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Old 3rd February 2011, 09:20 PM   #1
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Default AD1865 DAC + 6SN7 analogue stage

I've read a bunch of good reviews on the AD1865 chip, so thought it may be a good experiment to test it against my CEC DAC DX71 MkII.

Having little DIY experience in the DAC field (I've only built a few small headphone amps and preamps), I thought an assembled board such as this would be a great start.

I notice on the page it says:

"Reserve the analog output from AD1865 chip directly for matching with HDAM, tube driver

***Means the kit can output from JFET stage, HDAM stage and Tube buffer stage to suit for you’re listening interests.****
"

Now - here's where it gets interesting (for me, at least!) - could I simply take the analogue output straight to some RCA sockets, and then use a very short interconnect, to a tube buffer like this?

Or is it not that simple?
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Old 3rd February 2011, 09:32 PM   #2
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Please note that the board is a NOS (non oversampling) dac; be sure you want something like this or look for info on OS vs NOS. This has nothing to do with the particular dac chip.
The AD1865 is a current output chip; it needs some sort of I/V conversion and voltage amplification. The tube buffer is a mere cathode follower (no voltage gain).
So actually not that simple.
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Old 3rd February 2011, 09:39 PM   #3
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Ok no worries - I am keen to hear the difference between OS and NON-OS as my CEC is 8x oversampling.

What is the difference between i/v and voltage amplification? It is is simply a matter of voltage amplification, one could send the analog outputs of the AD1865 straight to a preamp....

Also, it looks like there is an on-baord JFET i/v stage, this could be used if the other ideas don't work....
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Old 3rd February 2011, 09:46 PM   #4
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The I/V is needed to convert the current output of the dac to voltage; it can be done by a simple resistor (passive) or some sort of active stage by means of an IC or discrete transistors. Some active I/V stages do the voltage gain also.
It would be good to know what the JFET I/V stage does in this respect, and how much voltage gain is needed to reach the standard 2 VRMS output.
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Old 3rd February 2011, 10:13 PM   #5
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Yes - the lack of specifications worries me. On a lot of DIY modules, we never read any details relating to key measurements such as output impedance, etc.

The problem is that there are very few AD1865 modules available - I'm tempted just to order it before the supply of chips runs out, then find an i/v stage which has some better documentation....

Any recommendations on a decent i/v stage (tube or otherwise)?
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Old 3rd February 2011, 10:17 PM   #6
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There are a lot of them: Zen I/V for instance, and Twisted Pear Audio has well documented I/V stages. Tons of information on I/V stages to be found on this forum to keep you busy for a long time....
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Old 3rd February 2011, 10:37 PM   #7
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Here's the datasheet: http://www.analog.com/static/importe...ets/AD1865.pdf

Also:

Quote:
Each channel is equipped with a high performance output amplifier.
These amplifiers achieve fast settling and high slew rate,
producing ±3 V signals at load currents up to 8 mA.
Doesn't seem a weak dac output, but it might benefit from a buffer.

You can also use a simple resistor for I/V-conversion.
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Old 3rd February 2011, 10:45 PM   #8
boudy is offline boudy  United States
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Default Take a look at the DAC-END threads

You'll find a schematic that can be used for reference (its not specifically for this board but its close). I think they use a 200 ohm Caddock resistor. I have the same board that you linked and I bypassed the output stage, using a 200 ohm Caddock (after disconnecting the onboard IV resistors).

I achieved a significant improvement in sound by replacing all of the rectifier diodes with MUR120 and replacing the big PSU capacitors. There's a thread (actually a sequence of posts buried within a thread for a different DAC board - ugh) on Audio Circle that describe these changes and more, you can find it by searching for AD1865.
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Old 3rd February 2011, 10:51 PM   #9
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OK great, thanks Boudy - I'll give it a search.

It's probably worthwhile just getting it, installing at as it is and listening to the FET i/v stage for a bit, then modding it...

That's the beauty of DIY!
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Old 3rd February 2011, 11:03 PM   #10
wa2ise is offline wa2ise  United States
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I've built "I/V DAC chip" to tube buffer/preamp circuits, mainly via modifying CD players that use similar chips to the AD1865
Click the image to open in full size.

More at my web page Modifying CD player DAC circuits
They all come out sounding quite good.
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