DG271 as means to decouple output. - diyAudio
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Old 30th December 2003, 09:57 AM   #1
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Default DG271 as means to decouple output.

Background:
My Proceed DAP, a DAC design from the 1993, based on CS8412 digital audio input receiver, PMD100 digital filter, 2x AD1864N-K 18-bit DACS and 8xOAPA2604 for IV-converter and buffer and DG271, High speed analog switches. The DAC has both single-ended and balanced output. I started modding the unit, 4-months ago, as a way to learn more about the basics as well as the Ďmagicí of DAC-design and how that and component choices relate to its sound.
What I liked of the DAPís sound was its solid sound (compared to the Meridian508 I had) and its firm bass and its pace combined with its ability to present micro-details. However what I missed was especially a rich mid-range.
At this moment I have replaced the OPA2604 in the IV-section with OPA627BPís, and in the Buffer I now use AD8066ís. I also changed the decoupling (was between +12V and -12V, and now between -12V and gnd). The mid-range and 3D has improved considerably. I changed the PS of the PMD100 and am working on some other mods, however they are not the subject of this thread.

The reason of this thread:
While re-engineering the design of the DAP, I found out it uses DG271 chips between the buffer and the single-ended output. These chips are High Speed analog CMOS swithes. The switch is controlled by the DG-pin of the PMD100 (DAC sample and hold deglitch ). That same signal is also used the latch the data into the DACís. The switch is closed when the DAC is latched and open during the time-slice the data is clocked into the DAC. I havenít found similar designs to decouple the ouput signal on this site as well as other I searched with Google. Compared to other decoupling schemes is this a good or bad one? And should I keep them in the signal path or bypass them.
The data sheet of the chip tells to properly decouple the power supply, which hasnít been done in the DAP (and is on my improvement list in case I donít replace them by something else).
What do the experts say?
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Old 9th March 2005, 05:27 AM   #2
dpaws is offline dpaws  United Kingdom
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Default Proceed CDP

Hiya ) I'm afraid I can't help with the DG271 problem, but many thanks for posting your Proceed mods - I've been contemplating attacking my CDP for a while. All info much appreciated - I don't understand how you changed the supply from -12/+12 to -12 and ground (Sorry, newbi )
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Old 9th March 2005, 06:30 AM   #3
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Hi,
I understand you have a Proceed DAP and have plans to mod it.
This thread has been started 8 months ago, I guess and in the mean time some other significan improvements have been made.

The DG271 are switches used for the variable volume, and not as reported in the first post for decoupling (I had some doubts when I wrote it down, and hoped to get feedback via this postage).

When you have plans to mod the the DAP and you considering you are a newbie (I still don'6 have a clue what experience you have with electronics), you should start with
- Replacing the I/V-opamps (opa2604 near the AD1864N-K) with LM6182 or LM6181 (single version using special 2-1 adapter-pcb from i.e Brown Dog.(http://cimarrontechnology.com/index....ATS&Category=2)
- improving decoupling of I/V (using good 0.1uF caps)
- Change ouput-opamps (opa2604; near the rear of the chasis), with opa2134.

More complex changes are:
- PMD100 (HDCD-filter, on the seperate PCB above the main-PCB, placed in an IC-socket). Split the Digital and Analog power supplies (on this site you can find the link to PMD100 datasheet). Add a 68uF OSCON to decouple each power-line.

- Replace the LM317/337 regs on the analog-PCB by for instance a ALW-regulators.

- CHange AD1864N-K with AD1865N-K (requies a separate +5V, as the power of the AD1864 is 12V and shared from the IV and buffer supply.

Regards Henk

PS. the
Quote:
I don't understand how you changed the supply from -12/+12 to -12 and ground
I didn't do that (actually I increased the voltage to +/-15V. I changed the decoupling caps op the I/V-opamps which are between + and - voltage (pin 4 and 8 of the opa2604) with 2 caps one beteen + (pin 8) and gnd and - (pin 4) and gnd.
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Old 11th March 2005, 03:00 PM   #4
dpaws is offline dpaws  United Kingdom
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Default Newbie qualifications...

Hiya Henk,

Many thanks for coming back to me so quickly. I'm afraid I have only a basic understanding of electronics. However, I understand what the different modules do, in what order they have to go etc - electronics by lego bricks I guess. I consider myself competant with the soldering iron....

I have the Proceed CDP all in one unit. Against everyones advice I have just replace the failed Philips CDM12 unit with a CD Pro2M supplied by Daisy and completed the clocking mods to the crystal as described on his site. Can't be done apparently but surprise surprise it all works perfectly

The Proceed CDP already has a very competant sound. However, I've been haunted by a previous demonstration of some Naim equipment, namely the CDX cd player. On its own the player sounded OK compared with my own Meridian 208. However, attach the optional £2500 XPS power supply and all hell breaks lose and everything gels together in an amazing way!

With that in mind, I know that the power supply to the analogue stages is crucially important in obtaining PRAT (pace, rthymn and timing). I've approached Proceed for the schematics of the CDP but despite best efforts they cant find any!

Having read all the threads I'm still not convinced that the Op amp is the way to go for I/V - everyone tweaks them to move the sounds flavour in their direction but no one actually seems entirely satisfied with their results.

Valves are often misunderstood as being sources of warmth and smoothness. If used correctly however, they should be more transparent and faster than their equivalent solid state. With that in mind, and also think about the KEEP IT SIMPLE STUPID philosophy I've been looking at this...

http://koti.mbnet.fi/siliconf/JukkaT...dac/part2.html

(Powered on board by a Jung type Superreg at 260V!) Using a board per channel to retain the balanced mode, losing the volume but hey, can't win them all...

Connection between the DAC and Jukkas board via transformer? Simplest I guess but do I risk losing dynamics?

Otherwise I'm very interested in the gains you've made by replacing the DAC and splitting the PMD100 supply - areas I hadn't considered. To be honest I'd only be happy once I had a schematic in front of me to identify the right bits! What may be obvious to you can appear rocket science to a newbie

External supperregs with larger lower esr supply caps again can only improve all of the 79xx's etc that are dotted around - mounting the circuitry close enough could be an issue - I guess adding 10cm of wire between the two sort of defeats the object!

Sorry, I'm waffling.... thanks again, Steve
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