This page is about an Apogee 12-channel DAC (they were available as cheap surplus). This page deals with some "reverse-engineering" of the original circuit, as well as the design of an ADAT input board to suit it.

The text below is a "blog" describing the work I have done with the circuit.

I have received the DA-12 today. Lots of very small SMD parts...

But here's the list of important parts:

CS8415A AES/EBU receiver
AD1852 DAC
And a lot of op amps. There's an OPA4134, an OPA2277 and an OP176 for each channel!

I'm not sure why it's 44.1/48kHz only as the receiver chip supports 96kHz and the DAC supports 192kHz. It could be because of the configuration of the DAC chips only, so 96kHz may be possible from the AES/EBU inputs. They have done some strange things like added a lot of extra wires and cut off pins on some chips, so I'll have to take a closer look to find out what's going on...

I have done some more "work" now (well a bit anyway).

What I know now:
  • The thing works (will make further work a lot easier...)
  • The sync source switch has to be changed from the default "0" setting. I thought "0" was for input 1, but the "1" setting actually is. The switch can only be moved with the power off, as the position is only read at power-up. These two things took a bit of testing to determine :)
  • It works just fine with S/PDIF signals. Just connect the inverting input to ground through a 10n cap and connect a 237R resistor from the input to ground.
  • None of the chips are operated in "strange" modes, and they are all operated in hardware mode.
  • The output signal is not muted when the sync is lost. Why? So with my CD player as the source I get a "chirp" when skipping tracks.

So far so good... The bad part is that it looks like the PCB has three or four layers, so a lot of the interesting stuff happens "inside" the PCB. It will not be easy to "inject" signals from an ADAT receiver on the DAC chips (as they are slightly tiny), so I hope the signal passes through some of the SOIC gates. I will have to look at that tomorrow...

If some of you would like to use the DAC with 96kHz AES/EBU signals, that should also be possible, but you will have to unsolder a pin on the DAC chips and connect resistors to them - that may not be very easy.

I finally had some more time to look at this. I have studied the clock/sync circuit a bit. The sync source switch on the back controls a '238 3-to-8 decoder (U14). The 6 47k resistors around it are mode setting resistors for the 8415As. The output from the '238 decides which of the six CS8415A receivers is operated in master mode, and which wordclock should be used as the sync source. The other receiver chips are operated in slave mode, controlled by the clock from the selected receiver.

Normally the "recovered clock" from the CS8415A is used as the masterclock for the DACs, but not in this case. The clock pins on the receivers chips have been cut off (why not just leave the pad unconnected?). Instead the "Clocked by Apogee" circuit inside the purple box generates the masterclock from the wordclock and/or bitclock received from the selected CS8415A.

I have now measured the signals on the different logic chips in different situations, and I have a pretty good idea what the different parts do. As mentioned above the '238 U14 decides which 8415A "delivers" the clocks.

The chips next to the receiver section U6 and U9 (both '02 quad NOR gates) are used to monitor some of the status signals form the receivers. Most likely one of the error signals and the AUDIO' signal. When both signals are low (valid audio and no errors), the output of the NOR gate goes high indicating a valid signal on that channel.

The chips U18 and U31 perform a similar function. Both chips are '00 quad NAND gates. One input on the gates is the status signal from U6/U9, and the other input is a status signal from something else - perhaps the clock circuit. When both inputs go high, the output goes low indicating a valid signal. U17 is a hex inverter that inverts the output signals from U18 and U31. The outputs from this circuit controls the mute pins on the DACs, and the front panel display.

The display is controlled by U24 and U36, which are just buffers for driving the LEDs.

All the signals on the above mentioned chips are DC - no data or clocks. They just switch the various status/control functions. Things get more interesting on U1 and U2...

These two chips buffer the clocks. U1 is a '365 hex gated buffer, and it is used to buffer the master clock and L/R-clock. U2 is a '04 hex inverter used to buffer the bit clock.

The circuit is not entirely error free BTW. Most of the unused inputs on the various logic chips are left floating, which is a big no-no...

