quest. on transformer sec. voltages

[troublerat]

I have some defunct audio equip. I am going through them and looking for power supplies I can use, I have a piece with a beautiful chassis tran.,input cord and I am questioning my self if I checked the secondary volt. correctly. Here is what I have. I connected the black lead of the dmm to the chassis ground, On the secondary side I have seven thinner wires on one side, three thicker wires on the other side. RED/ BLACK/RED,10.80, 1.25, 13.26 red/red, yellow/yellow, blue, white/white .371, 12.87, 5.88, 12.52,24.9, 11.64, 12.66. In that order I think I did it right as this your run of the mill am/fm/cd input etc and I dont think that the voltages would be that high. If I connect the dmm to primary black wire and check the sec. with red lead I get higher voltages and all of them between 36 and 78 volts. The thicker black wire on the one side with three I am suspecting is the most neg. wire[1.25] I hope this makes sense.

[Chuck H]

Well, I didn't actually see a question there other than "did I do this right?". To measure the secondary DC voltages you would need to run the secondaries through a rectifier of some sort. And idealy you would simulate the load to figure approximate voltage.

Chuck

[troublerat]

Question on trans. sec.

I" m sorry, there was a BIG question there. Without doing the rect. and simulated load was I even close on my method of measuring the voltages. This part of this hobby to me is somewhat of a mystery and to tell you the truth I"m just this side of being clueless. I have limited myself to 9 volt circuits but seeing all this artwork on the web with tubeamps, chipamps and the like has got me to lusting after a little more. Thanks for your response.

[Steve Conner]

If it's a solid-state stereo amp you've dismantled, then the transformer will be good for building... another solid-state stereo amp of the same wattage. You can use it to power those LM3886 amp chips or whatever.

In other words, you will have one large center tapped winding, from 25-0-25 in small amps, to 40-0-40 in large ones. This is rectified using a bridge rectifier and two large can caps, to produce two DC rails, +/-35 to 55V. Ie, 70 to 110V DC total. 110 corresponds to about 120W/channel into 8 ohms, most amps will be less than this.

Then there will be a bunch of other little windings to produce +/-15V power supplies for preamps, etc.

The three thick wires will probably be the center tapped winding mentioned above, and the thinner wires the preamp supply windings. Just look at the existing wiring to see how it hooks up. The voltages you measured all seem wrong, I'm guessing you measured the transformer wires (which carry AC) with your meter set to DC volts. Or maybe you chose wires belonging to two different windings, and are getting a reading through capacitive coupling.

Disclaimer: Some really old amps used a single ended supply, the transformer doesn't have the center tap, and this makes it unsuitable for powering chip amps and the like.

[troublerat]

q on trans sec.

No it was set to A/C thanks steve.

[troublerat]

figured it out[transformer voltages]

I figured this thing out. The red black red are 24 0 24,, theres one pair of yellow wires that give 24 volts at a lower current, the one smaller blue wire is some kinda center tap. My friend said I could use one of the big red and black for 24 volts or both reds together for 48 volts and this makes a little more sense than the way I tried to do it.

[troublerat]

update on transformer voltages

Done moved way past the volt. question and on to rect./smoothing caps/ bleeder res. and stable 36.66 + 36.66- and divided it down to an healthy 18 + 18- pre-amp supply. Heat sink/transformer/fuse/mounted on a chassis coutesy of the guy that threw this cheap stereo away.

[troublerat]

update on lm3886 build

Well the chips been mounted and the power amp section is up and running, very quiet also. Actualy upon start up I thought it wasn"t working as I didn"t hear the usual noise but then thought to turn up up the guitar vol. and heard that familiar kerang and was very pleased.