ProtoDAC TDA1387 X8 project

[wolf7250]

A friend of mine at work suggested something like this as a way to achieve low noise power input:
PSU Noise Filter

He is convinced it would work with a standard RPi PSU to reduce noise before powering a RPi or I guess the ProtoDAC....

[hifinet]

Could you share any details of your plans?

I have previously used a 1200mAh 2S LiPO battery (an airsoft battery) with the LT1963. Very quiet, of course. The dynamics are slightly restricted, and I think this is due to the 70 or so ESR of LiPO.

This SMPS DC filter has been popular at diyA. I have the parts, I guess I should try it out. Thread here. I input DC for the stack to the Allo Kali, which does have two input inductors and probably similar circuitry. 

[wolf7250]

Sure. My plan was to:

• Cut the end off a RPi PSU
  
• Solder the wall plug end onto the In side of the filter and
  
• Solder the USB end onto the Out side of the filter.
  

Then use it to power the RPi or ProtoDAC.... I personally have no idea how well it would work.
Issue is that some of the components are out of stock on DigiKey and Mouser... so a bit hard to build at the moment.

The SMPS DC filter you sent a link to looks really interesting! So you have the parts? Be very interested to hear what you think!
How would you use it? As an intermediary step between the PSU and the Allo Kali?

[hifinet]

For my current setup, it would go between the SMPS and the Kali. These DC filters attenuate noise, but they don't eliminate it. People have used multiple filters in series. But then you are affecting the transient response of the power supply. It would be best to filter, then go into a capacitor or ultracapacitor reservoir.

There is still noise generated by the Raspberry Pi CPU and also the switching regulators of the RPi. Only the original RPi 1 had linear regulators. Some people would actually do microsurgery on RPis with switching regulators, and replace with linear regulators.

That noise from the SMPS gets everywhere. Not only on the positive line, but in the grounds and radiates back onto the AC line. Some EMI as well, but much less significant. Electronics need to pass EMI standards set by the FCC and European regulators.

Until I heard the 5V ultracap supply of the PurePi, I didn't fully appreciate the haze of noise that the Allo basic 5V SMPS was casting, even with the built-in filter of the Kali. I knew there was some noise, but didn't realize it was that much. For some reason, the Allo SMPS creates a larger soundstage, so it has that benefit in its corner. The LifePO4 battery and/or ultracap supplies plus isolation (to deal with the noise from the RPi) seem to be the best solutions.

[hifinet]

Before using PurePi, I thought the LifePO4 batteries were also supplying the ultracapacitors. That is not the case.

[hifinet]

Issues of pops. Some issues with ProtoDAC reported to me by a kit customer.

A pop when switching the preamp/amp input to ProtoDAC, with ProtoDAC powered. This is due to the fact that ProtoDAC is missing a "polite part". This part is a resistor from the center RCA to ground. It can be anything from 100k to 1M. It is not included for simplicity and sound quality.

Since ProtoDAC output is not tied to ground with a resistor, a charge can develop when the input of the preamp/amp is switched to another component. Then when the input is switched to ProtoDAC, that charge is suddenly discharged to ground and is amplified. The simple solution is to turn the volume down before switching to the ProtoDAC input.

The signal not passing through a signal to ground resistor is your signal. No resistor is a perfect resistive device (metal foil get close). It also has inductive and capacitive properties. Signal to ground resistors will affect sound quality, especially before an amplification stage. Some advocate for metal foil resistors in all signal to ground positions. That can get expensive. My recommendation is to just turn the volume down before switching the input.

The same customer reports a pop when turning on ProtoDAC with the amp already on. It is best practice to turn the amp on last (ProtoDAC-->preamp-->amp), and turn it off in reverse order.

The third pop is at the start of the first playback track. I think this is module related. My TeraDak TDA1387*8 always made a sound of a record skipping grooves at the beginning of the first track.

I can rarely get a tick after the last track of the playlist.

[wolf7250]

For my current setup, it would go between the SMPS and the Kali. These DC filters attenuate noise, but they don't eliminate it. People have used multiple filters in series. But then you are affecting the transient response of the power supply. It would be best to filter, then go into a capacitor or ultracapacitor reservoir.