It looks like the data does not pass through any gates/buffers, so you have to connect the wires to the 8415A pads (you'll need a good magnifier ). As I see it the easiest would be to remove some of the receiver chips and solder some wires to the pads (not many at least). Adding AES/EBU transmitters to the ADAT board would be even easier of course...

I have ordered a sample board for the ADAT receiver, so I'll know soon if it works...

I have an updated schematic ready. I think this should be close to the final version - I just have to check the last few things...

Here it is: Schematic

The differences from the standard "OptoRec" application circuit were made to adapt the circuit to the Apogee. With this circuit you should be able to use 8 channels with the ADAT input and the remaining four channels with the AES/EBU inputs.

The other "features" are:

- it controls the mute circuit, so the DACs should mute when the ADAT signal is lost.
- you can select either the ADAT clock, the AES/EBU clock or an external wordclock - and using the small switch that's already on the back panel

And here's a new PCB layout: PCB

I have just tested my ADAT boards, and they just worked the first time I powered them up!

My test setup was S/PDIF signal from my PC > CS8415A receiver > ADAT transmitter > ADAT receiver > CS4340 DAC.

Now the next "problem" is getting the receiver board integrated in the DA-12...

Here's my nice and orderly test setup: Photo

And here's a closeup of the ADAT receiver board: Photo

I'll do some more work later today, when the temperature in my workshop drops below sauna-level...

I think I have figured out where to connect the different wires. The only small problem is how to get rith of one of the receiver chips without damaging the board...

Correction: The chips don't have to be removed. They can be disabled by lifting a pin on each.

Even though I posted that you don't have to remove the chip, I did it anyway - the pin-lift operation was unsuccessful...

But so far it looks like you have to pin-lift or remove one chip for every two channels you want connected to the ADAT input. You also have to remove 2 SMD resistors, but they are large 1206 types, so that shouldn't be too difficult.

I will try to connect the ADAT board tomorrow, and post the (hopefully good) results...

I got a bit further (but not much...).

I managed to solder wires to the TSSOP pads. Even neighbouring pads! I put some glue on those wires to hold them in place, so I can't touch that for some hours now.

The other wires for connecting the ADAT board are also ready, so I guess I should know if it works or not tomorrow...

I have started the testing. So far I have just connected the ADAT board to the clocks and power. The ADAT board powers up, and the error LED switches on (which it should). So far so good...

But one strange thing is that the 5 and 6 LEDs on the frontpanel turn on for some seconds when I turn on the DAC. Why? I thought those LEDs were controlled by the error signals of the receivers, but they are not. Those are only used for muting the DAC chips it seems. So what triggers the "Signal" LEDs? Power drain from the receiver chip? Something happening on the clock pins? The clock pins are inputs, so I don't understand what could be detected from those...

A schematic would be nice!

Success! It works!

I also solved the LED mystery - the LEDs are controlled by the DACs. The receiver-less DAC must have received some random noise, causing it to think there was a signal.

The way I (will) have my DAC setup is like this:

Input 1 and 2 are used for standard AES/EBU signals. The remaining 8 channels are used for the ADAT input. The sync source switch is used to select the clock source. The options will be the two AES/EBU inputs (1 and 2). 3 will select the ADAT input, and 4 will select the wordclock input.

You can use any of the eight channels for the ADAT board - not nescessarily the same setup as I used. You can also use all channels for 1½ ADAT input, or perhaps use two DA-12s for a 3*ADAT setup.

The next problem I have is mechanical. Where do I install the board? The optical receivers/transmitters are unfortunately quite high. So there isn't space above the DB25 connectors. So the board either has to be fitted between the analog and digital section, or close to the power input. I guess close to the power input would be best, to avoid injecting noise in the audio stage? The board will have to be mounted upside-down (because of the DB-25s again).

I have been asked about the pinouts of the DA-12 DAC. Here they are.


Comments? Questions? Just send me an e-mail!

MailMail: mcs@post5.tele.dk