Balanced Line Transmitter : How to Generating a Perfect Differential Signal

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In the previous post we discussed the balanced / differential transmission system , which discussed the basic mechanism and advantages of using a balanced / differential transmission mechanism. With this mechanism, common-mode noise received by both transmission cables is eliminated at the output.

In this post we will discuss the principle for generating a differential signal - two equal amplitude signals but are 180 degrees out of phase. There are many ways to achieve this, from simple methods to highly sophisticated ones.


Legendary Drivers in the Industry

Some of the most legendary balanced line drivers in the industry, especially in professional audio, include:

  • The DRV134 and DRV135 from Texas Instruments (TI), which are also produced by many other major IC manufacturers due to high demand.

  • The SSM2142 from Analog Devices.

  • The 1606 family from THAT Corp., which was developed with various improvements and extra features and was well-received by the industry.

  • And there are many more modern balance line drivers that are increasingly perfect.


Construction and Cross Feedback Principle

While modern models continue to be refined, the basic design of a professional balanced driver remains the same: it uses two drivers, one non-inverting and the other inverting .

Professional drivers utilize a cross feedback mechanism. The signal from the non-inverting driver's output VOP is fed back to the inverting driver's input. Conversely, the signal from the inverting driver's output is fed back to the non-inverting driver's input. In the diagram, this is shown by resistor R6 from VON to U2and R10 from VOP to U3.


This cross feedback construction creates an interlocked between VOP and VONIf you analyze the relationship between VOP, VON and VIN using Kirchhoff's Current Law (KCL), you'll see that their difference, VOP - VON, is always equal to 2 x VIN.

The purpose of this interlocked design is to ensure that if a load deviation occurs on one output (within certain limits), that deviation is reflected onto the other output. This keeps both outputs in a perfectly balanced state, which is crucial for signal integrity.

For an additional reference, you can try building your own balanced driver. Elliot Sound Products is a legendary site for DIY enthusiasts and the audio industry.

I hope this is useful. TABIK.


(This post is parallel to the status on the FaceBookGroup The Art of Electronics with the same Topic)

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