There is still noise generated by the Raspberry Pi CPU and also the switching regulators of the RPi. Only the original RPi 1 had linear regulators. Some people would actually do microsurgery on RPis with switching regulators, and replace with linear regulators.

That noise from the SMPS gets everywhere. Not only on the positive line, but in the grounds and radiates back onto the AC line. Some EMI as well, but much less significant. Electronics need to pass EMI standards set by the FCC and European regulators.

Until I heard the 5V ultracap supply of the PurePi, I didn't fully appreciate the haze of noise that the Allo basic 5V SMPS was casting, even with the built-in filter of the Kali. I knew there was some noise, but didn't realize it was that much. For some reason, the Allo SMPS creates a larger soundstage, so it has that benefit in its corner. The LifePO4 battery and/or ultracap supplies plus isolation (to deal with the noise from the RPi) seem to be the best solutions.

Thanks! This is all really great information. And has prompted me to investigate further into the products available on the Ian Canada website...
I found these (which you may already be aware of):

1. Isolator Pi which seems to handle the issues of noise on the Raspberry Pi GPIO etc - think that the FiFoPi Q7 has this built in??
   
2. Shield Pi Pro which seems to fulfil two functions:
   a) Act as an EMI shield (less interesting based on your observations of EMI not impacting performance as much) and
   b) Power Supply noise filter
   

If I was to go with a FiFoPi MA (and any Pi other than a Zero W) then I think the Isolator Pi could be useful.
The shield Pi Pro looks like it could be a really good alternative to the Power Supply noise filters we've already been discussing.
What do you think? Were you already aware of these products?

[wolf7250]

Issues of pops. Some issues with ProtoDAC reported to me by a kit customer.

A pop when switching the preamp/amp input to ProtoDAC, with ProtoDAC powered. This is due to the fact that ProtoDAC is missing a "polite part". This part is a resistor from the center RCA to ground. It can be anything from 100k to 1M. It is not included for simplicity and sound quality.

Since ProtoDAC output is not tied to ground with a resistor, a charge can develop when the input of the preamp/amp is switched to another component. Then when the input is switched to ProtoDAC, that charge is suddenly discharged to ground and is amplified. The simple solution is to turn the volume down before switching to the ProtoDAC input.

The signal not passing through a signal to ground resistor is your signal. No resistor is a perfect resistive device (metal foil get close). It also has inductive and capacitive properties. Signal to ground resistors will affect sound quality, especially before an amplification stage. Some advocate for metal foil resistors in all signal to ground positions. That can get expensive. My recommendation is to just turn the volume down before switching the input.

The same customer reports a pop when turning on ProtoDAC with the amp already on. It is best practice to turn the amp on last (ProtoDAC-->preamp-->amp), and turn it off in reverse order.

The third pop is at the start of the first playback track. I think this is module related. My TeraDak TDA1387*8 always made a sound of a record skipping grooves at the beginning of the first track.

I can rarely get a tick after the last track of the playlist.

Thanks! Really great information and advice 

[hifinet]

I think FifoPi MA and IsolatorPi II sounds like an ideal combination. Isn't this combo performing the same function as the Q7, but for $51 cheaper? The isolated GPIO of IsolatorPi provides 5V. Perfect! FifioPi MA can sit on the the IsolatorPi, and with J4 jumpers attached, receive power from the isolated GPIO and pass through 5V to ProtoDAC. Nice!

I think the ShieldPi Pro would be not be needed, since the 5V to the MA and ProtoDAC is isolated.

What about the ShieldPi? It's free, so sure, throw it in the mix. Is it free for a reason?

[wolf7250]

Yeah! That was my thoughts exactly. I think that the Q7 might have better clocks??? but not sure about that and I don't think I will be able to hear a difference... and in Australia $51 USD is more like $78 AUD with the exchange rate.

So yeah, to power this you would just provide 5V to the FiFoPi MA J3 (and short J4 as you said) and that would power the entire stack? Would you just use a standard PSU? Or incorporate one of the filters we've discussed? Like the kit from DiyAudio?

Yeah I saw the ShieldPi is free if available... haven't seen a reason why it would be free..