AUDIO WORKSHOP

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.

Examples

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.

The schematic

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:

http://www.ti.com/lit/gpn/drv134

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:
http://www.farnell.com/datasheets/391390.pdf

The XLR connectors are Neutrik – NC3MBH series:

http://www.neutrik.com/en/xlr/b-series/nc3mbh

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.

The complete manufacturing files are available for download HERE


Complete schematic is available for download HERE

Whether you’re trying to put a bit of a personal touch to your PCB design or you are in need to use a company logo on your product one thing is for sure. You will need to somehow transfer the image from the graphics software to the EAGLE environment. Here is one approach I’d like to share.
This is a typical workflow of  how to put images onto a finished PCB design. Once completed this approach will allow you to quickly and easily insert your logo or any other graphics into your finished design.

First thing we have to do is to chose a proper format for our image which will be later transferred to EAGLE. When designing you logo or choosing an input image you need to follow some basic rules.
– Use a two color image. Preferably black and white image.
– Use a high resolution image. Low resolution images will result in uneven and distorted edges of the output. This will look bad on the PCB.
– Don’t use a image that is too complex for your PCB manufacturer to print on the PCB. You should be aware of what are the manufacturer’s capabilities for printing graphics on the PCB’s.
– Don’t use images or logos that include shadows or reflections. These can not be rendered in two colors and later on transferred on the PCB. If shadows or reflections are an important part of your logo design you better not use it on the PCB and stick with a simple text printing.

Following those simple rules will ensure you have a great end result. You PCB’s will have a professional look.

Now lets get started.

For this demo I will be using a simple schematic that is already transferred to a PCB and finished. Here is the PCB ready for a logo:

Now for the logo part. We will need something simple and understandable for EAGLE to put onto the PCB. This means we need a two color monochrome image. For this demo I have created a simple demo image:

It is important to say that you need a *.BMP file that is monochrome. Beware that some image files are not two color though they look so. For example, the image shown is not two color. It was created as a 24 bit *.BMP file. EAGLE can not read this image. What you need is to convert this image to 1bit monochrome *.BMP file. This is easily done using Windows Paint. Just open the image and use
Save As -> Other Formats -> Monochrome Bitmap
You will want to save the file with a different name and not overwrite the original *.BMP file. Paint will warn you that saving in this format will result in a loss of information. This is not critical. Please note that for this demo I am using a large resolution file. It is 2545px by 615px.

Now that we’ve prepared the input file it’s time to transfer it to EAGLE.

First thing you need to do is to create a new library. For this in control panel select File => New => Library

Use Save As to save this new library with a suitable name. For this demo I’m saving it as new_logo.lbr Inside the library editor click on Package button to open the package editor. Type the name of your logo in the popup window and click OK. Click Yes to confirm the new package creation.

Now that you have the new package you can import the *.BMP. In the command line type the following command:

Click OK and a browse window will popup asking for the input file. Find the location of the file, select it and click Open.

Next you will have to select the color which will be used to render the output.

Experiment here to see what is the output result 😉

After selecting the input color another window will show. This one allows you to scale the output render to suit your needs. For this demo I’m using the following settings:

Under Format table I use DPI setting. This is an easier way for me to predict the render actual size. Units go to Inch as DPI means DOTS PER INCH. Under Dots Per Inch table you must enter the scale factor. This is – how many pixels will fall into 1 inch. For the demo I have used an image that is exactly 2545 pixels in width. This means that if I wanted a 1 inch wide logo on my PCB I would enter 2545 here. To get a larger output you simply need to put less pixels per inch. For example a 5 inch wide logo will require 2545/5 = 509 pixels. This is what you enter here. Under Chose a start layer for 1st selected color enter – 21. This is the tPLACE layer which is used to create the silkscreen printing. When you are finished entering all the values click OK and wait for your PC to calculate the script. On slow PC’s this might take some time. This is what you should see once the script calculation is finished:

This means that you are almost ready. Hit Run script button and watch the logo appearing on you screen.

You might spot a small line at the bottom left corner of the logo. This is actually a text that shows the input file used for the render. Go ahead and delete it as this is not used.

If you have reached to this point you are almost done. Now all you have to do is to save the library changes (Ctrl+S).

Go to Control panel and use View => Refresh. Find the new library that you have just created and click the little green dot on the right of it. Now go back to the layout editor when you see your PCB. Click Add and find the library. The logo should appear as a component now. You just enter it straight onto the PCB. You can rotate it as you want.

This is how the PCB looks now:

Though this method is a bit long it gets really easy to use once you have created a set of logos you need. I hope you enjoy this feature of EAGLE and use it to create more professional looking PCB’s