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opc's NTD1 project


This is a wiki page to provide a simple entry point to find out more info about the NTD1 project as designed by diyAudio forum member opc. This wiki’s primary aims are help you to:

- Find out what the project is
- Decide whether it’s something that would work with your existing components
- Spec out a BOM by listing options and alternative components in addition to the official BOM
- Work through problems that other people have encountered
- Avoid clogging up the thread with questions that have been asked many times

What is the NTD1?

It’s a DAC output (or IV) stage based upon the Nelson Pass D1 (https://www.passdiy.com/project/other/zen-i-v-converter). It is designed for current output DACs that require an IV stage (to turn the current output into a voltage output that will drive an amp) with a very low input impedance to match their own very low output impedance (sometimes just a few ohms). Its design provides a balanced output.

All intellectual property for the project is Owen’s (opc).

Will it work with my DAC?

DACs particularly suited to be paired with the NTD1:

- Current based DAC chips, for example:
- PCM63
- ESS chips like ES9006 or ES9018

Different versions

Version 1

Date: 2010.
This was developed to work directly with the BII.

Version 2

Date: 2012.

Version 3

Performance Measurement

Version 1

Version 2

Version 3

Schematics, BOM,useful docs and links

Build tips

There're plenty of SMD components in this build. If it's your first SMD project check out how to solder SMD before you start. Useful youtube video here.

Think about having to remove the complete build from the heatsink before you start. Obviously soldering the heatsink-mounted components will be the last thing done during the build but it will be difficult to desolder without unmounting the heatsink. So, make sure that the screws/bolts you use to mount the components can be removed with the PCB in place and the component soldered. Usually this just means making sure that the screws are short enough to fit between the PCB and the heatsink or that they can fit through the mounting holes in the PCB. see http://www.diyaudio.com/forums/digit...ml#post3462526

Don't forget to insulate the FETs when they are mounted on the heatsink. Keratherm is good as is mica

You don't need to insulate the heatsink-mounted resistors but do use thermal paste or similar

Don't snip the FET leads when they've been mounted on the PCB as it's easier to clip multimeter leads for testing.
If you're using zfoils then advisable to put a block of solder onto the knee of the bend under the pcb to strengthen the bend. http://www.diyaudio.com/forums/digit...ml#post3458833

Make sure that the 4 pots on the main circuit (not the power supply) are all turned down to as close to 0R as you can get them. Measure the resistance between the centre pin and the pin furthest away from the trim screw.

Putting it all together

Owen's case plan

bigpandahk's build in case

First power up

Measure your AVCC regulator voltage on the DAC board. It will probably be 3.3v but don't assume. You will use AVCC/2 to set your Source voltage lower down (where I use 1.65v below just replace this for your own AVCC/2 measurement).

- DON'T plug the dac board in!
- Tweak trim pots to give 1.65V at ground-Source on Q1-Q4
- You should have about 21 - 23 V ground-drain on Q1-Q4
- You should have arrouns 20v Drain to Source on Q1-Q4
- The heatsink should heat up to about 50 degrees over 30 minutes
- After 30 minutes re-adjust the voltages to AVCC/2 (or 1.65v)
- Power off and connect your dac

Monitor your dac chip temperature. It must be below 70 degrees otherwise you will cook your chip!

Trouble Shooting

Part Selection

Heatsink discussion

Transformer discussion

Matching FETs and resistors

It's important to use matched quads of FETs and ideal to use 0.1% or 0.01% matched resistors.

See post here: http://www.diyaudio.com/forums/digit...ml#post3808491

Early group buy matching data - historical interest only (in case you come across a quad from this group).

Original testing results post here: http://www.diyaudio.com/forums/digit...ml#post3225376

Full testing data: https://docs.google.com/spreadsheet/...0hPSzRhQkN4MVE
Final set numbers: https://docs.google.com/spreadsheet/...BOFBYekE#gid=0

Output Capacitor Selection

IV & gain resistor selection

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