Rochey

8 channel AD converter idea/discussion
« on: November 17, 2005, 11:46:33 AM »
heya guys,

I'm toying with the idea of an 8 channel AD converter for use with one of those behringer control surfaces with ADAT i/O.

For increased functionality, I'd like to build it from 4 x stereo ADC cards. (which could be used for other things in the future)

My initial thought would be to use the front end from the EVM (because our EVM's are capable of the full 118dB) and have S/PDif outputs for each channel as well as the capability to use an ADAT optical for 8 channels of 44.1KHz or 48KHz

I personally don't think it'd be a LOT of hard work (especially if one or two of you can help! - especially with the analogue) and I'd be more than happy to put the schematics on here or sell PCB's.

Anyway, to the point of my post... I really don't fancy building a PLL to generate the various clocks from the word-clock. So, for word clock generation, I have a few options (and was hoping for your input).

1) Onboard crystals - the rest of the system would have to accept my word clock through either S/PDIF or ADAT.
2) Take an S/PDIF input from the PC.
3) Use an external ADAT input.

I have concerns with each of them though....

1) What if my crystal isn't good enough to clock the system?
2) What do I need an S/PDIF input for - would you really want to waste one of your S/PDIF outputs?
3) I'm not wasting an ADAT output for a clock!


Arghhh... any inputs you guys have would be appreciated.

Cheers

Rochey
Expat Audio Home: http://www.expataudio.com


clintrubber

Re: 8 channel AD converter idea/discussion
« Reply #1 on: November 17, 2005, 12:18:19 PM »
Quote from: "Rochey"
heya guys,

I'm toying with the idea of an 8 channel AD converter for use with one of those behringer control surfaces with ADAT i/O.


It won't be the input you're looking for and you know it already, but let me state the obvious:
why bother while that ADA8000 just begs to be used for that BCA2000 ?

Rochey

8 channel AD converter idea/discussion
« Reply #2 on: November 17, 2005, 12:43:56 PM »
sure, lets use some of those ultra-low-noise mic pre's that behringer make instead of building Green Mic Pre's  :grin:

I accept your point, but surely, one of the driving points of DIY is saying to yourself "I made that"  :grin:
Expat Audio Home: http://www.expataudio.com

Rochey

8 channel AD converter idea/discussion
« Reply #3 on: November 17, 2005, 12:56:09 PM »
A HA!

First step in World Domination -  By using the wavefront AL1402, even if there isn't a DAC in the box, it's possible to use it as a PLL to generate all the clocks needed from an external wordclock...

http://wavefrontsemi.com/index.php?topic=82

woohoo  :green:
Expat Audio Home: http://www.expataudio.com

clintrubber

8 channel AD converter idea/discussion
« Reply #4 on: November 17, 2005, 02:23:50 PM »
Quote from: "Rochey"
sure, lets use some of those ultra-low-noise mic pre's that behringer make instead of building Green Mic Pre's  :grin:

I accept your point, but surely, one of the driving points of DIY is saying to yourself "I made that"  :grin:


Fully agreed, I understand what you want to do. It's just that I've been 'contemplating' that BCA2000 a while ago and had to say that apart from a few cut corners,  Beh. did a nice job on that one. Imho nice expandable setup-possibilities without having to buy everything at once.
(I didn't jump though :wink: )  

Bye,

  Peter

mcs

8 channel AD converter idea/discussion
« Reply #5 on: November 17, 2005, 06:26:37 PM »
Quote from: "Rochey"
First step in World Domination -  By using the wavefront AL1402, even if there isn't a DAC in the box, it's possible to use it as a PLL to generate all the clocks needed from an external wordclock...

I have actually thought about posting that option before. But I didn't want to post it without testing it first...

Best regards,

Mikkel C. Simonsen

Rochey

8 channel AD converter idea/discussion
« Reply #6 on: November 17, 2005, 06:38:27 PM »
Well,

i think I've just about botched together a plan for the card(s)... to make the design as modular as possible (so you only build what you need) This is what i had in mind...

Stereo ADC Card
118dB 24bit, working either from onboard crystals to generate clocks to drive an S/PDIF Transmitter up to 96KHz or in a slave mode that will support up to 48KHz.
Basically, it can act a standalone ADC with S/PDIF or as part of an ADAT 8 channel ADC.

This might make a nice addition to a Green Pre to use that extra S/PDIF input on your soundcard :)

ADAT Card
Can generate all clocks from an external word clock or ADAT input. 4 connectors onboard to distribute clocks (Fs, 128fS and 256fS to the adc card) and bring back the Data from each of the stereo ADC's. The card will then convert the data from each stereo ADC into a single ADAT stream.



The digital side of all this I've got under control (i think) and I'll post some schematics over the weekend. The Analog side I'll most probably pinch from the EVM for the PCM4202.

Any feedback would be appreciated.

 :wink:

R
Expat Audio Home: http://www.expataudio.com

mcs

8 channel AD converter idea/discussion
« Reply #7 on: November 17, 2005, 07:24:10 PM »
I have tested ADAT boards (both ways), as you may have seen.

I also have ADCs, DACs and AES/EBU transmitters and receivers. I think I have posted most of the schematics here...

I have just made a new DAC board (based on one of the top Crystal chips), but I haven't tested that yet (waiting for parts).

I also have a semi-complete design based on the TI 4-channel chips :grin:

I normally like to keep everything on small modules - that makes it a lot easier to replace just some of the parts, without having to re-do everything...

