Soundcards, DACs, and OP Amps

[slymartins]

Hey folks,

Pardon the noob question, but search only yielded discussions that got increasingly technical with every post, which didn't do me any good.

What is the simplest/cheapest way to amplify your PC soundcard headphone or line stereo output to drive a couple of X/Y scanners? I'm looking for a bare-bones approach: left channel going to a X scanner, right to Y scanner (be that an old HD galvo, cone speaker, mechanical relay, lama hair, whatever).

An output in the range of -10v, 10v would be great since I'm mostly interested in investigating different scanning methods and that's a good voltage range to plug in any mixture of different type actuators.

Appreciated.

 

[Bionic-Badger]

Yeah, unfortunately while that thread has a lot of good information, there is a lot of drama, many deleted posts, and even some now-banned people in there.

Still, there are some gems in the rough in that thread. One is Benm's correction amp that will properly correct the output of your sound-card (remember to solder before the output capacitors) so that it can drive your galvos. I have personally built and used that correction amp and it works great.

Also, on the third page Benm posted a circuit for connecting a sound-card to a current sink for analog control of non-DPSS lasers. That works good for red and blue lasers. Otherwise you'll need another correction amp to the input of your analog-controlled laser.

I used this sound card for my USB DAC.

 

[slymartins]

Thanks Bionic, but that's the type of "circuitry" and amp-building wizardry I was hoping to avoid. I have no idea what a correction amp is or a current sink is for. I'm sure these will grant much better results, but I see guys on youtube doing this with just a single op-amp (though none actually explains how they wired the damn thing), some guys using the speaker signal from small powered speakers, and others even pluggin' in the HD galvos directly to the 1.5v or whatever it is from the soundcard output.

I could sacrifice a pair of speakers and go that route, but seems like a waste.

 

[Bionic-Badger]

It's not wizardry, it's basic electronics. I wouldn't even begin undertaking a project like this without learning more about basic electronics. If that's still too much, you ought to just save yourself the effort and just buy a ready-made solution.

 

[Things]

For a start, you'll need a method of applying an offset voltage to the soundcards output, as usually they float a volt or 2 above 0V. After that, it's as simple as using the opamp as an amplifier, and providing it with around +/-12V power (Unless you can get a rail to rail opamp).

 

[slymartins]

For a start, you'll need a method of applying an offset voltage to the soundcards output, as usually they float a volt or 2 above 0V. After that, it's as simple as using the opamp as an amplifier, and providing it with around +/-12V power (Unless you can get a rail to rail opamp).

That was actually very helpful, thank you. I'll continue my research from there and maybe turn it into a simple tutorial if successful. Still amazed I can't find one on it... seems like the most obvious avenue hobbyists choose to ease into lasers and projectors.

Your projector reference site is outstanding, btw. I kept on going back to it time and again. I'll be sure to make a donation. Also, let me know if you need help with any illustrations on the site. Will be glad to help with such a great resource. :beer:

 

[Bionic-Badger]

There is no tutorial because the correction amp is doing exactly what you need. The problem is that you're trying to jump to the solution without understanding the fundamentals of electronic circuits. Go study some basic electronics. The equations aren't hard for op-amps, and the correction amp's operation will become clear once you learn.

 

[slymartins]

I appreciate your attempt to help.

 

[Meatball]

Do the TL082s actually drive the scanners themselves? Or do they drive the input to the OEM scanner amps?

I would be surprised to learn that an op amp could power a load-bearing inductive load.

 

[Bionic-Badger]

No, the galvos have their own PID-based amplifiers/drivers that use differential inputs (+/-10V) for setting the position. Often the final stage drives the inductive load with a power amplifier chip. A popular power-amp that you'll see on almost all the commercial galvo drivers is the TDA2030; it can deliver about 14W of power, and the amp itself is usually fixed to a large heatsink. Power-amps have the same basic operational characteristics as a typical small-signal op-amp but are designed to drive larger loads, perhaps without the same fidelity as the smaller op-amps.

A good explanation of a home-made galvo driver circuit and galvo mechanical parts can be found here. The final stage of the driver uses a LM675 as its power amp -- which can supply about 3A.

Most galvo drivers are designed to work with the ILDA standard, and that's why they're differential. The differential input also helps ensure that you don't need a common-ground between the various inputs to the system (a good thing). Still, unless you can program offsets and such for your drivers, it's good to have a correction amp anyway to ensure that you have complete control of the range and offset of your drivers. For sound-card DACs this is critical because they output voltages for audio-ranges (~0-2V) which doesn't work for those galvo inputs.