USB Audio DAC mod + LaserBoy Correction Amp

[drlava]

Just to add to what steve said, you will need the offset for blanking also. I applaud the effort and there are improvements to be made with the original schem, but I don't see them in yours, there are problems. Channel inversion is done in EzAudDac.ini settings, no re-wiring required. Your schem has only 1k input impedance so you'll get significantly more loading and distortion than with the original schem. 741 is a poor choice for galvo amps, the original suggested op amps are better in about every way. Like steve said, cheating split supply with 2k resistors will just get you poor results.

If you have time, feel free to take another shot, and I agree on one improvement.. X and Y really should have differential outputs similar to what you show.

[DMills]

Here is my shot at it, Gain and offset controls are around the first opamp, with the second just being an inverter.

The power supply is 9V AC from a wall wart or small transformer which is converted to somewhere around +- 12V or so by the two diodes and two electrolytics. The reference voltage for the offset controls is set by the zenner diodes so that supply voltage variations do not change the settings.

The 100 ohm resistors at the output decouple the opamp from the cable capacitance and help ensure stability and the common mode choke and 330pf caps stop RF from getting back into the circuit.

If I was doing it in anger there would be clamp diodes from each output to each rail to protect the opamps from downstream faults, and I might have used a different PSU (7815/7915 or similar).

The power supply could run several sets, so the whole thing does not need to be repeated for each channel.

Have fun.

Regards, Dan.

[weartronics]

Using the C-Media chips, the DC offset is the same for every output channel. It means you only need one potentiometer to null it and one op-amp to buffer it, regardless of the number of output channels you're using. Also, every software has a method to set the scan angle (and you can even use the volume control otherwise), so there is no need for any gain adjustment potentiometers. It means only 12 op-amps with fixed resistors are necessary to offset, scale and buffer 6 channels (X,Y,R,G,B,I) with fully differential outputs.

[drlava]

Much better, and heh, I love it when people use the blue box for the amp, too. nice job

[Stoney3K]

Here is my shot at it, Gain and offset controls are around the first opamp, with the second just being an inverter.

The power supply is 9V AC from a wall wart or small transformer which is converted to somewhere around +- 12V or so by the two diodes and two electrolytics. The reference voltage for the offset controls is set by the zenner diodes so that supply voltage variations do not change the settings

Why are you using a half-bridge for the PSU instead of a full one? Would help a lot with the ripple on your DC lines. Great job on the zeners for voltage reference and the RF choke filter on the end.

Weartronics: I like your way of using SMD components on the bottom of the stripboard. At first, I figured there were some bits missing

Are those huge resistors (100k gain pot) not going to wreak havoc on your signal by introducing noise? The output on the C-media is line level, but I plugged in many a headphone on the thing, so a load of a few hundred Ohms to 1K should be fine.

[ElektroFreak]

Using the C-Media chips, the DC offset is the same for every output channel. It means you only need one potentiometer to null it and one op-amp to buffer it, regardless of the number of output channels you're using. Also, every software has a method to set the scan angle (and you can even use the volume control otherwise), so there is no need for any gain adjustment potentiometers. It means only 12 op-amps with fixed resistors are necessary to offset, scale and buffer 6 channels (X,Y,R,G,B,I) with fully differential outputs.

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

[Stoney3K]

Using the C-Media chips, the DC offset is the same for every output channel. It means you only need one potentiometer to null it and one op-amp to buffer it, regardless of the number of output channels you're using. Also, every software has a method to set the scan angle (and you can even use the volume control otherwise), so there is no need for any gain adjustment potentiometers. It means only 12 op-amps with fixed resistors are necessary to offset, scale and buffer 6 channels (X,Y,R,G,B,I) with fully differential outputs.

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?

[mixedgas]

Wow, a actual tech thread on pl longer then 5 posts, keep it up guys!
That has not happened in a while
Steve

[DMills]

I just had a further thought:

As there is no need to swing beyond +-10V (And thus getting an extra 6db of headroom is not important), there is no reason to bother with the second opamp in my diagram at all, just ground the end of R4 that was fed by the opamp on the board, job done.

The line is still balanced, but you have saved half the opamps (A balanced line is defined by impedance, not equal and opposite voltages).

Regards, Dan.

[Stoney3K]

The line is still balanced, but you have saved half the opamps (A balanced line is defined by impedance, not equal and opposite voltages).

Regards, Dan.

The line would be matched to the cable impedance in that case, but not balanced, since ground and the negative end of the signal line coincide. You'd make it a matched single-ended line, not a differential balanced one.

I don't think you're going to run in a lot of trouble, only if you expect quite some common-mode noise or AC hum. Aside from the fact that you're strictly operating out of ILDA spec.