Topic: Howto: simple Differential Amp + TTL ?

[Xer0]

Hello Folks!

I have a 6ch CMedia ready and want to build an quick & dirty correction board.

Could you help me?

I drawed a plan with the patient help of Nicolas (weartronics) and his great Differential Amp Design. As i cant get the ±9V DC Converter here, only 5 to ±12V, which wont get enought power from usb; i thought of taking one of the 12V wallwarts i have colleting dust, and creating ±5V from it by splitting regulated 10V. the dropout of the IC is 2V so it fits, or not?

Also, i have a only TTL modulated Laser (Pf-114) so i dont need real linear amplifying by now, it should just be able to blank the Laser with Spaghetti and HE. Maybe someone could give me a solution that uses only Transistors for the RGB chan or even raw unamped Soundcard Output.

Sorry im a total noob in Electronics. i can solder even SMD without problems but have no clue how the stuff works and how to design a circuit from scratch and what parts i need etc...

Thanks
TC

[James]

USB 5 to ±12V will be fine.

Or just use the power supplies from your scanner amps.

Or get a powered USB hub for more current to the DC/DC.

That circuit is a bit more complex than it looks.

You are better off paying a little bit for trim pots.

Differential output will gain you nothing.

The sound card is single ended.

The offset NULL voltage that gets mixed in each op-amp is always going to be negative.

James. 

[Xer0]

Like i understood his design, this circuit creates Symmetrical signal from the DC of Soundcard, assuming idle Voltage 2.25 is Zero, 3.8 is +5/-5 and 0.7 -5/+5 ... or so 

Where should i place trim pots there? instead the 150k resistor?

Mhh i wanted to build it as small as possible and put everything in the DB25 plug ^^

[James]

His design is exactly the same idea as Dean's original single op-amp design.

One stage of op-amp is all that is needed to mix two things together: 1. The signal with a positive DC offset and 2. A regulated nagative voltage to NULL the positive offset.

Dean's amp design has 2 trim pots on each op-amp; 1. To trim the offset NULL voltage and 2. to set the gain of the op-amp.

This allows you to know exactly what you have and adjust it even between the channels.

You could simplify this design a bit and elliminate the offset trimmers, but you will not measure zero volts on all of your corrected channels at the same time.

James. 

[Fanny Pack]

Differential output will gain you nothing.

The sound card is single ended.

Differential output is for canceling out cable noise and is very useful regardless of what the soundcard has.  Add it!

[James]

How far apart will your DAC be from your scanners?

If it's less than 20 feet of wire, don't worry about it.

Is your home stereo all XLR connected?

James. 

[Xer0]

How far apart will your DAC be from your scanners?

Apart? ehm... my plan was to integrate a small circuit in a DB25 plug, tie the SC to it and plug it into the Scanner. like these Scart DVB-T Receivers :<

You could simplify this design a bit and elliminate the offset trimmers, but you will not measure zero volts on all of your corrected channels at the same time.

How much will they divert from 0?

If its just some mV... screw it. it will maybe shift the projection a tiny bit; what i can correct in most software; or just add 2 pots to xy. for the colors... they are TTL, who cares.


So there will be no Difference if i use Differential ±5V or Simple 0..10V ?
How its then "understand" by the Scanner Amp?

[Fanny Pack]

Who knows how far it will be?  That's why you use differntial outputs and don't worry about it.  I used good shielded cables with my home stereo and they are short.

[James]

How many sound cards have you modified and attached to correction amps that you made, tuned and tested on your own laser system, Gary?

If all you have is TTL color then, you're right. You don't need that kind of precision. And also you are correct for X and Y. The center of the projection will only move a tiny amount for offsets in the millivolts. You probably won't even see it.

How different are the output voltages on the DC side of all of the DAC channels now, without adding any correction? This will probably get worse with a bunch more fixed resisters and gain will exagerate it.

You connect the single signal end of the correction amp output to either the positive or the negative input on the scanner. The other input phase of the scanner should be grounded. The difference between putting the signal to the + or the - will flip the image.

Do you have an electronics prototyping board like this?

http://www.akrobiz.com/laserboy/p_laserboy_38.html
http://www.akrobiz.com/laserboy/p_laserboy_39.html

James. 

[Fanny Pack]

What kind of question is that?  Just because you play with soundcards in your basement doesn't mean you know what you are talking about.  I have modified two sound cards to answer your question.  All of my professional laser DACs have differential outputs.  At work all of our adapters have differential outputs.  Lots of things have differential outputs.  How many things with differential outputs have you worked with on a daily basis that makes you think you know what you are talking about?

[Xer0]

Do you have an electronics prototyping board like this?

Soon. i want to get all parts plus one breadboard from Reichelt. but i have to get the nice coloured bridge wires somewhere else, are out of stock now.

@ Gary: How does it help to cancel the Noise?

[James]

Get a nice hunk of telephone wire for the jumpers.

Solid copper AWG 24 with thin plastic insulation is what you need. Any Ethernet cable with solid conductor wire is perfect.

Long lines can sometimes pick up electronic noise like an antenna. If you pick up the same noise in two wires and they come together as + and - signals, the noise is cancelled out. +noise -noise = no-noise. The output impedance of the correction amp in combination with the input impedance of the laser projector inputs, at that length of wire, makes differential just about pointless.

BTW 60Hz hum doesn't come from an unballanced system. It comes from a system with grounding problems.

James. 

[Fanny Pack]

Differential outputs have two wires, usually twisted with opposing voltages.  So, while one might be +5V, the other would be -5V.  In this case, the voltage difference is 10V. 

When noise is introduced in lines from crossing over a power cord or being near a motor or whatever, it is typically introduced equally in both wires so if the noise happens to be +1V (that is very extreme and just an example), the +5V line would go to +6V and the -5V line would go to -4V.  The difference between them is still 10V!   So, even though 1V of noise was introduced, it didn't effect the received differential voltage at all.

On the other hand, with signal ended systems, one line is grounded and stays at 0V.  That 1V noise would change a +5V signal to +6V and the 1V of noise is seen at the receiving end. 

Noise is usually in the form of something like 60Hz hum and that is very noticeable visually.  That is why it is important to eliminate it if you can.

[Fanny Pack]

Even ethernet uses opposing signals in their lines to do the exact same thing as differntial outputs.  Ethernet cables aren't some sort of magic wire than just prevents noise on its own.  James, if differntial outputs are pointless then why is it industry standard on just about anything that transmit data over wire? 

[James]

Because that data is moving at well into the Giga Hertz through bundles of thousands of wires running hundreds of meters.

Get a clue.

James.