dam1021 R-2R DAC MODs
BEST FREE MOD: Board Isolation
Eliminate ground loop
To eliminate the ground loop I was experiencing with the external RCA connectors I had to isolate the RCA connectors from the chassis. That did not explain the source of the ground loop though, it merely eliminate the additional path to signal ground. I had no supply ground wires connected to chassis and the only ground connection to chassis was the safety EARTH ground, so where did the ground loop come from?
After some poking with a voltmeter, I figured the real source of the ground loop. The mounting holes of the DAC board are encircled with ground pads. These short to the mounting posts and the mounting posts short to the chassis.
If you don’t have an alternative path (for the signal) to ground, then you don’t have a ground loop. Having signal ground connected to the chassis is probably a safety feature but for audio, it is best to avoid it. I have built all my electronics with the signal ground (the supply ground) isolated from the chassis and to ensure safety I use a 3-wire power cord with the EARTH ground wire (the green wire) connected to the chassis.
If you don’t have the chassis connected to ground, then having the signal ground connected to the chassis could create a dangerous situation in the even of a short, energizing the chassis.
If using the chassis to shield sensitive electronics, then it is certainly a good idea to not share the chassis ground with the electronics/signal ground.
So I think it is always a good idea isolating the board from the chassis (and using earth ground to ground the chassis)
To isolate the mounting posts, I made poly washers for the mounting screws and put them on the top side and bottom side of the board:
These were made from presentation transparencies (remember those?) with a regular 3-hole paper punch and then using a smaller craft punch for the center hole. The came out very nice.
Mr. dimdim measured the noise improvements after isolating the board [link]:
… The installation of the α20s had coincided with me changing my plastic IEC with a metal one, that also provided filtering. Metal (IEC) meant that the main’s GND would be connected to my chassis. You may remember that the Soekris does not have insulation on its mounting holes, so if you are using metal standoffs to mount it you are effectively connecting your signal ground to your chassis.
To make a long story short, isolating the DAM from the chassis improved the noise floor by a whopping ~23dBs. It also made significant difference in the dynamic range, THD, etc.
There you have it! The best free mod.
INITIAL POWER MODS
The power on/off POP seems benign if listening through headphones. The power ON pop is not loud at all but the power OFF pop is loud. If connected directly to an amplifier, it can potentially damage the loud speakers (I have not tried it, so far listening with headphones).
During Power off, the positive rail drops faster than the negative rail and this creates a voltage imbalance resulting in a huge POP. At some point, the device is practically powered by the negative rail [link]. The effect can also be visually seen:
The left LED is on the positive supply. It goes dim first when cutting the external power.
I attempted to balance the power consumption for the two rails hoping to reduce the turn-off POP.
Adding capacitors to the positive rail
By trial and error, incrementally adding capacitors up to 13,000 uF I was able to balance the power decay between the positive rail and the negative rail by visually matching the turn-off rate of the two LEDs in the supply. The POP was somewhat reduced (using headphones) but not eliminated.
Adding resistors to the negative rail
I then tried to match the current consumption of the negative rail with the positive rail. The power consumption of the DAC is: 
- Positive Rail: .18A, 10V
- Negative Rail: 0.06A, 10V
I added enough resistors to increase current consumption by 120 mA. I used 10 1 Kohm resistors on the -12V leg of the supply. Each would dissipate 12 mA of current and .144 W of power -they are rated at .25 W, so we are safe.
The result was better than adding capacitors to the positive rail, the POP at power off was further reduced but still not eliminated. I liked this mod better than adding capacitor, so I made it permanent in the supply.
Note: only tried with headphones. And yes it is a waste of energy, but it is only 1.4W 🙂
BYPASS THE BRIDGE?
An apology to my readers for putting up wrong information.
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 filter/smoothing caps. This would not destroy the board, but it is not the right design/thing-to-do either. Thus there is no good reason to power through J2.
The white arrows point at the +/- analog power traces:
While examining the back of the board and realizing that J2 cannot be used as input power, I figure it is the perfect place to add capacitors to the power supply lines. Decided to add 330 uF Oscons to the digital 1.2V and 3.3V and Panasonic FMs to the +/- analog lines. This mod is completely reversible. Just cut the leads. I first soldered a row of pins and the capacitors are soldered to the pins as shown in the photo below.
Adding capacitors to the analog lines further improves the filtering because it adds capacitance to the RC filter.
There is still the +/- 5V lines but there is no more space. I also chose not to add capacitors to these lines because the 5V regulators already have large output capacitors, and because I thought they would benefit the least: if you examine the backside of the board, you’ll notice that the connectors at J2 trace back from different places in the board. The longest traces are from the +/- 5V regulators.
The capacitor added to the 3.3v supply connects to the 3.3v power plane through a short trace (the straight vertical trace under the pins of the left capacitor).
These power mods are “approved”: [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.
Didn’t want to remove the existing capacitors and risk damaging something. It is too early to do any kind of surgery on the board :-). Plus the existing filter capacitors don’t seem quite easy to remove given that there is very little space between them.
FULL DIGITAL INPUT ISOLATION
These are the Silicon Labs digital isolators on the board [link]. I believe the original ones in the engineering board were TI isolators [link]. Both isolators use the same capacitive coupling technology. I suspect the Si parts have equivalent performance when it comes to noise isolation.
According to the diagram below, the parts can also be used for logic level translation and thus they can be powered with 5V on the input. This will make the input compatible and tolerant to 5V signals such as those present in a serial interface of a standard Arduino microcontroler.
5V tolerance is also needed if interfacing to the I2S/serial signals of older generation CD players when they used spend good engineering money in making CD-only players. I like older CD-only players as compared to modern multi-format players because they are better made, start up/play much faster and the drawer mechanism is also much faster. You don’t have to wait for the device to check the format, read the contents, etc before it starts playing. If you click “eject”, it immediately ejects; no need to wait for the device to do who knows what before it ejects the disc.
To reduce overall system power consumption, many of today’s high-speed logic devices (e.g. FPGAs) operate from supplies of 3V or less. Lower bias voltages (and consequently lower logic thresholds) complicate interface with 5 V devices, creating a need for a fast and robust logic level shifter… the Si86xx isolator (can be) used as a logic threshold level shifter where each side of the isolator is biased to match the local logic rails. Note the common ground on both sides of the isolator since alllogic supplies are assumed to be connected to a common ground.
However, I don’t have to assume that the logic supplies are connected to a common ground and instead I am going to isolate the two grounds as much as possible by powering the input side with a completely separate supply, including using a separate AC transformer.
I salvaged another transformer (this one gives 9V DC unloaded) and a 7805 series LDO regulator
I had to rearrange the supplies in order to fit them in the chassis. This arrangement is even better as the line voltage components and wires are confined to the left-most area of the case and as far as possible from the low voltage electronics.
MY OTHER POSTS ON THIS DAC
|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.|