Home > DIY HiFi, POWER > dam 1021 R2R More Mods…

dam 1021 R2R More Mods…


I Have been pretty happy with my current implementation of the Soekris DAC, using a basic “workhorse” 7812/7912 based power supply. These are legendary for their ruggedness and still the most widely used regulator family for most commercial HiFi equipment.

But not having a lot of time and/or patience to try different mods, I mainly rely on good engineering and the collective wisdom of the people at the diyaudio forums. And lately, improvements in sound have been reported using low noise supplies with the Soekris DAC board [link].

In addition, there is always the attraction for “new and improved” and the introduction of low cost, low-noise supplies in recent years makes upgrading an almost a “must do” thing for diy people like us🙂

I decided to upgrade the power supply section with a TPS7A47 based power supply kit from diyinhk and an R-core transformer. The TPS regulator has very low noise and a wide operating range. The R-core transformer has multiple (4) secondaries that can feed multiple supplies.

Some users are using shunt regulators for this board. But with the local regulators being of the series type, I don’t think the benefits of a shunt regulator (very low impedance, with “instantaneous” power delivery) can be realized. (Besides I would need to build from bare PCBs and that would take a long time).





The diyinhk kits are based on the factory recommended PCB layout and as shown in the evaluation boards. As such they should perform as specified in the datasheet.

The kits implement some enhancements over the evaluation board such as built-in AC rectification, large filter capacitors, heatsinks, screw connectors and noise-reducing film capacitor. I am using the +/- 12v-15v bipolar supply kit.

The bipolar supply uses TPS7A47 regulators for both the positive rail and the negative rail. I had inquired diyinhk why not use the TPS7A33 [link] for the negative rail and they replied saying that the chip was very frail and had destroyed a few during prototyping. These kits have been available for a while, implying that they in fact work well.


The transformer has 4 secondaries. I can add two more supplies later… These transformers have the following specs:

  • Primary : 0-115vx2,
  • Secondaries : 0-9X2(0.7A),0-15X2(0.7A)


What is so good about R-Core transformers?

Here is an excellent resource comparing the different types of transformers [link]. I have summarized here the main advantages of the R-Core:

Parameter R-Core
Toroidal EI
Leakage flux The Balanced winding cancels the leakage flux. The total amount of leakage flux is extremely small. (1/10th compared to EI) The winding is not balanced hence there is lot of leakage flux. There are gaps in the magnetic path and the winding is not balanced resulting in very high leakage flux. Requiring shielding.
Efficiency Very low losses result in better efficiency. Efficiency > 90%. Efficiency is better than EI transformer but less than R-core transformer. More losses result in poor efficiency.
Exciting Current Exciting current is minimal as magnetic paths align with the rolling direction of the steel. Also gapless core Less than that in EI transformer, but higher than that of R-core  due to lack of balancing of winding. Higher exciting current is required due to presence of magnetic gaps, the inability to use the effect of rolling direction of grain oriented steel, etc.
Heat generation Heat generation is minimal due to low Iron loss. The large surface area of the coil allows for better heat dissipation. The core is not exposed at any point. Thus the heat generated has no area for dissipation. Due to iron loss the heat generation is more. Large part of the winding is covered inside the core resulting in poor heat dissipation.
Size 40% smaller than EI transformers

I first tested the supply with two 75 ohm resistors (at 12v this results in 160 mA of current -which is approximately the current consumption of the DAC board). It is always a good idea to test everything while building the circuit. This will prevent frying some component (or even frying yourself). Always exercise caution when dealing with line voltages.


5V Auxiliary supply (for the “dirty” side of the isolators)



I have been using the on-board output buffer connected directly to high-impedance headphones (Senn HD-580, rated at 300 ohm). The output buffer, the LME49724 [link] is specified to drive a load as small as 600 ohms while meeting specifications. At 300 ohm, it should be pretty close to spec.


Using the Philips Fidelio X1 (with a measured impedance of 37 ohm [link]), the direct connection seemed less pleasant (than the Senn headphone). Thus the need for a “real” head amp or at least a buffer with sufficient driving capability.

The diyinhk D1X is an inexpensive head amp implemented with a couple of LME49610 high performance audio buffers.



The kit comes with “basic” dual opamps. I had some OPA627s which I obtained when they first appeared in the market, and used those instead.


