Soekris dam1021 Build

Please also refer to:

• The Soekris R-2R DAC: Technical Details [link]
• R-2R DAC For The REST of US [link]

Here is my board (S/N 000003) and even personally signed by Soren :-). I will document my build in this post.

POWER SUPPLY

Just finished building a +/- 12V power supply and had it powered-on an entire day in order to ensure that  nothing was wrong with it. Most of the components are from my “pile of electronics”: some now, some recycled.

The transformers were taken from “surplus” unregulated wall supplies and are rated at 15V AC (which are too high of a voltage to use directly on the DAC board).

It is as basic and standard as a linear supply could be.

The AC rectifying part is basically the dam1021 input section

• Two transformers (AC 15V)
• Single bridge (2A)
• Smoothing capacitors. In this case 35V, 2200 uF standard Nichicons, one per rail

The regulators are fixed output voltage LM7812 [link] and LM7912.[link]. Could have used adjusttable, but did not have a negative LM337 at hand.

The film caps are additional bypass close to the regulators.

The “hardest” part of the build was manually placing the components on the board for a good fit. Maybe a CAD layout tool would had been a great help.

Not very pretty, but works fine. The thicker wire in the center is GND.

If building the PS from scratch is too much of a hassle, there are many kits from eBay for very little money. (You still need to source the transformers). The most popular ones are the adjustable ones based on the LM317/LM337. One good example is the following kit [link].

POWER SUPPLY MODS

For now, I am not thinking about PS mods, but here is a post with a lot of good information and measurement on the DAC in general and particularly on the power section [link].

After reading the post, these are some of the things that come to mind:

Reducing Power On/Off POP

During Power off, the positive rail drops faster than the negative rail (remember that the positive power consumption is 3X negative power consumption) and this creates a huge POP when the device is practically powered by the negative rail. – Using the additional capacitance of the regulated supply would probably make this problem worse because the capacitors are equal in size between the positive rail and negative rail.

The left LED is on the positive supply. It goes off first when cutting the external power

Possible solutions:

• Increase the power draw of the negative supply to be equal of the positive supply: you can do this by adding resistors to ground on the output of the supply.
• Adjust the amount of capacitance between the two rails so that during power off, the voltages on the two rails decay in approximately equal rate. This may or may not work, but it sounds that it could.
• Separate the analog supply (opamp and voltage reference) from the digital supply which only uses the positive rail. This requires major surgery of the board.

Snubbers on the transformer

The shouty sound can be somewhat tamed by filtering the power line and using optimal snubbers for the power transformer. (Check the Quasimodo/Cheapomodo threads for an excellent snubber measurement jig by Mark Johnson.)

Bypass bridge rectifier (this also “skips” the smoothing capacitors)

Update: I took a closer look at the J2 connections on the backside and the board and  the +/- analog power connections are connected to the power lines after the RC filter. If you power through J2, there will be a 12 ohm resistor to the smoothing caps. This would not destroy the board, but not the right design. (It is actually safe [link]). This is probably only useful if you have a well filtered and regulated DC supply and you are operating near the headroom required by the 5V regulator which is around 7V. In any case, it is better to power through J1.

If using a regulated supply for input power, it is possible bypass the built-in bridge rectifier which is used for AC input. For DC input it is basically serves no function except it adds extra protection in case you inadvertently apply the wrong polarity.

I measured the PWR A+ and PWR A- connections in J2 against the + and – poles of the input filter caps and I measure continuity. 

It would be better to use the GND connections of the power input (the ones in J1)) since they provide a solid and hefty connection to the GND plane. The ground connections on J2 are through thin traces.

But I need to respect the manufacturer’s warning, so here it goes [link]:

I Repeat:

I can only recommend to supply any power on J1, the diode bridge used on the input is a low noise schottky type.

J2 is NOT for supplying power, it’s for testing or for sourcing small amount of power for external input circuits. Applying power will probably blow the board.

