USB Audio DAC mod + LaserBoy Correction Amp

[buffo]

Any last minute suggestions you feel I should take in to account?

Just to be patient when you tune. It's a time-consuming process the first couple times you do it. Also, for best results, you might want to manually set your scan angle to something like 6 degrees, rather than relying on the method listed in my tutorial. (The "watch the circle" method I mention in the tutorial is a quick and dirty way to get close to where you want to be, but if your tuning is way off to start with, that method might lead you to scan a bit too wide.) If you tune at a scan angle lower than the scanners are capable of, you're fine. But if you tune at an angle greater than they are capable of, you'll have lots of problems. So a slightly smaller scan angle is helpful when tuning.

Note that by "the angle they're capable of", I'm referring to the angle they can correctly scan the ILDA test pattern at. This angle is different for each speed, and it will vary slightly even between scanners of the same make and model. It's not a number that is published, and it's much less than the max rated angle listed in the docs. It *should* be around 8 degrees or so for the rated speed of the scanners, but there's no guarantee...

Adam

[DPSS]

Thanks Adam, I'll be sure to follow your suggestion.
As soon as I find the time, I'll retune and post back my results (with pics ).

Domingo.

[DPSS]

A little help please, I'm having trouble tuning the scanners following Adam's tutorial.
I'm trying to tune the 12k/30k ILDA test pattern at 12kpps at an angle of 6 degrees.
Using the ILDA test pattern and the quadrature pattern, I've dialed back the scanners to the point when HF and LF damping are completely down to zero and gain is just enough to see the square before it collapses.
Then I start increasing the gain and turn up LF damping to compensate for the overshot, until I start seeing some undershot, moment in which I start adding HF damping.
After a few rounds of adjusting gain, LF damping and HF damping, I switch to the ILDA test pattern but, no matter now much I increase gain and adjust both the dampings, the circle remains outside the inner square. I eventually get to maximum gain on both scanner amps and it's still outside.
¿What am I doing wrong?

[drlava]

try dialing back down and bumping your point rate up to 20kpps.

[mixedgas]

A little help please, I'm having trouble tuning the scanners following Adam's tutorial.
I'm trying to tune the 12k/30k ILDA test pattern at 12kpps at an angle of 6 degrees.
Using the ILDA test pattern and the quadrature pattern, I've dialed back the scanners to the point when HF and LF damping are completely down to zero and gain is just enough to see the square before it collapses.
Then I start increasing the gain and turn up LF damping to compensate for the overshot, until I start seeing some undershot, moment in which I start adding HF damping.
After a few rounds of adjusting gain, LF damping and HF damping, I switch to the ILDA test pattern but, no matter now much I increase gain and adjust both the dampings, the circle remains outside the inner square. I eventually get to maximum gain on both scanner amps and it's still outside.
¿What am I doing wrong?

Increase the scan speed and tell us the PPS when it just collapses inside the square.

Steve

[DPSS]

It gets gradually smaller as I increase the pps and at 28kpps it's arround the same size as the inner square. I could of sworn I tried that and it didn't work .

Domingo.

[DPSS]

So isn't the tuning process the same when doing it at 12Kpps?

Domingo.

[buffo]

Yes, the tuning process is the same. Your problem is that your gain is too high. You've overshot your optimal tuning point for that speed.

Remember: With 30K scanners, your tuning point is going to be *very* low if you tune for 12K speeds. What has happened is that while you were going up the stairs (and only looking at the quadrature square wave and LaserMedia patterns), you over-shot your tuning point for the ILDA test pattern. In other words, if you had switched to the ILDA test pattern sooner, you would have been OK.

To fix this, you need to walk the scanners back down the stairs with the ILDA test pattern displayed. When the circle shrinks enough that it's inside the square, stop. Now start tuning up from this point. It should be easy to see the point where the gain is just high enough to keep the circle touching the sides of the square.

Adam

[Stoney3K]

I've had a look at the Laserboy amp and, since I've been in electronics for quite some years now and have tons of spare parts lying around, I'll try to whip up some amps on a breadboard the coming week. I have a 6-channel Trust USB sound card with a C-Media chipset, which will probably work with Laserboy or Spaghetti just fine.

I've whipped up a schematic with some improvements on the 'original' Laserboy amp:



Now, the thing seems pretty simple right now, but it doesn't have things like offset adjustment and the likes yet. I might or might not implement that, it's just a matter of adding a few resistors to the first amp to add DC. My galvo driver board has offset adjustment already, so I might not need it at all.

The output from the original LB amp was single-ended and inverted, this one outputs a nice (right way up) differential voltage which is compatible with all ILDA hardware. If the output happens to be inverted, just swap the + and - wires and the image will magically mirror itself

I've dropped the whole power supply and intend to use a single 12V wall plug style adapter. Note that the adapter ground is not connected to the actual unit (and ILDA) ground and therefore also not to the casing, instead, it supplies -6V since the ground is right in between. There is hardly any current draw so the resistors will be OK.

The gain will max at 10, but the op-amps will probably hit their rails way before you reach that point. If the max gain proves to be too much, R1 could be changed for a bigger resistor (2k2 or 2k6 will give a max gain of about 5). 741's are used here, but feel free to use any piece of sand that is pin compatible.

[mixedgas]

I think you misunderstand the purpose of the galvo board offset control. It sets the operating point of the CLASS AB power amp. Using this to offset a image reduces headroom and causes un needed heating of the driver stage.

BTW, you need to rethink using 2Ks to set the ground of the 741s, and go with a real split supply for your cable driver.

Steve