Due to the size of the dual chip carriers, some of the capacitors had to be installed on the back side.


I mounted it on the back of the case (because there is already a hole for the plug).


Direct connection to the DAC’s raw outputs:



After hooking up the power supply to the DAC Board and head-amp, the temperature at the heatsink of the power supply was running up to 75C and this is with the case cover off. Although likely a “safe” operating temperature since the specified maximum junction temperature is 125C, this is way too hot for my liking. In addition, with the cover on, it will be heating everything inside the case which will make run even hotter. Regardless of whether it can run at this temperature at extended periods of time, it is just not good practice running your parts are this high temperature.

The 15V outputs of the transformer measured about 18V AC (This results in about 25V DC). This means that with the TPA, being a low drop out regulator, approximately 12V is applied across the regulator itself. With a 12 V output, the amount of power dissipated by the regulator is just as large as the power dissipated by the DAC board – no wonder it gets so hot.

Actually, this would be a very good transformer for a Salas Shunt implementation. From the manual of the Salas, it specifies:

Voltage drop across regulator: Get a transformer that for DC gives you at least 5V extra from your target voltage. 7-10V is better against mains fluctuations, transformer regulation with high current etc.

But the TPA regulators which is a low drop out regulator only requires about .3 V of headroom and 12 V extra is totally unnecessary.


I therefore used a different transformer, this one has 2X 9V AC secondaries and a primary specified to run at 220V. According to the label, each 9V output can supply 0.85A which is plenty for our application (30VA total power).

I rewired it for 120V operation (I don’t remember exactly what I did, but likely I changed the series input connection into parallel connection). The measured output was almost 12V AC loaded. Perfect.




Now running at 35C… Perfect!



The correct way is to use a properly designed snubber, but the second best and much easier to do is to use a small value capacitor across the secondary [link]

The ringing is caused by the parasitics of the transformer + rectifier circuit. The parasitic inductance of the transformer windings and routing set up a resonant circuit with the stray capacitance of the circuit. When the current in this resonant circuit is interrupted, as when the diodes turn off, the energy left in the resonant circuit will cause it to ring until the energy has been dissipated in the parasitic resistance of the circuit. The ringing tends to be in the 100s of kHz to low MHz range – so well outside the audio band.

There are several ways to address this ringing. One can add a small capacitor (100 nF is usually a good starting point) across each secondary winding. This lowers the frequency of the ringing and effectively suppresses any coupling into the amplifier.

100 nF film caps across the secondaries:


I also added a capacitor across the primary:



(At least with headphones)

In addition to several enhancements over the original D1 headamp, the D1X implements an output signal relay:


The new distinctive feature is our customized power on delay and instant off output relay circuit. It can prevent the annoying power on/off clicks and pops, and with the DC servo circuit, expensive headphones are further protected from damage.

Especially for this DAC, this feature comes as a real bonus. The incredibly loud POPs during power on/off are totally eliminated.


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.
  1. Coris
    June 14, 2015 at 08:49

    The main reason one should not use a negative regulator as TPS7A33 in connection with the positive version (TPS7A47), is the quite big differences between these chips, when about noise level and PSRR. A much better (balanced) tandem for a bipolar PSU is TPS7A49/30.
    I would prefer this alternative, than having regulated the ground line, in an inverted approach as diyinhk use for this bipolar PSU.
    Not always the lowest noise figures is the only best for a regulator device…

    • BlgGear
      June 15, 2015 at 19:28

      Thanks for the tip. Checked out the datasheets but the TPS7A49/30 are low current device (150ma), so they cannot be used to power the DAC board.

  2. Derek
    June 16, 2015 at 20:12

    Nice. Do you have enough room in the chassis to fit an RPI and a 5VDC diyinhk PS for the Pi? Seeing as you have that extra pair of 9V outputs on the R-core….although, from memory, the R26-34 is 0.7A on the 9V outs?? Still, it might work if you don’t use anything USB on the Pi. Or you could parallel the two 9Vs.

    I find the i2s out from the Pi to the DAM a surprisingly good interface. Should be easy to try in your chassis. And it then has that addictive iPad interface….

