USB Audio DAC mod + LaserBoy Correction Amp

[weartronics]

Still, I think that's using a (somewhat unreliable) software workaround to fix a chip that sends out improper signal levels. Volume control should be clamped (at something like 50% to prevent clipping) and the software scan angle should be kept at max in your show software, which would give a peak-to-peak voltage swing of 10V on your ILDA outputs. (5V on the +, 5V on the -). If it's not, adjust the correction amp gain and not the output level of the card.

(That would be saying: Hey, our amp doesn't work right with the set design specs, let's just modify a design constraint to make it work!)

You don't start tweaking your galvo amps because your projected image is too big either, right?

Of course, the volume control and amplifier gain must be correlated to produce 10V p-p output, but I respectfully disagree with the need for hardware adjustable amplifier gain. There is an ideal amplifier gain, which maps the full-scale DAC output to 10V p-p. Any higher volume control/lower amplifier gain will produce clipping, and any lower volume control/higher amplifier gain will waste resolution. I never want either of these results, so I fixed the amplifier gain at the ideal level, and I set the volume control accordingly.

[DMills]

I am not really talking about matching transmission line impedance (irrelevant at a 100Khz or so over lines this short), and no the ground and neg end do not coincide as there is current flow in the neg leg of the balanced line and thus voltage developed across that resistor.

The diagram shows the arrangement where R1 and R2 are the line source impedances with R3 and R4 being the input common mode impedances of the receiver, V noise is the difference in ground voltage between the two ends of the link.

Obviously perfect balance is obtained for R1/R3 = R2/R4 and this is not dependant upon injecting the signal in anti phase onto both lines, just supplying the signal as a single ended drive in series with R1 will suffice (of course 6 db more swing would be available with anti phase drive, but as we only want 5V swing, who cares).... .

The sneaky bit is that if R3 == R4 and both are very much greater then R1 and R2 then smallish differences between R1 and R2 do not matter, so with a sufficiently high Z input stage, even bonding one leg directly to ground at the sending end will still leave reasonable CMRR!

Regards, Dan.

[DMills]

I tend to go with Weartronics on this, as long as you are prepared to measure (or look up on the data sheet) the peak output for whatever card you are using.

I don't see the need for huge numbers of pots (get it within 3db or so above ILDA spec and tweak from there in software).

Regards, Dan.

[Stoney3K]

Of course, the volume control and amplifier gain must be correlated to produce 10V p-p output, but I respectfully disagree with the need for hardware adjustable amplifier gain.

It may be perfectly usable for your situation (your own laptop) which probably has all of the settings just right and will stay that way. However, if you use someone else's laptop and the Windows sound system is a bit different, or you run Linux, your setting may be out of whack.

DMils: You're still claiming that having 6dB less of voltage swing doesn't really matter. Don't forget that it's half your scan angle, galvos are linear, not logarithmic like our hearing!

Op-amps and resistors are dirt cheap, so what budget are you cutting if you decide to make the output stage single-ended? I doubt you want to save the $3 on parts.

[DMills]

Well, we only need 10V total swing, (5V for the colour) so knocking 6db off is no big deal, because even a single opamp will swing more then 10V.

I would probably do a proper stage for the X and Y lines (if only so that fitting an X invert switch at the projector will still work if some numpty is making scanner drivers with single ended inputs), but the real point was that the colour signals could be effectively balanced at the cost of 3 resistors at the sending end.

Regards, Dan.

[Xer0]

Just a question... if someone dont needs the whole ILDA stuff at first, would it be possible to use just 2 correction stages for feeding the galvo amps, and for rgb just using the plain dac output and instead designing the driver to run at 100% with the max sound voltage?

[drlava]

It's possible, but why.. remember there still would be an offset on the sound card output

[Xer0]

Couldn't this be biased away on the driver itself?

It would save some parts and space, i dont need full ILDA compatibility for the beginning.

[weartronics]

I really like that amp board.. do you have a schematic for it you would be willing to share?

With pleasure, I have drawn the schematic for you this morning:


All channels (X, Y, I, R, G, B) share one offset buffer, and all channels use exactly the same differential amp circuit. The X, Y channels are bipolar differential, and the R, G, B, I channels are unipolar differential, but this is accomplished in software.

I suggest that you use a +/- 5V power supply with rail-to-rail op-amps. It means the output voltages are limited to safe levels without needing any additional circuitry. I used the IA0509S DC-DC module (from the original correction amp kit) to provide the +/- 5V power supply from USB, and TLC074 op-amps.

The offset adjustment potentiometer can be any value between e.g. 10k and 100k. The 47k resistors can also be any value between e.g. 10k and 100k, as long as they are all exactly the same. The 100R resistors are not critical either - their purpose is only to limit transmission line effects and short-circuit current.

I'm aware of many ways to improve this design, but I choose not to implement them because additional complexity is unnecessary. The schematic above works well.

It may be perfectly usable for your situation (your own laptop) which probably has all of the settings just right and will stay that way. However, if you use someone else's laptop and the Windows sound system is a bit different, or you run Linux, your setting may be out of whack.

As you can see from my screen captures in other threads, my work is in Linux. I tested the DAC using C-Media cards based on CM106L and CM6206. It works well using the ALSA-provided USB audio driver as well as my own kernel-mode driver on Linux. I also used a Windows computer to run some of the free Zoof-ware through the drlava EasyLase wrapper without any problems. What "setting may be out of whack" specifically?

[Stoney3K]

The offset adjustment potentiometer can be any value between e.g. 10k and 100k. The 47k resistors can also be any value between e.g. 10k and 100k, as long as they are all exactly the same. The 100R resistors are not critical either - their purpose is only to limit transmission line effects and short-circuit current.

Throwing in the same resistors on the input and feedback of an op-amp, will give you a gain of, er... exactly, one. That would work great as a buffer/inverter stage but you're not doing much amplification... what's the max output swing on the C-media boards?

What "setting may be out of whack" specifically?

If you only use your own system and tailor your correction amp/DAC for that, you've got no problems and this will work without any trouble. Migrating to another hardware or software platform might be a bit challenging, for example, the volume control in Windows or on a Mac is far from exact, and it takes a bit of guesstimating to get that right. Plugging the board into another computer might also cause some unexpected effects, some USB controllers and their power supplies have glitches.

The basic amp board is fine, but you could improve a bit on making it more universal.