ProtoDAC TDA1387 X8 project

[hifinet]

Questions received from a ProtoDAC builder (with answers):

1. Do I mount GPIO header on the back of PCB?

The header (plastic piece) mounts on the bottom of the PCB. The pins are soldered on top (labeled ProtoDAC TDA1387 X8). 

2. You said "and then do your best job on the working GPIO pins (2,4,6,12,35,40)”.  Does that mean I only solder those pins that you mentioned?

It is best to solder all of the pins for good mechanical support. Just be sure to do your best solder job on the working pins.

3. Do I always read the pin numbers from the top of PCB? The pin 1 is marked by a square. 

Yes, that is correct. The square pad is pin 1. 

4. Should I pair Nichicon 1000 microF 6.4 V with those Panasonic ones that you gave to me for free?

I personally think that two black Panasonic FR-A sound best. Some people like the Nichicon UKA. The total capacitance of the parallel capacitors C3 and C4 should be 2000uF . If you use the blue Nichicon UKA 1000uF, use it with a black Panasonic 1000uF. Later on, feel free to experiment with other capacitors with larger or smaller capacitance. It does affect the sound quite a bit.

5. When I was looking for the capacitors, "voltage rating?" was one of the questions been asked? How do I choose the correct one?

For the output film capacitors, get the lowest voltage (probably 35V). The most voltage they could possibly see is 5V. The lower the voltage, the smaller the capacitor. For the electrolytic capacitors (the black Panasonic or blue Nichicon), they should be rated no lower than 6.3V. They see the power supply voltage of 5V, and you need a safety margin. Again, the higher the voltage rating, the larger the capacitor for a given capacitance.

6. What is the purpose to have output capacitors? instant high current(low impedance), or the last step to filter out DC offset?

They are needed to block DC voltage at the output. 

7. How do you ground the PCB board? I couldn’t see any connection on the board.

The PCB is grounded through GPIO pins 6 and  14 when it is attached to the Raspberry Pi. There is also a ground pad between the RCA jacks, labeled GND. It is possible to power ProtoDAC and the Raspberry Pi by applying power to the 5V and GND pads between the RCA jacks on ProtoDAC. Note that this bypasses protection circuitry built into the Raspberry Pi, and any failures in ProtoDAC can also kill the Raspberry Pi.

[Tim Curtis]

Questions received from a ProtoDAC builder (with answers):

1. Do I mount GPIO header on the back of PCB?

The header (plastic piece) mounts on the bottom of the PCB. The pins are soldered on top (labeled ProtoDAC TDA1387 X8). 

2. You said "and then do your best job on the working GPIO pins (2,4,6,12,35,40)”.  Does that mean I only solder those pins that you mentioned?

It is best to solder all of the pins for good mechanical support. Just be sure to do your best solder job on the working pins.

3. Do I always read the pin numbers from the top of PCB? The pin 1 is marked by a square. 

Yes, that is correct. The square pad is pin 1. 

4. Should I pair Nichicon 1000 microF 6.4 V with those Panasonic ones that you gave to me for free?

I personally think that two black Panasonic sound best. Some people like the Nichicon UKA. The total capacitance of the parallel capacitors C3 and C4 should be 2000uF . If you use the blue Nichicon UKA 1000uF, use it with a black Panasonic 1000uF. Later on, feel free to experiment with other capacitors with larger or smaller capacitance. It does affect the sound quite a bit.

5. When I was looking for the capacitors, "voltage rating?" was one of the questions been asked? How do I choose the correct one?

For the output film capacitors, get the lowest voltage (probably 35V). The most voltage they could possibly see is 5V. The lower the voltage, the smaller the capacitor. For the electrolytic capacitors (the black Panasonic or blue Nichicon), they should be rated no lower than 6.3V. They see the power supply voltage of 5V, and you need a safety margin. Again, the higher the voltage rating, the larger the capacitor for a given capacitance.

6. What is the purpose to have output capacitors? instant high current(low impedance), or the last step to filter out DC offset?

They are needed to block DC voltage at the output. 

7. How do you ground the PCB board? I couldn’t see any connection on the board.

The PCB is grounded through GPIO pins 6 and  14 when it is attached to the Raspberry Pi. There is also a ground pad between the RCA jacks, labeled GND. It is possible to power ProtoDAC and the Raspberry Pi by applying power to the 5V and GND pads between the RCA jacks on ProtoDAC. Note that this bypasses protection circuitry built into the Raspberry Pi, and any failures in ProtoDAC can also kill the Raspberry Pi.

Nice Q&A. Maybe it should be included in the build document?

[hifinet]

Excellent idea.

[Tim Curtis]

I'll post a commit in next few days :-)

[hifinet]

Buyer beware on eBay caps. There are 10uF 63V MKC344 caps listed as polycarbonate, but they are PETP based on the part numbers and the chart below. The 15 indicates PETP and 10% tolerance. No Philips MKC <100V and no value larger than 6.8uF. The blue 6.8uF 100V 344-21 look correct. Note that lead spacing on the radial PC caps is 27.5mm, and the PCB takes 37.5mm, so a wire is needed.

The size and dimensions can be helpful in determining authenticity. PETP(polyester/mylar) < PC < PP < PTFE(teflon). 

Philips MKC Chart

Philips MKC data sheet

ERO (Vishay) MKC data sheet

WIMA MKC data sheet

[Nutul]

OMG, you guys are dissecting the thing with surgical skills... I am in awe.

[hifinet]

The new ProtoDAC testing board with pin sockets to test different C3/C4 capacitors. It also has pin sockets for the I/V resistors. Plan to try the S-foil TX2352 (the original 'naked' foil audio resistor!). They are a few bucks less expensive than the TX2575.

https://flic.kr/p/2oU4KYa

[Tim Curtis]

The new ProtoDAC testing board with pin sockets to test different C3/C4 capacitors. It also has pin sockets for the I/V resistors. Plan to try the S-foil TX2352 (the original 'naked' foil audio resistor!). They are a few bucks less expensive than the TX2575.

https://flic.kr/p/2oU4KYa

Super cool idea -> cap and resister rolling :-)

[Skip Pack]

Nice insertion posts for the caps in your image there. Do you have a part number available?  I put a second PDAC together,  this time with the z-foil output resistors and the polycarb cap pulls I purchased from you. The improvement from the 'plain' minimal model is clear, and I'm running 24hrs a day  to get some time one it. I think I'll do the next one with the ERO MKC caps I got. Since the lead spacing is a bit short, I'm thinking about running those caps upside down running two wires down to the board, thinking it might be more stable particularly if I have to offset it at all due to the 40 GPIO pins poking up from below. 

Thanks again for all your work on this,

Skip

[hifinet]

I don't have a part number. I bought them years ago when I used them as sockets for the wire leads of subminature tubes. I was using these, but they were hard to find and expensive. I have ones is plastic DIPs like this or these in gold plating. The ones in the picture came mounted on a DIP metal frame carrier. The leads on the TX2575 and Panasonic caps are a very tight fit, requiring needle nose pliers for insertion. I also used them as sockets for the TDA1387 module on the protoboard HAT version. Machine pin socket seems to pull them up in an eBay search. Here is one on DigiKey.

I suppose the PCB should have 27.5mm pad spacing for radial MKC caps.