[Pageygeeza]

A short while ago I thought of the idea of isolating the send and return feeds through an headphone amplifier.  After some discussions, I found out this was how balanced amplifiers work.  This then led me to another obstacle:  How can you turn a non-balanced source to a balanced one?  Basically you can't.  Which left me last night pondering about using a DAC as a balanced source.  If the DAC could output isolated send and return feeds, the rest is relatively easy.
 
I've trawled the net for a decent design, but haven't really got anywhere.
 
This is where you lot come in.
 
I'm not looking at getting someone to make it FOR me, I'd just love to see this idea come to life and hear it for myself.
 
It'd be great if all the great minds here in "Head-fi" could come together to realise this, with everyone's expertise this could become quite the diy project.
 
If this has been documented before in a diy project, then my apologies.

 

[Kees]

Turning an unbalanced source into a balanced source is done like this.

 

[mugdecoffee]

Almost all DACs output balanced signals (both send and return aka positive and negative outputs).  Most single ended DACs either don't use one pair of outputs or combine positive and negative outputs into a single output relative to ground.
 
In addition to transformers (Kees linked to an example above), there are opamp circuits that duplicate the signal and then invert it which are easy enough to build.  You might also be interested in active ground / 3 channel circuits where the signal return is buffered.

 

[Pageygeeza]

I like the idea, but admittedly all that's being done is separating something which has already been combined.  Kinda like merging two streams of water into one and then splitting it into two again, basically what comes out isn't what went in as it's been mixed.
 
I need the channel currents to be 100% separate.
 
Thanks anyway.  

 

[DaKi][er]

PCM2706 -> PCM1794 -> i/v converter -> 4channels of your favourite headphone amp
 
Read the datasheets of the parts I linked, start playing with a PCB layout editor, spend a few months playing with designs, look and research other peoples designs, and by the end you'll have something that'll probably work

 

[Avro_Arrow]

I see what he is saying...the D/A converter usually has balanced outputs but most people combine these at the first amplifier/filter stage.
If you use four op-amps in stead of two for the first stage then you could have balanced outputs. This is probably how they do it in pro audio.
You could then use the balanced output to drive a scaled down Nelson Pass Zen balanced amp and drive bi-wired cans.

 

[Pageygeeza]

Quote:

I see what he is saying...the D/A converter usually has balanced outputs but most people combine these at the first amplifier/filter stage.
If you use four op-amps in stead of two for the first stage then you could have balanced outputs. This is probably how they do it in pro audio.
You could then use the balanced output to drive a scaled down Nelson Pass Zen balanced amp and drive bi-wired cans.

Spot on Avro!

 

[nikongod]

 
Quote:

Turning an unbalanced source into a balanced source is done like this.

You could also use a differential pair in a discrete amplifier.
 
Quote:

I need the channel currents to be 100% separate.
 

You may find a substantial performance benefit by connecting things rather than seeking 100% isolation. Locking the 2 phases of a given channel rigidly together may help with forcing the balance in the gain and output stages.
 
 
Quote:

If you use four op-amps in stead of two for the first stage then you could have balanced outputs. This is probably how they do it in pro audio.

Most pro audio gear runs single ended "in the box". It is nearly impossible to build a balanced *adjustable* equalizer or mixer. In the vast majority of pro-audio gear balanced outputs and inputs are done (faked as many on head-fi would call it) with the use of transformer or SS devices.

 

[Pageygeeza]

This is the reason I'm looking for 100% separation.
 
I'm not looking to un-split the ground, I need ground for L/R to be 100% separate.  After looking at current flows on a normal amp and noticing at certain stages L+R WILL be mixed together.  Even though this is happening at 44/48 thousand times per sec, it's still changing the original waveform.  Certain combinations of current flow will change the original waveform and others will make the ground work harder.  All four channels will be working with their separate currents.

 

[jamato8]

Yes, you can get "faked" balanced but it does work. I have done it both ways and when you get the positive and negative output from the AC musical signal, the generated signal from a transformer is a balanced type that gives you some nice swing voltage and very nice control over the drivers or when used from a preamp to a balanced input amp. 
 
Quote:

 

You could also use a differential pair in a discrete amplifier.
 

You may find a substantial performance benefit by connecting things rather than seeking 100% isolation. Locking the 2 phases of a given channel rigidly together may help with forcing the balance in the gain and output stages.

Most pro audio gear runs single ended "in the box". It is nearly impossible to build a balanced *adjustable* equalizer or mixer. In the vast majority of pro-audio gear balanced outputs and inputs are done (faked as many on head-fi would call it) with the use of transformer or SS devices.

 

[pabbi1]

Are you just looking for a 1 box solution, or DAC built into the same board? I sorta went this way on my Millett Maximus in my sig, with an Twisted Pear OPUS and dual Millett Max boards, so, balanced and differential, I guess.

 

[Pageygeeza]

Yeah, one boxed would be a lot handier.  Trying to keep the connections between each stage at a minimum.
 
Basically once all four channels come out the DAC, none of them ever get joined.  So there is no interference between any of the channels.

 

[Avro_Arrow]

When I get some free time I will draw something up for you.
Do you have a favourite DAC or is PCM1794 good enough?
Are you going to use software volume or do you need a volume
control? Lastly, are you using low or high impedance cans?

 

[Pageygeeza]

I was looking at the PCM1794 pdf, but all the examples were merging the grounds.  I know it has a LOT of separate outputs for L/R and basically that's where I'm confused.  I'm looking at amping both +/- of both L/R channels.
 
As for the volume, I was looking at rotary encoders to do my bidding, so yeah software.
 
Most if not all of my cans are 32ohm, how easy would it be to implement a switch to select different impedance cans?
 
Cheers man.  
 

 

[dsavitsk]

1794 output needs a reference to generate a voltage, and that reference is ground.  Something like the wm8741 can be used without reference to ground -- into a transformer, for instance: http://www.ecp.cc/QDWM8741.html
 
I think you are not understanding what balanced is, and why you want to use it.