But on the other hand, just go ahead, I’ll be happy to sell you a couple of new boards 🙂

Connecting the input power this way, bypasses the bridge rectifier (and skips the smoothing capacitors. I say “skip” because it does not bypass them since they are still connected through a 220 ohm resistor).

Replacing and adding on-board PS capacitor

This one is endorsed by Soren [link]

If you insist to improve the on-board power supply, try replacing the 6 electrolytic capacitors with aluminum polymer types, 1000u 16V exist in same 10mm SMD footprint and t.ex. digikey stock them at $2.20 each. Should be easy to replace.

You can also add a small polymer electrolytic on the 3.3V output, but please note that the clock oscillator power already have a filter, so I doubt it will make any difference.

Here is the 3.3V regulator. You may add a capacitor between Vout and GND (pins 1 adn 2)

At this time I would be doing a standard installation before thinking of any other mod.

CASE

Old electronics are excellent for these projects. Not only they are free, but at the minimum you get athe power cord/socket installed and some even come with power filters.

This one has a power socket for a detachable power cord and ground safety (the green wire)

Lots of useless buttons, but not bad looking at all 🙂

Reusing the RCA connectors: one set for single-ended raw output and one set of single-ended buffered output

Perfect fit.

I like to use the analog-audio cable assemblies found inside (older) PCs. They have three leads (for left, right and ground) and are shielded.

OTHER COMPONENTS

Volume Potentiometer

It is recommended to use a 10K pot. (although a different value one might work as I beleive it is measuring voltage)

I am using one from a gutted Sony analog surround processor (when Japan used to make stuff :-)) which happens to be 10K potentiometer.

Toslink Module

I will be using what I think is the best Toslink Module: the original one from TwistedPear Audio. This one has the Toshiba TORX142 module (25Mb) with supports up 192KHz sample rate (data sheet: torx142l), but Toshiba stopped making them. I purchased this several years back with the OPUS DAC [link] (which got me started in this audio DIY thing). The current one that is on sale at the TwistedPear Audio store is specified to support up to 96KHz [link].

The module has an integrated 3.3V regulator [link] and thus it is compatible with the 3.3V input limit of the R-2R DAC. In addition, the regulator is specified to accept 5-12V DC (with a spec max of 18V). It can therefore be powered directly by the 12V supply. No need to pull the power from the DAC.

The case even has a cut-out for the Toslink module…

USB to I2S Module

I am going to use the original DIYINHK XMOS-based module [link]. This one is USB powered.

ALL TOGETHER

Wanted to first check out the DAC with a basic and default configuration: Toslink Input, volume control, and SE buffered output to headphones.

Power connections, volume control connections and SPDIF connection.

Single-ended buffered output to a front-panel headphone jack.

The output opamp, the LME49724 [link] can drive a high impedance headphone directly. It is specified to drive a 600 ohm load meeting full specification.

Since the Senn HD-580 I use has an impedance of 300 ohm, I expect only a slight deviation from the specification, if any.

Front panel: power LED (in the power button), toggle power switch, headphone jack and volume control

I also installed an RCA jack for coax SDPIF. The Toslink module is powered by the +12V supply. The USB-I2S is powered by USB.

Back panel: SPDIF Coax, SE Raw output, SPDIF Toslink and USB-I2S

Just powered it up, original filters. Denon Multiformat player, Toslink output. Senn HD-580 (300 ohm) and Fidelio X1 (30 ohm).

INITIAL POWER-ON

No sound

First there was no sound either from the raw outputs or the SE buffered outputs.

• The signal lock LED was steady. In auto-input mode, the signal locks quite fast, as it should
• The voltages on J2 were all correct
• Removed the volume potentiometer connections, leaving just the SPDIF input
• Double checked all the output connections

It turned out that I had soldered the wrong connectors on the RCA jacks and the Headphone jacks!

