of the rail, that's still a top level of 24 VAC, which is... 72 watts into 8 ohms, which is above the rating of the amp (and I'll probably never go that loud) anyway... am I going to be okay here?
Thanks again, everyone, for your help.
I took the thing and wired it up and measured the voltages. The wires look like this:
white white black red red
The inside red and white both showed 27 VAC against the black; the outside showed 47 VAC.
Once I get the filter caps in there, it becomes 27 volts DC, right? My meter measures the RMS voltage, which is the same value as the filtered DC, right?
If I power my LM3875's with +/- 27v, is that enough? Will I kill something? If the chips don't swing to within 3v of the rail, that's still a top level of 24 VAC, which is... 72 watts into 8 ohms, which is above the rating of the amp (and I'll probably never go that loud) anyway... am I going to be okay here?
Thanks again, everyone, for your help.
The two different voltage may be for 4 or 8 ohm speakers, though 27 V seems disprportionately low for 4 ohm speakers, compared to 47 V for 8 ohms. could be though. Is there a switch on the back of the receiver for 4/8 ohms?
27 VAC does not become 27 VDC when rectified and filtered. An AC voltage reading is the RMS value of the AC sine wave. Some of the time, the voltage is lower than that, and some of the time it is actually higher. The RMS voltage is somewhat akin to an average (it's actually the square root of the average of the squares: Root-Mean-Square). The peak is at 27 * sqrt(2) = about 38 Volts. This will be the DC voltage once rectified and filtered. So with black as ground, you'll have a +/- 38 VDC supply.
So, that's exactly what I'm going for, right? I mean, +/- 38 V is perfect, isn't it?
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