Best regards,

Mikkel C. Simonsen

Rochey

8 channel AD converter idea/discussion
« Reply #8 on: November 17, 2005, 07:31:29 PM »
doh! you beat me to it :)

I may still have some tricks up my sleeve though :)
Expat Audio Home: http://www.expataudio.com

mcs

8 channel AD converter idea/discussion
« Reply #9 on: November 17, 2005, 07:46:11 PM »
But I would like some help with the analog input/output circuits also :grin:

I guess I should clean-up one of the schematics I have and post that tomorrow...

Best regards,

Mikkel C. Simonsen


Rochey

8 channel AD converter idea/discussion
« Reply #10 on: November 18, 2005, 04:51:26 AM »
MCS & all -

do you prefer to work in surface mount or thru hole? I was thinking that as the IC's are surface mount, it may be easier to put the larger surface mount resistors and caps on the boards as well. This would also help shrink the pcb as well (usefull when your trying to cram as much into a eurocard as possible!)

Either that or use thru hole resistors standing - but I understand that this isn't really recommended.

any inputs?

cheers

R
Expat Audio Home: http://www.expataudio.com

mcs

8 channel AD converter idea/discussion
« Reply #11 on: November 18, 2005, 06:53:56 AM »
I normally make the digital parts with SMDs - you can't get DIL chips anyway, and the layout is often critical. Some parts I keep through-hole - E96 resistors for instance. They are hard to get in small quantities in SMD.

The analog parts I keep through-hole. Trying different op.amps. is much easier that way :grin: And many of the good audio capacitors (and electrolytics) are not available in SMD form.

Best regards,

Mikkel C. Simonsen

mhelin

8 channel AD converter idea/discussion
« Reply #12 on: November 18, 2005, 07:43:57 AM »
mcs already has a nice layout for the ADAT transmitter board. What we still need is the word clock input card with the AL1402 (to be used in slave or master mode to generate the clocks), a BNC connector (or two for daisy chaining the WC) and the 4x headers for the clock signal outputs, right? The Olimex (aren't these Mikkel's pcbs made by Olimex?) pcb I got looks nice, but isn't there the extra VAT that should be paid for customs if you impoert them from Bulgaria (they have otherwise low prices though)?
Mikko

Rochey

8 channel AD converter idea/discussion
« Reply #13 on: November 18, 2005, 08:47:55 AM »
mhelin

thanks for the input. I will spent tonight putting together such a pcb & schematic - shouldn't be too tough. I'll also put the pads down for the ADAT generator, that way, if you want to use it, you can, if you don't... then it'll just be a bit of spare pcb  :wink:  

Regarding Olimex, I've done 2 sets of PCB's through them now, and have been pleased with the results. They aren't the same quality as full production run type PCB's, but for the stuff we're doing, I'm very impressed.

I'll post schematics and layouts tonight for your feedback. (MCS especially!)

cheers

R
Expat Audio Home: http://www.expataudio.com

Rochey

8 channel AD converter idea/discussion
« Reply #14 on: November 18, 2005, 09:42:31 AM »
Quote from: "mhelin"
a BNC connector (or two for daisy chaining the WC)


I don't think you "daisy chain" wordclock - i thought it was more a bus with T connectors.
Expat Audio Home: http://www.expataudio.com

mcs

8 channel AD converter idea/discussion
« Reply #15 on: November 18, 2005, 10:06:20 PM »
I had a look at the DAC output filters. Something like this is usually recommended in the datasheets:

http://stiftsbogtrykkeriet.dk/~mcs/DACFilt_sch.gif

I stole the filter values from a Crystal manual. The -3dB point should be at 38kHz.

Now my questions are:

- Is a circuit like this a good choice, or could you do better?
- Wouldn't you need different -3dB points for different sample frequencies? Most filters I have seen in datasheets are somewhere in the 35-45kHz range.

And now the stupid question of the day: Why are these low-pass filters needed at all? The DACs include low-pass filters already. This is the frequency response of one of the Crystal DACs:

http://stiftsbogtrykkeriet.dk/~mcs/Crystal_filter.gif

And no, I don't intend to use a 1458 :grin:

Crystal uses the LT1038 for the expensive model demoboards. I guess the good-old 5532 would be good also?

Best regards,

Mikkel C. Simonsen

Rochey

8 channel AD converter idea/discussion
« Reply #16 on: November 19, 2005, 07:52:59 AM »
Again, my analog is a little on the poor side with DAC's...

On the PCM1792 there is an internal digital filter, but it's recommended that you add an external analogue filter to ensure that any high end noise is filtered out, as well as having the balanced to unbalanced conversion.

I suspect that these days, you could do a lot of hte output circuitry with something like the OPA1632 - filtering and conversion in one step... although it'd take someone with a lot more design skills than me to tell you exactly how  :green:
Expat Audio Home: http://www.expataudio.com

Kev

8 channel AD converter idea/discussion
« Reply #17 on: November 19, 2005, 05:05:53 PM »
one of my friends is a salesman and have just moved jobs

in his product list is Mutec  www.mutec-net.de
and in their list they have  Interfaces for Musical Instruments


Has anyone been down this road before ?
Kev
DIY Factory

designmaster

8 channel AD converter idea/discussion
« Reply #18 on: November 19, 2005, 05:45:43 PM »
@Rochey

Hi very interesting project, same stuff like I want to build since a couple of month now.

How about this (not fully finished yet):
http://groupdiy.twin-x.com/albums/userpics/10057/adc-part.pdf

tmbg

8 channel AD converter idea/discussion
« Reply #19 on: November 21, 2005, 12:47:20 PM »
hey Rochey, mcs

sorry I've been away so long guys... you may well remember this was one of my pet projects too, and sadly I never got anywhere with it.

Anyway, sounds like you're making progress on speccing components!

I'd say definitely go SMD, I've done a bunch of SMD layout, so I can be of help if you'd like.

:)


 

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