Topic: Howto: simple Differential Amp + TTL ?

[meandean]

  Jim, I don't think that you get what a bridged (differential) output is... (+2.5V) - (-2.5V) = 5V; (-2.5v) - (+2.5V) = -5V. It's simple algebra... It's how much car stereo gear works...

[James]

OK. I guess I get that. I get a bit mixed up now and then. I thought from the beginning of this thread that the point of doing it this way was to achieve +/-10V swing on the galvos AND differential output as per the ILDA spec.

James. 

[Xer0]

I bet the DC/DC converter you already have has ground isolation; meaning, you need to tie the input voltage ground to the output voltage ground.

To be true, i dunno. Has V+/V- (usb gnd) In but V+, Common, V- Out ...That's the Datasheet:
http://www.datasheetarchive.com/AM2D-0512DZ-datasheet.html

With the 5V... thats hte pity. i didnt expect that my Laptop's USB is so "underspec"
With the iPOD Charger i get even over 5V so it goes to -5V without loss.

btw, the DC-DC i bought for +-12V is unregulated too...
Without Load its puts out 20V on each channel also almost 40V p-p

[James]

This is the DC/DC I use.

http://www.newark.com/xp-power/ia0509s/dc-dc-converter/dp/88M0693

It works GREAT, even with sagging USB +5VDC input.

The input GND and the output GND are not connected inside the device, but they are connected on the board of my amp.

James. 

[Xer0]

Looks almost identical to my one even with same method of naming in the part number.

shoulda be the same inside...

[meandean]

btw, the DC-DC i bought for +-12V is unregulated too...
Without Load its puts out 20V on each channel also almost 40V p-p

  Wow...

  Life would be a bit easier if you could get a good DC-DC converter like the one James uses, but if you keep the ICL7660,
make sure that the specific one you choose has enough current output- some are as little as 10mA (that might not be enough),
others can provide more.

[James]

Hook it up and measure it again, knowing that the GND in and out are not common.

In other words, tie the GND in and out together and measure it.

James. 

[Xer0]

Ok, two things:
1. It didn't explode when shorting -Vin to CmnOut and gives now ±20V over GND
2. The 20V output is a load case only. with just 40k in series between + and - Out it goes down to ±13,3V. So my question is: will it blow 16V elko's? i bought a bunch of them just for the case. or should i drive for 35V to be safe?
btw: James, could you measure Voltages on your DC-DC without any load (idle) please?

[Xer0]

Has an Substactive OpAmp Setup (with positive Vref) advantages versus the Additive you all use here; besides that it dont need an inverter or negative regulator to make Vref?

[meandean]

Has an Substactive OpAmp Setup (with positive Vref) advantages versus the Additive you all use here; besides that it dont need an inverter or negative regulator to make Vref?

  Not sure, since I haven't tried it that way yet. The summing amp does allow the gain to be turned all the way like a volume control, if you need that functionality. Non- inverters have a minimum gain of 1.

 Experiment with the single-ended Weartronics circuit and see what you get...

[meandean]

So my question is: will it blow 16V elko's? i bought a bunch of them just for the case. or should i drive for 35V to be safe?

 I like to have at least 50% voltage headroom on eletrolytic caps as they sometimes fail.

[meandean]

 HA! I found an IA0509S (5 to +/- 9v), and without load it produces +/- 12.3v...

 Some regulators do require a load to work properly.

[James]

Whenever I build an amp board, I always test it before I put the chips in the sockets. I'm pretty sure that leaves the DC/DC wide open with no load. With a +5.4VDC wall-wart for testing, I get a bit more than +/-10VDC; which seems to be perfectly normal.

I use 16v 10uF Tantalum caps, as recommended for the DC/DC and the LM337 negative voltage regulator.

James. 

[Xer0]

Ohh dammnit!
I have it all set up now, have the right offset and gain set, and adjusted it while playing max.wav

And then the Shock: ILDA uses female plugs and i ordered default male DB25 ones -.-
But my projector has ILDA Out too in which the plug fits. afaik its just simple passthrough, so i thought this shoulda work when i connect it to the output instead. but nada!

While playing "The Circle" in Spider, the Laser makes a standing dot to one side and dont moves. It switches side when switching +- pins on breadboard. And it turns the other colors on when you plug out signal cables on board. So there seems to be something switched or so...

Any hint why it doesnt work? Meanwhile i will return the D-Sub Cable for changing to F-M one.

[James]

First of all, do you get mesurable signals, when you play wave, from the output of the sound card itself?

Unplugging the sound card signals will take away the positive offset and you will be left with the voltage ref with gain; NOT zero volts.

The Circle is a sin wave in one channel and a cos in the other. You should be able to read it with your meter set to AC volts. The silent DC offset on the DAC is zero volts AC. The colors should show a big change on your DC meter when you play and stop the wave. If you can measure the silent DC offset on all channels, you will know that you tapped the capacitors correctly.

James.