Noise from HDMI port, Pi2, HiFiBerry DAC+ [SOLVED]

[rikardo1979]

[PeteB]

@Rikardo:

You could get something like this, plus an adapter for microusb:



It is not an LPS, but these were designed for medical instruments, and they meet fairly strict specs, even the commercial version meets the FDA specs. Less than 50 mV ripple. I have used them on the job, there are several manufacturers, all with good specs, for around $20 in quantity one.

I just tested one of these against an old HP filtered supply, and they test almost the same, within my ability to measure, so I got it for one of my two Pi 2 combos. Clean power on a <$20 budget. I am sure there is a Euro version of the same thing, your regs are stricter than US.

As a plus, they are 4-amp supplies, and their load regulation is very tight. I use it with the Pi that has a WD hard drive attached. It never deviates more than 100mV from 5V, even if the spec says +/- 5%. 80% or 85% efficiency, depending on what you pay for, and meet all EMC guidelines. Few of the RPi supplies can come close to that (nobody can for under $10, which is what the pi market demands).

Pete

[rikardo1979]

[dunghnguyen]

@PeteB: tks for your share about the mouser adapter. It looks good for people using Pi.

[rikardo1979]

[rastus]

[rastus]

[rikardo1979]

[PeteB]

Voltage:
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The Pi is specified for 5V +/- 0.25V... (5%). The Pi by itself can run on a wider range, but many USB devices can't.
If the voltage drops too low, the Pi has a circuit to detect it and will not run.
If the voltage is too high, there is a risk of damage to the Pi circuitry, or a USB dongle, or a memory device.

As the load on a typical power supply increases, the voltage drops a little, and vice versa. At no load it is a bit higher, at full load a bit lower. This is called load regulation. A cheap 5V supply has poor load regulation, and may measure only 4.7~4.8V at the USB port, too low for some WiFi dongles. The voltage also drops a bit in the typical cheap supply cable.

The reason why some people use 5.1V~5.3V power supplies is to compensate for poor load regulation. They are counting on the voltage drop from a cheap regulator to bring the voltage down to 5V at the Pi. This is a common practice, but not a good one. A good 5V 2A supply should measure 5.0V at 50% load, which is typical after startup. Unfortunately a "good" wall-wart would cost over $10, not the $5~6 most Pi owners are counting on.

6V is definitely too high for a direct connection.

The reason why we have 6~7V, or even higher supplies is because some Pi DACs and Pi Amps regulate this higher voltage down to 5V, filter it, and supply the Pi through the 40-pin connector. This way the Pi DAC or the Pi Amp gets clean power, and the Pi gets a separate power supply that does not add noise to the DAC or Amp. This is a good solution (like Audiophonics uses), much better than supplying the DAC through the Pi, but more expensive.

Current:
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A direct connect linear supply can be rated at anything over 2 Amps, no matter what, it will supply clean power if designed and filtered correctly. Running a 10-amp supply at 1 amp will do no harm

A direct connect SMPS should be rated at over 2 Amps, but less than 10. Some SMPS supplies need about 10~20% load to meet their noise specs. So a 10 amp SMPS will work safely at 1 amp of less, but it may not meet its stated regulation or noise limits at a low current.

[rastus]

Very nice explanation PeteB, thank you.... and I do mean that, thank you brother. Makes sense to me.