Isolated Line Out

[stephenl]

I was looking around for an isolated line out circuit and found this. What do you guys think, will this work well?

[tictac]

For the application in your drawing Jensen makes a 600:600 (1:1) transformer for a line out from a speaker output. They have a 200 ohm / 20 ohm network across the speaker terminal and feed the line out transformer from the junction of those two resistors.

The 10k:10k line you have is used for the input side of amps and is seen in splitters and A/B boxes etc. R.G. has a few schematics on his site for this.

In any event you need to isolate your line out tranny from the speaker output via a large enough resistance that it won't effect the speaker Zout.

TT

[stephenl]

Thanks Tic Tac, I found the schematic on the Jenson site.

In my attached schematic, the 109T transformer has a dcr of 700 ohms and a 10k impedence on the primary which would "isolate" it relative to the speaker load...right?

[JazzGuitarGimp]

Thanks Tic Tac, I found the schematic on the Jenson site.

In my attached schematic, the 109T transformer has a dcr of 700 ohms and a 10k impedence on the primary which would "isolate" it relative to the speaker load...right?

It works just like an OT: it's ratio is 1:1, and you have a 25K-ohm load on the output, so 25K is reflected back to the speaker output. The actual load will be slightly less than 25K, depending on the impedance of the amplier input the line out is connected to, and the setting of the line out level pot.

[tictac]

The signal coming off directly off the speaker is not a good idea.

You need attenuation between the OT and the line out transformer or you could end up frying the line out.

You could put a potentiometer on the primary side of the line out but you should still isolate it with a resistor.

TT

[FUCHSAUDIO]

I was thinking: Why not do the level control on the input end, so the output would be a consistent low-z send ? Add a resistor in series with the pot into the transformer primary. Should work okay if you watch levels and then you won't torch the matching transformer.

[labb]

You guy's might enjoy this little discussion.

http://music-electronics-forum.com/t25684/

[tictac]

excellent discussion thanks....

[stephenl]

Thanks all.

Mr. Fuchs, under what conditions do you think the matching transformer, in the original post, will smoke.

[pdf64]

under what conditions do you think the matching transformer, in the original post, will smoke.

Perhaps if the line out vol was up full and the line output got connected to a low impedance load, or accidentally shorted.

[R.G.]

Easy enough to add a 1K resistor in series with the secondary.

[FUCHSAUDIO]

Thanks all.

Mr. Fuchs, under what conditions do you think the matching transformer, in the original post, will smoke.

The lack of a load, very high powered long time operation ? I think adjusting the input to the transformer solves two issues: not potentially zapping the transformer and keeping the output drive impedance low and consistent. I might be off-base, but I think I make two valid points here. You could try it both ways I suppose.

[labb]

This works

http://www.tedweber.com/media/import/wbalunsch.JPG