Signal lock LED

• Steady on: signal lock
• Blinking: no signal or no lock

Hum

Then there was a hum coming from the raw output connected to the RCA jacks (RCA-Head Amp-Headphones). I figure there got to be a ground loop. I measured continuity between the chassis and the RCA GND.

Had to remove the metal plate that was grounding the outer sleeve of the RCA jacks to the chassis (the little metal tabs on each RCA jack).

Volume control rustling/crackling noise

I hear a faint static-like click when adjusting the volume. It is like the old analog amps when the pot was dirty or worn out. But this is digital. I shall investigate this further…

I added a 0.1 uF capacitor between 3.3V and Gnd and between the wiper and Gnd. No improvement. I think the problem could be in software (the FPGA volume software)

HOW DOES IT SOUND

Using Toslink source from Denon multiformat player, single-ended buffered output to a Senn HD-580 headphone. Compared with the analog output of the same player through a Fiio top of the line headphone amp.

The immediate most noticeable thing in an A/B comparison with the Denon player is the larger soundstage of the R-2R DAC. Further listening indeed shows a more expansive soundstage, like each instrument and voice fills more of the space around it.

The bass is also more “full bodied” and more impactful. Beautiful bass. I am using the original filter and do not detect any “harshness” as others have.

Perhaps comparing with the Denon is not such a good comparison, but it was easy and shows the that DAC performs very well.

I shall try other filters. Perhaps then I would feel the original filter results in some harshness…

Also, with direct output, I prefer the HD-580 to the Fidelio X1.

GOT THE I2S GOING

Finally connected the DIYINHK XMOS I2S module (this is the original model, different from what Soren is using -I believe the current isolated one).

Listening with iTunes and having iTunes upsample everything to 176.4KHz. Sounds good…

The DAC automatically selects the active input right away. On the XMOS device, once you apply power, there is bitclock and the DAC locks to it.

I am still using original firmware, so the slight clicks when changing sample rate are there. The rustling/crackling sound of adjusting the volume is also still there. I am sure these will be fixed, it the meantime, they don’t bother the listening experience.

In order to power the input side of the isolator, I had to build a small supply with a 5V regulator and a 3.3V regulator (didn’t have anything else suitable). The 3.3V regulator is a surface mount device so I had to solder legs and small heatsink tab. Just a crude but effective job 🙂

How does it sound?

Whereas in the previous comparison, the DAC showed a more expansive soundstage, and the bass was more “full bodied”, more impactful, using I2S input (Windows 7 laptop, iTunes 12 with upsampling to 176.4KHz 192KHz [Check my post on adjusting the iTunes playback sample rate [link] and diyinhk XMOS interface) results in another step forward towards better sound and a enjoyable experience. This time the improvement is presented as music with more clarity or “crispiness”  (more details?) as though the instruments give out a more “defined” sound. I think this reviewer gives a pretty good description of what I want to say: [link].

MY OTHER POSTS ON THIS DAC

TITLE	SUBJECT	LINK	DESCRIPTION
Soekris dam 1021 R-2R DAC ILLUSTRATED GUIDE	Users Manual	[link]	Users manual for the Soekris DAC.
Soekris dam1021 Build	Build Guide	[link]	Details of my initial build of the Soekris DAC.
dam1021 R-2R DAC MODs	Mods	[link]	Mods I have performed on the DAC build.
dam1021 R2R More Mods	Mods	[link]	Later mods on the DAC build.
Digital Filters for Soekris R2R DAC	Digital Filters	[link]	Extensive list of DIY filters from the diyaudio filter brewing forum thread.
R2R Benchmark Filters (for now)	Digital Filters	[link]	Latest set of filters developed and shared in the diyaudio filter brewing forum thread. The best filters of the bunch.
R-2R DAC For The REST of US	Technical Details	[link]	Introductory post describing the innovations and capabilities implemented in this DAC.
The Soekris R-2R DAC: Technical Details	Technical Details	[link]	Additional technical details of the Soekris DAC that were not covered in the post above and collected after I had the DAC on my hands.