Step 4
![[Image: 52688519647_47a5d35347_b.jpg]](https://live.staticflickr.com/65535/52688519647_47a5d35347_b.jpg)
The DAC is nearly completed. The last steps will take a little ingenuity, because it involves placement of the output coupling capacitors. These capacitors can vary considerably in size, quality and price. You may want to press fit these in place to try different types before soldering.
The value should be around 10 uF, but can be as low as 4.7 uF. With standard 10k impedence, 10 uF will give a low frequency -3dB of 1.59 Hz. 4.7 uF will give a low frequency -3dB of 3.38 Hz. You want the low frequency cutoff to be well below 20 Hz, ideally less than 2 Hz.
A metal foil and film capacitor will be the highest quality, but physically larger and more expensive.
A basic polypropylene metallized film capacitor is a WIMA MKP4 or MKP10 with radial leads. The lead spacing will be either 27.5mm (1.08") or 37.5mm (1.48"), which is slightly different from the 0.1" spacing of the board. The size is very compact for the capacitance. Digi-Key has MKP4D046806F00JSSD which is a 6.8uF 100VDC currently in stock. Price is about $4.29 each. WIMA MKP are commonly available, competent sounding and are a good choice for this project. Generally, most polypropylene capacitors will be satisfactory. Panasonic ECW and EZP PP caps are good, and relatively small for capacitance. PP caps can also be purchased from speaker components suppliers, where they are used as crossover caps. For example, Parts-Express, Madisound, Speaker City USA, etc. It is best to get the lowest voltage rating available, which will reduce the size. Panasonic, Solen and WIMA PP caps tend to be neutral sounding without any annoying qualities. Humble Homemade Hifi has some capacitor ratings.
Boutique film capacitors can be very expensive and are often too large for the available space on the HAT. Metallized polypropylene tend to be the best value.
Solder the right channel output capacitor to the hole of the red line near pin 6 of the module. Note that the right channel I/V resistor is also connected to pin 6.
Solder the left channel output capacitor to the hole of the white line near pin 25 of the module. Note that the left channel I/V resistor is also connected to pin 25.
Solder the left and right channel PC mount RCA output jacks near the top (near the display cable notch in the board). Connect the coupling capacitor to the signal (positive) of the RCA. Connect the ground of the RCA to the ground strips on the board.
You can also connect 1-2M ohm resistors from the signal (positive) RCA to ground, if you like. They are "politeness parts" and can prevent pops when you select then DAC with the preamp switch. They do affect sound quality, and I typically omit them.
I use RCA jacks as pictured below. They are available on eBay with search terms: RCA PCB DAC. They have a part number RJ-255. High quality parts use gold plated brass and teflon (PTFE) white insulator around the signal. Teflon has superior insulating properties, and is also extremely heat resistant. Typical plastics will melt with the heat of a soldering iron. I found some inexpensive parts at ELECbee , which are listed as gold plated brass, but the insulator is POM or ABS (i.e. not teflon). I have not found a Chinese supplier that uses teflon signal insulator.
If you are using chassis mount RCA jacks, it is best to use the type that have a soldered ground and mounting nuts on the outside. The ground is more secure soldered. With the nut on the outside, you can easily remove the RCA jack from the chassis without needing to desolder.
You can cut the trace on the board to separate the two poles, or bend the center signal wire to the next strip. You will need to drill two holes for the mounting post. Be careful not to drill into the 5V power tracing on the right channel side of the board, which is close. Plan the placement of the RCA and drill any holes before mounting any components. The left channel post hole center is 3mm from the top edge and 12mm from the side edge. The drill is 5/64". It is best to use a drill press. The right channel post hole will drill out most of the "G" in GND (centered 3mm from the top edge). Keep the drill clear of the "N", which is right over the 5V trace. Refer to the board layers in the picture below. Expect that you may need to do some fine filing to fit them straight. Soldering the RCA jacks is one of the final assembly steps. You can save board space by omitting the RCA jacks, and soldering wires from the outputs to external RCA jacks or a 3.5mm plug.
![[Image: VAMPIRE-88330.jpg]](https://www.partsconnexion.com/media/product_images/VAMPIRE-88330.jpg)
With a simple circuit like this, sound quality depends on 1) I/V resistor quality, 2) coupling capacitor quality and 3) power supply quality. With quality resistors and caps, I have run the DAC directly on the RPi, getting power from the RPi with a cheap SMPS (wall wart), and it sounds excellent. It sounds even better with an Allo Kali reclocker for low jitter and a better power supply.
![[Image: 71WYP0lUCIL._AC_SL1000_.jpg]](https://m.media-amazon.com/images/I/71WYP0lUCIL._AC_SL1000_.jpg)
Changing the main Vcc electrolytic decoupling cap will affect the sound considerably. With a well regulated 5V supply and no cap, the sound is very 3D, with large soundstage and excellent depth, but also anemic with weak bass and no dynamics. The proper capacitor will add bass and dynamics, and maintain a good soundstage. If the capacitor is too large, the depth of the image will become very thin. There is also some dynamic range compression and peak limiting. The bass remains strong. Some people prefer this. The smaller the I/V resistor, the larger the capacitor. I would recommend 1800-2200uF Vcc capacitor with a typical 430R I/V resistor (audio grade Nichicon UKA are very good). It would make sense to press fit various capacitors in place and audition several values before final soldering.
I have tried stacking boards (by removing all the pins and soldering connecting wires through 1, 2, 3, 5, 6, 25, 28). The I/V resistor value needs to be halved and the Vcc cap doubled. This will reduce output impedence and noise, but I didn't think it was worth it. Noise is reduced 3dB with every doubling.
