PicoPSU cause audio distortion

[guybrush]

Got a PicoPSU 120W for my A1200; and after connecting everything, I noticed that the Amiga run fine, but the audio does not work as expected.


The audio output is exceedingly noisy and feel like I am playing music on a broken speaker. If I use the regular PSU, the audio is just fine using the same exact speakers.


I saw plenty of youtube videos where people modded their Amiga to use a PicoPSU, and I do not see any negative comment about the audio distorted. Do I have a broken PicoPSU? I connected the PSU to the floppy disk to get the load, then I have the +5, +12 and -12V pins connected to a square Amiga PSU connector; so nothing out of the ordinary here.

[Daedalus]

The issue is most likely the power supply you're using with the PicoPSU. Some PicoPSU models require a direct 12V input, others allow a range like 12-24V. If yours specifically requires 12V, then it doesn't do anything with that rail at all - it simply passes it directly to the Amiga, which then uses it directly for the audio circuitry. Many cheap power bricks are very noisy, and that noise is being passed directly into the Amiga's audio circuitry, resulting in the distortion you hear.

Do you have a multimeter? It might also be worth checking that the 12V rail is what it should be - you can measure it at the floppy power connector. The -12V rail should be generated by the PicoPSU, it's worth checking that that's ok too, though you'll probably need to check that at the PSU itself.

I've observed both video and audio noise with different power bricks feeding various PicoPSU models, including what should be decent Dell laptop chargers. The PicoPSU's advantage is its size, but that comes at a cost of filtering, so it really needs a very clean input supply to work well.

[pandy71]

Originally Posted by Daedalus

If yours specifically requires 12V, then it doesn't do anything with that rail at all - it simply passes it directly to the Amiga, which then uses it directly for the audio circuitry.

Analog audio part is decoupled by rudimentary RC filter (10 Ohm + 220nF + 22uF) and original PSU is also quite noisy...
Perhaps issue is -12V line as you already suggested...
Anyway multimeter (or perhaps scope could be better) to verify required voltage present is recommended.

[Daedalus]

Indeed, the 12V supply doesn't have to be perfect, but it does have to be decent. The basic filtering you mention is enough to deal with the noise from the original PSU, but it's easy to find much worse, and where I've seen noise on the audio output of that nature, it was obvious with a 'scope that it was a subharmonic of the switching frequency of the original 12V supply that was amplified and output. Swapping to a slightly better 12V PSU eliminated it.

[tech3475]

How close is the Pico to the circuits?

I'd try moving the Pico away if possible or shielding it to see if it's producing EM interference. I bought a cheap Pico last year which some people reported as having issues, but it was good enough for my needs.

Also, going purely by your post, have you checked ground?

I tried modding my Mega Drive/Gen 1 last year for rear Stereo and I had a similar audio issue, which I believe is due to ground issues with the connector I was using.

[guybrush]

Thanks for your replies; let's see if I get this right

The issue may be the AC adapter I use to power the pico? I bought both the AC adapter and the pico together, so I assumed they were good paired; but it seems they were not.

The seller gave me the 120W when I told him I was running an Amiga 1200 with CF card adapter and a Blizzard 1230 MK4; and the AC adapter is a 12V 10A

I tried this big 10A PSU, then I tried a 5A PSU and finally a 3A PSU; all of them boot the amiga fine; and I get at the square terminal connector the correct voltages (+5, +12 and -5); but none of those give me the good audio. Only if I use the original PSU I get the audio that is not distorted.

I guess at this point the issue is to find an AC adapter that is stable enough for the Pico? I like the form factor but if the pico is small and the AC adapter that power it is huge anyway, at that point I keep the original PSU or get a standard ATX one I put the pico inside a PC case (I transformed my 1200 in a tower basically); so getting an ATX PSU is not the end of the world; I just want to make a small build with the 1200 in a desktop case, and this is why I bought the Pico.

[whaka]

hello, you may really have a weak -12V as already suggested.
don't forget than modern psu still have the -12 for... nothing on modern pc.
so, it's usually given for 0,1A but i'll not be surprise if cheap psu don't really respect that.

i have a picopsu for various testing (but not amiga), and the -12V happily fall at -10,80V with no load on it...
this may be low for the opamp of an amiga.

