Bare correction amp board

[dooiespons]

I did for the rear and sub+centre channels, but the front channels are a bit harder.
The lines are coupled with the headphone jack on the front, so i have to check that with my multimeter.

By the way it isn't a PCI card, it is this one: http://cfs4.tistory.com/image/9/tist.../47d4f5ec6fae7
Thanks for the info on the KHz part- i'll check this week if it really is 96KHz, just to know.

About the copper clad board: I wanted to try this, but i didn't have the time to make a circuit for it.
Maybe i'll try it tonight anyway; because im getting a bit impatient about seeing my galvos in action

[james]

Are you absolutely sure you have a DC coupled signal?

Have you tried making a wave of the white circle in LaserBoy and playing it to measure a nice solid constant (negative) voltage from the color channel lines? You know it needs to be negative because the correction amp is an inverter. Actually, it will not be a true negative voltage. It will be a voltage that is positive but close to zero. Since the DAC puts out all positive voltage, the center of the signal (digital zero) is the DC offset and the negative swing of the signal takes the voltage down close to zero. Once you build the correction amp (and apply the offset null) with a true differential power supply for the op-amps, you will get true negative and positive voltages.

The reason I ask is because I have tried to tap Soundblaster cards in the past and had no luck. They're weird. There is usually a lot of electronics after the DAC.

When I started on LaserBoy it only worked in Linux, so I just looked in the Linux kernel configuration for sound devices that were supported there that could do 6 or 8 channels. I found that both C-Media and VIA make good DAC chips that are found on both PCI cards and external USB devices. Those are easy to tap for DC signals.

Another thing that is very important is the actual DC offset voltage that you find on the DAC channels with no signal being played. Some of them are about 2.25 to 2.5VDC. Some of them are lower, like 1.2VDC. If you find the lower voltage, the original design of the correction amp will not work! It was designed to null out a voltage in the 2.2 to 2.5VDC range. The original design uses either a 5.1V zener diode or a fixed negative 5V regulator. I redesigned the amp with different resister and pot values for finer adjustment and changed the negative reference voltage unit to be a variable negative voltage regulator. So it works with just about any kind of sound device with a DC offset.

BTW, if you make the right kind of original art, you can display it on scanners without having to do the DC mod! Spirograph type images are made from periodic functions (sine waves and derivatives) and if they are centered at the origin of space, they will scan correctly even if there are decoupling caps in the signal paths. (Just an idea since you want to get your scanners humming!)

James.

[dooiespons]

Im not absolutely sure yet, since i don't got a fully working correction amp yet
I did make a version on a breadboard though, with 2 tl072's and 3 in/outputs (x/y/red) but i didn't have time to fully test it yet.
I wired one output from soundcard->correction amp->multimeter, and everytime i play a signal, the signal goes from 0.00 to -1.00~, and back to zero.

And this soundcard contains indeed a LOT of electronics.
I noticed that there are many capacitors on the board.. so i guess it will take some time to find out where to correctly place my wires.

And that last idea is really good actually, maybe i'll try that tomorrow!

I also thought about hacking an old HP terminal i got (T5720) to do some testing with.
The fun part is, this small terminal actually has an PCI port, and can run linux easily.

So many ideas... so little time



By the way mixedgas: I've send you the money by paypal and a pm, but im not sure if you received the pm. My inbox is acting a bit strange

[james]

If you use LaserBoy to make waves, you can make them inverted. That means that positive scanner coordinates and color values get stored as negative numbers in the wave file. A negative number results in a drop in voltage from the sound card DAC and that translates to a positive voltage on the output of the correction (inverting) amp.

LaserBoy also lets you make quite a few math generated figures that will scan properly with a capacitor decoupled audio signal. But you will probably still need gain to boost the signal voltages high enough to drive the scanners.

James.

[james]

Contact me on Skype and I will help you with everything from building it to testing it and adjusting all the trim pots.

Skype contact = james_laserboy

James.

[dooiespons]

Wow, thanks for the support! I'll add you tomorrow.

I've also just found a old sweex 5.1 PCI soundcard, with the CM8738 chipset.
It has a lot less components on it than my usb soundblaster, so i hope this one will be easier to mod.

[james]

Yep! Done that one!

[dooiespons]

I've added you on skype for some small questions