Topic: Who's GOT? :: Who NEEDS?

[Fanny Pack]

I read over on PL some dude had an idea to use a single pot to set the offset for all opamps.  Seems like a good idea unless there is reason to believe that the sound card would have different voltages for each channel.  Further more, it seems like you could use a single gain pot for the colors and a single gain pot for X&Y.  That would reduce the number of pots to 3 per board instead of 3 per channel.  Cuts your price down to $2.89 for pots.  A savings of $14.44 per board.

[James]

I read that too.

It cuts it down to 1 trim pot for offset. You still need 6 pots for gain, unless you go with fixed gain and mess with the levels in software.

Also, his design does not use a voltage regulator or a zener diode for the negative reference voltage, so it will change with supply voltage.

Actually, it is possible with some sound cards to have no pots for offset. Some of the sound card chips out there have a positive voltage reference pin that is set to the voltage of the offset.

However, this correction amp design is generic and will work with any sound card with just about any DC offset.

And, yes, I have measured different offset voltages on every channel of the same multi-channel DAC. I think it has to do with the tolerance of the resistors in the analog outputs of the sound card.

I think I did pretty well finding trim pots at 3 for $4!

James. 

[meandean]

 I measured the raw out of my USB DAC and found the offset difference between channels was no more than 20mV, worst case. The idea of one offset pot may not be 'perfect' on paper, but may be a real practical alternative with only about a percent or two of error (but it is very possible that the op-amps could interact in a bad way depending on exactly how you wired it, I would want to do some tests to be sure- it might involve adding some fixed isolation resistors). As for use of a single pot to control gain across more than one channel, I'm not sure how to pull that off...

 The funny thing is I scribbled up a design for an audio correction amp years ago, and I've yet to wire one up for myself (I suppose it would help if I had something to connect it to).

[Fanny Pack]

Anything you can do to reduce size, cost, and complexity makes it more attractive to potential hobbyists.  I think 18 pots might scare some people away.  If you can get it down to just a few and come up with a self tuning utility app then you might have something.

Hint:  Maybe you can use ground and 5V from the USB as a reference signal for setting the output.  If you can have a switch that will allow showing a zero signal and let the user compare that to the offset zero value they can use that to set offset.  And then they switch between a 5V ref signal and DAC output to set the gain.  No oscope or multimeter would be needed!  Just a simple app and a couple switches or test points or something.  Figure it out.

[James]

13 pots; 2 per channel and 1 for the variable negative voltage regulator.

I think your idea of using USB zero and +5VDC as a target for setting color gain is a good idea, but it would be better to use a +5V regulator on the positive output of the DC/DC converter.

USB voltage sags like crazy.

I guess it could be done with a cheapo little needle meter. You know... another kind of galvonometer.

What the hell. A multimeter goes for about $4 these days. You could build one right into the DAC package!

James. 

[Fanny Pack]

Yea, better to use a true 5v output than USB but USB.  But, really, it should be either 5V or 10V depending on the galvos. Most are probably +-10V now.

No meter is required.  They can use their scanner.  Send a 0V signal and let them draw a mark on the wall.  Then change output to DAC and let them tweak until the mark is at the same spot.  Then show a 5V mark and let them mark it.  Then they adjust gain until their mark hits the same point.  I am not too concerned with overdriving the galvo amps and causing damage since the signal would be constant.  If it was a sinewave that caused the galvos to whack back and forth to their limits I would worry.  But, any overshoot can be avoided by instructions to dial down both gain and offset pots to min before starting the tuning procedure.

Not sure why I was thinking 3 pots per channel.  But 2 per is still a lot when you have 6 chs.  I bet you could even just choose some resistors that are on the conservative side and not deal with pots at all.  At least if it is for the same soundcard.

[meandean]

I bet you could even just choose some resistors that are on the conservative side and not deal with pots at all.  At least if it is for the same soundcard.

  I did some more checks on my USB and found that the channel balance was within 1%.
I expect it would be possible to cook up something with no pots other than a gain for
X&Y. I like how James added stereo pots at the final output for both X&Y and audio.

  LWave has plots just for level setting purposes; James has an app of his own, but
it's not integrated into LBoy- add that to the to-do list.

USB voltage sags like crazy.

  I think yor laptop has a problem- the audio card gets un-glued every time I plug in a USB flash
drive...

[no-esc]

A friendly bump...

[James]

I got one of the shipments of stuff today!

Pretty amazing. All the way from Thailand.

So now I have TL072's, sockets, pin headers, tantalum caps, and enough trim pots to make two boards.

Waiting on the 1% metal film resisters.

James.