Large Distance Optical isolation for DAC.

[Brian Guralnick]

I already found a good USB2.0 chip, except, Firewire offers a better supply current through the cable & TI's 400Mb/s 1394.a interface chip is only 2.04$

http://focus.ti.com/docs/prod/produc...umber=TSB41AB1

As for optical transmition, I found an 880nm IR led from sharp which has a 40MHz bandwisth. Price = 0.49$. As for photo transistors, they are too slow & the fast ones are too expensive. I'm going with photo-diodes. Typical 100 Mhz bandwidth photodiode costs 0.21$. +, with the transmition distance of 1 inch, just a resistor off of the photodiode is good enough to cleanly drive CMOS I/Os. Same goes for the IR emitting LED.

Try to find a laser-diode, or fiber optic emitter, that cheap.

40Mhz cheap LED!
http://smaecom1.sharpsec.com/sma/pro...ed/gl496_e.pdf


My last headache is the analog side. I want a real pre-amp, non of this built in DAC pre-amp BS. I'm currently leaning toward a photo vactrol design.

[Electro]

My last headache is the analog side.

The headache won't end on the analog side. It will end after finishing programming the USB driver for your device.

Brian Guralnick, do you have any links on photo vactrol design.

[Brian Guralnick]

Forget USB2.0, I found a 2$ 1394.a (fire wire) controler chip. Full duplex 400 Megabit. Made by TI. As for drivers, for now, I plan to use the port as a dummy com conection with the dummy com developement drivers from TI, &, Ill use my own CDPlayer software which will send straight to the com.


As for the vactrol design, I'm creating my own from scratch. I'm goind to work it out on a bread board this weekend. I believe I found the perfect way to absolutely compensate for all variations in the vactrols to always give you ferfect balance & perfect volume of 0 - 100%.

I think I'll start a new thread for this 1, under Source/Digital since this pre-amp will be driven from the balanced outputs of the Crystal DAC which I plan to use.

[tiroth]

Sounds fun. I'd be very interested in you posting your progress with the IEE1394 interface--this always struck me as a pain to work with, but it sure has some nice qualities. It would be great to have a trailblazer for this technology.

[Brian Guralnick]

The best thing about 1394, is that you can draw 7 watts from the buss powersupply. Try doing that with USB.

For the all - digital side of my sound board, 7 watts is plentiful.

Items which need 2.5v to run:

2 x GL496 IR leds, 40 MHz BW - L/R clock & Data bits for I2S DAC
2 x cheap IR leds - sends controlls to the pre-amp DAC & I2C buss of audio dac.
Altera EP1K30 - PLD
Altera EPC1 - boot prom
TSB41AB1 - TI 1394/fir wire/i.link One port transciever.
MT46V32M4 - 128 megabit ram buffer.
--------------------------------------------------------------------
After the 1 inch opto-iso gap, everything here
is run off a separate supply, isolated from the PC.
--------------------------------------------------------------------
CS43122 - Crystal 122 dB DAC.
GPS grade 12.288MHz oscilator.
1 x GL496 - send clock back to the all digital side.
4 channel 16 bit I2C dac for attenuation/pre-amp control.
+ all of the analog stuff.
---------------------------------------------------------------------
(The digital side will be modified to fit another project, a BQ video grabber, but, that's for a diy video forum.)

My software player will be doing all the up-sampling to 192KHz. for cheaper, slower PCs, it can be run in triangular oversampling, for those of us with 1.7GHz & faster, there will be a true Fourier sampling transform with programable high frequency tapper off.

[peufeu]

I used a transformer to send the clock from the DAC to the CDP. I think 3E25 toroid core, 1.2 cm diameter, 10 turns oneither side.

And I used another transformer to convey the SPDIF back from the CDP to the DAC. SPDIF has no DC so it goes through transformers.

Of course the SPDIF receiver is connected directly to the DAC chip but I don't think it matters really. Most of the noise comes from the player and the transformer takes care of it.

It sounds so good.

[Brian Guralnick]

I'm keeping mini toroid transformers as an option. I will still need 3 since I do not want any decoding logic on the DAC side. The only other digital components on the DAC side are the crystal oscilator, &, attenuation DAC for the Vactrol based pre-amp.

[Brian Guralnick]

Just finished 10 hours of new part entry into my CAD software, Protel99se. Building the schematic next weekend should be a breeze. The schematics will finally be posted.

[alvaius]

If you insist on applying isolation, which for a lot of good reasons presented here, I would try to avoid, you may want to consider Analog Devices ADum1100. It is based on a mems transformer with signal conditioning. It will actually do 100mpbs, though it is more the jitter that is the issue. The data sheet does not have jitter specs that I can remember.

Alvaius