Here is a quick project on using the DRV134 for converting an unbalanced to balanced signals. This type of signal transmission is usually associated with the professional audio or PA systems. It is a widely spread common “knowledge” that balanced signals sound “worse”. Those things are not top priority in most of the so called audiophile systems. However I don’t suffer from such prejudice.
I think balanced transmission lines are actually superior to the unbalanced once if done properly. Sometimes you might hear that balanced isn’t worth the efforts for such a short distance, like home audio. That might be true. However It’s a common knowledge that balanced cables have almost (if any at all) no impact on the sound quality. Properly terminated balanced line will give the almost perfect connection between two stages. And the best part is that you don’t need to shell out your one month salary to buy those super-duper unbalanced audiophile quality cables anymore. All it takes is a good studio quality cable and some nice well made connectors. Personally I’m more than happy with Neutrik connectors.
To make a good balanced line you need a proper termination for the both ends of the line. That’s where the unbalanced cables shine – they don’t need any termination. If you have a poor termination to the balanced cable you will end up with poor sounding system.
So how to do it? Well one way is to go fully discrete. That will give some great results but requires a lot of skills and will cost a lot. There is a more conventional way using opamps. Here is an example:
This one is the most basic way to make a unbalanced to balanced convertor. This one however does not present an equal output impedance to the bots sides of the line. Its only pros are that is cheap and can do the job if you have nothing else to use.
A more advanced circuit is shown here:
This one is used in my professional CD player – the Sony CDP-2700. IC403 is the input buffer, which provides impedance match to the low-pass filter of the previous stage. The conversion is done by the two opamps (actually it’s one dual opamp IC404) and it’s pretty straightforward. You have one inverting and one non inverting amplifiers with the same gain and mutual feedback. The trimmer at the output is there to allow setting for maximum performance. It helps to balance the two stages by improving CMRR and minimizing distortion. This is because you can never have discrete resistors precisely ant tightly matched. This circuit will require a scope and distortion analyzer for the best performance. The other way is to buy some large quantities of resistors and measure each and every one. This will of course come at a price and will require a lot of time and efforts.
The best (and easiest) way is to go fully integrated and use one IC to deal with all. These have internally laser trimmed resistors and allow for the best common mode rejection ratio (CMRR) and minimum distortion.
One such IC is the TI’s DRV134 and DRV135 AUDIO BALANCED LINE DRIVERS. These offer a complete solution for balanced audio signal transmission. This is how the circuit looks like:
The schematic is pretty much the suggested in the datasheet here:
For this project I’m using only high quality parts. By high quality I don’t mean overpriced audiophile dream parts of course. All electrolytic caps are Panasonic FK. Industrial quality, long life, high temp reliable capacitors – http://www.farnell.com/datasheets/731857.pdf
Those are improved version of the FC series.
Low value smal caps are ceramics. Standard X7R caps. Nothing fancy here. All resistors are metal film in 1206 case size. For this project I’m using these Panasonic resistors:
The XLR connectors are Neutrik – NC3MBH series:
This is how the board looks like:
The project is finished on a two-layer board to allow maximum shielding via ground plane and lowest noise.