Balanced line

Well i was wondering if there is a circuit out there that i can build or use for my problem here..

i have a aleph p1.7 preamp that has rca jacks or balanced i also have a aleph 5 balanced amp that im building and want to take advantage of the balanced fetures. now my cd player is rca is there a circuit that will convert rca to balanced ( xlr ).. i have a schematic from my dad that uses a cap to make balanced to rca but i dont know if that is backwards compatable. is it .. ?
 
JasonL said:
Well i was wondering if there is a circuit out there that i can build or use for my problem here..

i have a aleph p1.7 preamp that has rca jacks or balanced i also have a aleph 5 balanced amp that im building and want to take advantage of the balanced fetures. now my cd player is rca is there a circuit that will convert rca to balanced ( xlr ).. i have a schematic from my dad that uses a cap to make balanced to rca but i dont know if that is backwards compatable. is it .. ?

Something you might want to consider is the <a href="http://www.jensentransformers.com">Jensen</a> DM2-2RX IsoMax unit. At $200 it's not exactly cheap, but one of the best ways I've found to convert from balanced to unbalanced and vice versa is with a transformer.

You can also buy the raw transformers (either the JT-11-DMCF or the JT-11-DMPC depending whether you want flying leads or PC mount) and do it yourself. The transformers are about $80 each so you don't save a whole lot but it gives you more freedom with the design.

se
 
Nelson Pass said:
The Aleph P is perfectly happy to do this conversion
for you without loss of performance.

How will his Aleph P convert the unbalanced output of his CD player into a balanced output?

The way I read his post, he has a CD player with unbalanced outputs and he wants to convert those to balanced so he can feed the Aleph P's balanced inputs.

se
 
If your CD player doesn't have a true balance output, I think that additional circuit can only degrade the sound and benefit of going from CDP to preamp balanced would be diminished.

So you can either use quality transformer for conversion (if you really want to go full balanced from CDP to Pre) or use Aleph P to do it for you;). I would say this is the main advantage of using preamp in that place, because in addition controlling the volume it does the conversion. Unless you want to build Aleph P inside your CD player and somehow place volume knob on a front panel.;)
 
The Aleph P ought to be fantastic at converting unbalanced to balanced. If you are interested in building something new, the Bride of Son of Zen with active current sources at the Source pins of the gain MOSFETs also does the job nicely.

I'd just stick with feeding the input with the unbalanced output of the compact disc player.

Erik
 
Peter Daniel said:
If your CD player doesn't have a true balance output, I think that additional circuit can only degrade the sound and benefit of going from CDP to preamp balanced would be diminished.

You can achieve a true balanced output from the CD player without additional active circuitry or even a transformer. Just need to mirror the output network of the existing circuit, creating in effect a balanced voltage divider.

se
 
Peter Daniel said:
Nothing comes free.;) Do you have an example of such circuit? I've seen the Ono balanced converter, but it looks rather complicated.

I didn't mean to suggest anything came free. Only that it needn't be terribly complicated. My personal preference is to use transformers (though I just use them at the inputs as they give better common-mode rejection from single-ended sources than active balanced inputs give from balanced sources).

As for a circuit example:

<center>
<img src="http://www.q-audio.com/images/drivera.jpg">
</center>

Let's say that describes the existing output circuit.

All you have to do is mirror it, thus:

<center>
<img src="http://www.q-audio.com/images/driverb.jpg">
</center>

If the original circuit has no shut resistor, just use a pair of 100k resistors. Also, if it has an output coupling capacitor, mirror that as well.

Voila. A true balanced output.

se
 

Nelson Pass

The one and only
Paid Member
2001-03-29 12:38 am
Actually it's a fine circuit. The - line may not be driven,
but it does experience the same noise pickup as the
+ line, and will give good rejection going into a balanced
input.

I think you don't see it often because using it requires
understanding of the nature of balanced lines, and also
because the public has been educated to expect both lines
to be actively driven.
 
Peter Daniel said:
But there must be a price here to pay, otherwise everybody would be using that circuit, and to tell the truth I never seen it in use. But maybe I was looking at the wrong schematics?;)

I think the reason you don't see such a circuit very often is because it seems most people don't quite understand just what constitutes a balanced interface.

Most are of the belief that it means transmitting two signals of opposite polarity. So you typically see designs using some sort of active inverting circuit in conjunction with a non-inverting circuit, such as:

<center>
<img src="http://www.q-audio.com/images/driverc.jpg">
</center>

However all balanced means is that the IMPEDANCE of each line with respect to ground is the same. That's it. And when you take an unbalanced output and create the voltage divider as noted in the previous post, you've got a true balanced output. If there's any price to pay, it's that you don't get the additional 6dB of voltage gain that you get if you use a second active inverting driver outputting a signal of the same magnitude as the non-inverting side.

I've seen designs that maintain separate non-inverted and inverted signal paths all the way from the input to the output. And while such a design is "balanced" you don't get any benefit from it.

The benefit comes from feeding the balanced line into a balanced DIFFERENTIAL input so that any common-mode noise on the line can be rejected. If you maintain separate signal paths from input to output, any common-mode noise on the line will simply be amplified and passed on down the line.

se