I have read that you can stack the chips directly, but SO8 are difficult to solder that way, and if there is a bad chip in the middle of the stack, you are in for an ordeal.
![[Image: img_1358-jpg.1049389]](https://www.diyaudio.com/community/attachments/img_1358-jpg.1049389/)
The DAC is nearly completed. The last steps will take a little ingenuity, because it involves placement of the output coupling capacitors. These capacitors can vary considerably in size, quality and price. You may want to press fit these in place to try different types before soldering.
The value should be around 10 uF, but can be as low as 4.7 uF. With standard 10k impedence, 10 uF will give a low frequency -3dB of 1.59 Hz. 4.7 uF will give a low frequency -3dB of 3.38 Hz. You want the low frequency cutoff to be well below 20 Hz, ideally less than 2 Hz.
A metal foil and film capacitor will be the highest quality, but physically larger and more expensive.
A basic polypropylene metallized film capacitor is a WIMA MKP4 or MKP10 with radial leads. The lead spacing will be either 27.5mm (1.08") or 37.5mm (1.48"), which is slightly different from the 0.1" spacing of the board. The size is very compact for the capacitance. Digi-Key has MKP4D046806F00JSSD which is a 6.8uF 100VDC currently in stock. Price is about $4.29 each. WIMA MKP are commonly available, competent sounding and are a good choice for this project. Generally, most polypropylene capacitors will be satisfactory. Panasonic ECW and EZP PP caps are good, and relatively small for capacitance. PP caps can also be purchased from speaker components suppliers, where they are used as crossover caps. For example, Parts-Express, Madisound, Speaker City USA, etc. It is best to get the lowest voltage rating available, which will reduce the size. Panasonic, Solen and WIMA PP caps tend to be neutral sounding without any annoying qualities. Humble Homemade Hifi has some capacitor ratings.
Boutique film capacitors can be very expensive and are often too large for the available space on the HAT. Metallized polypropylene tend to be the best value.
Solder the right channel output capacitor to the hole of the red line near pin 6 of the module. Note that the right channel I/V resistor is also connected to pin 6.
Solder the left channel output capacitor to the hole of the white line near pin 25 of the module. Note that the left channel I/V resistor is also connected to pin 25.
Solder the left and right channel PC mount RCA output jacks near the top (near the display cable notch in the board). Connect the coupling capacitor to the signal (positive) of the RCA. Connect the ground of the RCA to the ground strips on the board.
You can also connect 1-2M ohm resistors from the signal (positive) RCA to ground, if you like. They are "politeness parts" and can prevent pops when you select then DAC with the preamp switch. They do affect sound quality, and I typically omit them.
I use RCA jacks as pictured below. They are available on eBay with search terms: RCA PCB DAC. They have a part number RJ-255. High quality parts use gold plated brass and teflon (PTFE) white insulator around the signal. Teflon has superior insulating properties, and is also extremely heat resistant. Typical plastics will melt with the heat of a soldering iron. I found some inexpensive parts at ELECbee , which are listed as gold plated brass, but the insulator is POM or ABS (i.e. not teflon). I have not found a Chinese supplier that uses teflon signal insulator.
If you are using chassis mount RCA jacks, it is best to use the type that have a soldered ground and mounting nuts on the outside. The ground is more secure soldered. With the nut on the outside, you can easily remove the RCA jack from the chassis without needing to desolder.
You can cut the trace on the board to separate the two poles, or bend the center signal wire to the next strip. You will need to drill two holes for the mounting post. Be careful not to drill into the 5V power tracing on the right channel side of the board, which is close. Plan the placement of the RCA and drill any holes before mounting any components. The left channel post hole center is 3mm from the top edge and 12mm from the side edge. The drill is 5/64". It is best to use a drill press. The right channel post hole will drill out most of the "G" in GND (centered 3mm from the top edge). Keep the drill clear of the "N", which is right over the 5V trace. Refer to the board layers in the picture below. Expect that you may need to do some fine filing to fit them straight. Soldering the RCA jacks is one of the final assembly steps. You can save board space by omitting the RCA jacks, and soldering wires from the outputs to external RCA jacks or a 3.5mm plug.
With a simple circuit like this, sound quality depends on 1) I/V resistor quality, 2) coupling capacitor quality and 3) power supply quality. With quality resistors and caps, I have run the DAC directly on the RPi, getting power from the RPi with a cheap SMPS (wall wart), and it sounds excellent. It sounds even better with an Allo Kali reclocker for low jitter and a better power supply.
Changing the main Vcc electrolytic decoupling cap will affect the sound considerably. With a well regulated 5V supply and no cap, the sound is very 3D, with large soundstage and excellent depth, but also anemic with weak bass and no dynamics. The proper capacitor will add bass and dynamics, and maintain a good soundstage. If the capacitor is too large, the depth of the image will become very thin. There is also some dynamic range compression and peak limiting. The bass remains strong. Some people prefer this. The smaller the I/V resistor, the larger the capacitor. I would recommend 1800-2200uF Vcc capacitor with a typical 430R I/V resistor (audio grade Nichicon UKA are very good). It would make sense to press fit various capacitors in place and audition several values before final soldering.
I have tried stacking boards (by removing all the pins and soldering connecting wires through 1, 2, 3, 5, 6, 25, 28). The I/V resistor value needs to be halved and the Vcc cap doubled. This will reduce output impedence and noise, but I didn't think it was worth it. Noise is reduced 3dB with every doubling.
I have read that you can stack the chips directly, but SO8 are difficult to solder that way, and if there is a bad chip in the middle of the stack, you are in for an ordeal.