  3. Anonymous
    June 19, 2015 at 05:19

    Hello, may I ask the sound better for R- Transformer?? or EI transformer the sound will be more clean?
    By the way, if using neutron star clock for the DAC, which model is right?


  4. Coris
    June 19, 2015 at 08:06

    The R core transformer is the best type transformer (lowest capacity secondary/primary) when to prevent induced noises from the main AC line into the secondary connected devices. The toroid is the worst one, while the EI type is in the middle.
    When this transformer is right manufactured (efficient shielding between primary – secondary on each coil), then this is the “quietest” possible transformer.
    Also with the highest energy efficiency, comparing with the rest of the types.

  5. Anonymous
    June 21, 2015 at 16:43

    Thanks Coris,
    I read Soren post on diyaudio that the total power consumption do the dam is 2.4w (positive side: 0.18A, 10w and 0.06A, 10w), so is it the ideal setup will be the R-core transformer with two 10w outputs, then connect to two DC shunt regulator. One with 0.18A output and the another is with 0.06A output? Then there will be not much works for the regulators and inside of the dam?


    • BlgGear
      June 22, 2015 at 15:24

      You need to determine what is the minimum input voltage required by the shunt regulator in order to produce 10V DC. If a transformer is rated at 10V AC, the DC voltage is 10×1.4=14V. The diode bridge will drop about 1 V leaving 13 V DC as input to the shunt regulator. This may or may not be sufficient depending on the design of the regulator.

      A quick look at the Salas BIB says: Voltage drop across regulator: Get a transformer that for DC gives you at least 5V extra from your target voltage. 7-10V is better against mains fluctuations, transformer regulation with high current etc.

      So if you are planning on using the Salas shunt, then you need an input DC voltage of 17 V and a transformer that gives you 18/1.4=13V AC. Keep in mind also that the bridge rectifier in the DAC board also drops about 1V, so if you want to have 10V powering the rails, then you need an input voltage of 11 V and a transformer that give you ~14V AC. The 15V AC dual secondary R-Core transformer would be a good fit for this application.

  6. June 22, 2015 at 12:51

    I do not understand why you use “D1X High Fidelity Headphone Amplifier”. You can directly connect your headphone to dac. It has enough power.

    • BlgGear
      June 22, 2015 at 16:39

      Yes, you can connect directly to the buffered outputs and that works very well. As I indicated in the post, the buffer opamp can drive loads directly and it is specified to perform as specified with loads down to 600 ohm. Many headphones have impedance much less than 600 ohms. In the case of the Fidelio X1, it is 32 ohms. I haven’t properly compared the two outputs, but wanted to try the DX1 especially with lower impedance headphones. As an added bonus, especially for this DAC, the turn-on/off POPs have been totally eliminated.

  7. July 8, 2015 at 17:15

    you have to make new capacitor mod, they said that it makes great improvement.

    Cant w8 to read your comments….

  8. December 10, 2015 at 19:18

    Hi BlgGear

    thanks for such detailed write up. Based on your write ups/ illustrated guides I felt confident that I could pull off building Soekris dam 1021 R-2R DAC. i have ordered the board and in the process of ordering the power regulator/ tranny. Had few questions, I would greatly appreciate if you could help

    a) did you hear discernible difference when you switched to the TPS7A47 regulator vs the 78xx based.
    b) wanted to confirm that its okay to go with 9v r-core transformer vs 12 or 15v, I found one on ebay rated for 30w but could only provide 0.7a current at 9v vs the 0.85 you had.


    • BlgGear
      December 11, 2015 at 06:08

      I can’t say that I can hear any differences, but based on value, I would go for the TPS power supply. It never hurts reducing noise.

      For the transformer, the 9V AC will give you about 12V DC (9×1.4 minus the voltage drop in the rectifiers which is in the order of 1 v) 12V may be insufficient to regulate to 12V but typically the transformer are underrated and you might more than 12V. Since the TPS only requires .3V headroom. Anything above 12.5V (loaded) should work fine

      If you read post https://hifiduino.wordpress.com/2015/06/14/dam-1021-r2r-more-mods/ you will find that using a 9V transformer (rewired from 220 to 120) was good enough for me. You will also find that a 15V transformer is too much for the TPS regulator (but OK for the 78xx regulators). You can ask diyinhk to see if a typical 9V RCORE is good enough…

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