Topic: Anyone tried the 6 channel card with +/-12 or +/15V?

[dar303]

I tried to exchange the switcher with a +/-15v one but it didn't work, I think it had to low current output. The negative rail dissapeared when I turned the scale pot to far down.
I'll make some new tests soon.

[James]

I tried to exchange the switcher with a +/-15v one but it didn't work, I think it had to low current output. The negative rail dissapeared when I turned the scale pot to far down.
I'll make some new tests soon.

Ask Hyena (carmangary) about that. He replaced his +/-9VDC DC/DC converter with a +/-12VDC one and I think it worked OK.

Just out of curiosity, why would you need that?

James. 

[Fanny Pack]

I never put the 12V DC-DC converter on the board.  I got two of them as free samples but they are still sitting on my desk.

But, the reason you would want them is because most scanners require +-10V to give you full scale.  All of my other DACs put our +-10v.   I think that the sound card DAC mod will have more street cred if you change it to have balanced xy outputs at +-10v.  The RGB outputs are fine as they are.

[James]

I thought about my choice of DC/DC converter voltage for a while.

Given the fact that USB +5VDC has a very limited current, I decided to get as much out of that "power" as possible by going with the lowest +/-VDC that would properly power the chips.

If you are going to use an external power supply, then I guess you could have anything you want!

So why bother with a DC/DC converter at all? Just make a little, linear differential power supply.

James. 

[Fanny Pack]

One thing I have noticed about all of my USB powered DACs is that the output voltage varies depending on which USB port and or computer I hook it to.  I used to love my USB powered DACs but when I found that they were ripping me off as far as laser power goes, I started using my EasyLase with an external power supply.  So, I don't really trust USB power anymore and I think that a seperate power supply is the way to go.

But, you're point about the limited current is one of the reasons I am reluctant to installing the 12V converters.  I noticed on their data sheet that their current output is less, which makes sense.  It would be interesting to find out how much power/current the sound card/correction circuit really needs.

[James]

(How much power it needs)

Dean could answer that question.

Power is power; Volts times Amps.

Less Volts, more Amps!

If you plan on using the same laptop all the time, you can tweak the trim pots on the correction amp to give you a solid +5VDC on all of the color channels at full-on. I have not noticed any significant change over time. I have not really looked into moving the USB device from one computer to another. I'll try that!

Also, with the way the correction amp is designed, it is possible to lift the +5VDC coming from the sound card and put another +5VDC power source in its place.

James. 

[dar303]

I think you can draw as much as 500mA from a USB2 if you request this from the port, that requires some extra data communication though.
If you are up to the programming you  should be able to put the port into battery charging mode and get more current.

From the USB specs:
"In Battery Charging Specification, new powering modes are added to the USB specification. A host or hub charger can supply maximum 1.5A when communicating at low-speed or full-speed, maximum 900mA when communicating at hi-speed, no upper current limit when no communication is taking place."

I would like to power the whole thing of USB but if I have to I'll use a real +/-15V supply to get the full +/-10V output.

How have you guys implemented balanced outputs? Cant you just invert the output signal with another OP and use that as the + signal (since the output from the 6-channel card is inverted)?

[James]

The output from the 6 channel card is not inverted! The correction amp is an inverter, so the result is inverted.

I'm not the electronics design expert that Dean is.  :

He has a special circuit in mind that not only gives you ballanced lines, but it also eliminates any possibility of ground loops. His idea takes power from the +/-15VDC scanner PSU. I haven't seen the plan, but is is completely ground isolated.

James. 

[Fanny Pack]

That'll work.  But if the card is close enough to the scanner amp that it can tap off the scanner amp's power then having balanced outputs is a wasted effort.  I think you'd be fine with just +-10V single ended output.  Balanced outputs are to prevent noise that can be picked up on long cable runs.

[James]

That's the first reason to use ballanced lines.

The other very good reason is to employ Dean's ground isoloation technique.

BTW... why not just increase the input sensitivity to your scanners so they swing from +/-5V ?

James.