[guybrush]

How do I check for current on load? Should I start playing a mod and then measure with the multimeter?
And is there a way to figure out which power brick would work with the picoPSU? I assume others are using this mod too; and while not all power bricks are created equal; there should be a reliable way to find one that works for sure with the Pico

I am tempted to just throw out of the window the Pico idea; although there is not much in terms of alternatives at that point, since even an ATX PSU may have the same exact issues of the pico at this point. And I am not that keen on using a 20+ year old PSU.

Considering that the amiga just need 12v and 5V for most of its circuitry, and -12V for the audio circuit, I find strange that nobody in the community made a small PSU; all I found are ATX mods or Pico mods

Looking at the power consumption, unless I have the numbers wrong, the Amiga 1200 with the Blizzard 1230 and a Gotek/floppy should use not more than 10-15W at full load; this means that if the power input to the Pico is 12V in, the current needed is less than 2A; so my 10A brick is supplying way more current than what needed; as such the issue must be in how clean the signal is. I just ordered a different PicoPSU from a different company; will give a try and see if that one does the same as the Pico I have; just so I can rule out the Pico issue itself.

I have 3 different power bricks, at 3, 5 and 10A; so the chances that all 3 are bad are quite on the low side; or they all lack something that the Pico is expecting (-12V rail); which means I have no clue how to fix this. The AC bricks on Mouser do not seem to have clear data about the -12V rail; unless I am missing something:

https://www.mouser.com/Search/Refine...128+4292906361

[Lisko]

Meanwells are also nice and compact psus suitable for the amiga, look at the rt-65b or the rpt-60b. On eBay you can find them also pre-built for the amiga. Be only sure that the meanwell they are using has a native -12v output (like the two units I noted) because some sellers use ones without and perform unsafe hacks to provide it. You can also put them in the original psu case or in a (way) smaller one.

[Daedalus]

Originally Posted by guybrush

I tried this big 10A PSU, then I tried a 5A PSU and finally a 3A PSU; all of them boot the amiga fine; and I get at the square terminal connector the correct voltages (+5, +12 and -5); but none of those give me the good audio. Only if I use the original PSU I get the audio that is not distorted.

Okay, are you sure you're measuring the correct pins? There's no -5V input for an A1200 - the centre pin should be -12V. Also, if you've tried multiple AC adaptors and are getting the same results, it's starting to point at the PicoPSU itself, or the adaptor being used.

The current consumption on the +/-12V rails is quite low and well within the PicoPSU specs - I've never had issues with that end of things with any PicoPSUs I've tried. The tolerance is also quite relaxed, so -10V shouldn't cause too many problems. There's still plenty of overhead for the audio circuitry at that level. But at this point I would start to suspect the PicoPSU itself or the associated wiring.

It's worth considering too that counterfeit PicoPSUs are fairly commonplace, so there's always the possibility that the one you got isn't up to snuff.

I guess at this point the issue is to find an AC adapter that is stable enough for the Pico? I like the form factor but if the pico is small and the AC adapter that power it is huge anyway, at that point I keep the original PSU or get a standard ATX one I put the pico inside a PC case (I transformed my 1200 in a tower basically); so getting an ATX PSU is not the end of the world; I just want to make a small build with the 1200 in a desktop case, and this is why I bought the Pico.

Yeah, the only real advantage of the PicoPSU is the small format, and if you're going to have a large external brick anyway, it sort of negates the advantage. As mentioned, there are various replacement PSUs that are probably around the same size as the PicoPSU AC adaptor but contain a Meanwell or similar module, and provide the power you need externally. It might be worth considering one of them.

[pandy71]

-12V must be -12V that's all, let say with +-10% (+-1.2V) accuracy under normal load (i.e. Amiga operational) so -12V (-13.2V...-10.8V) is OK, everything else is NOK (and PicoPSU seem is able to deliver only 50mA on -12V line but -12V is used in Amiga by audio operational amplifier (LF347 sink approx max 11mA accordingly to TI data sheet) and by serial driver MC1488 (accordingly to Motorola/ON Semi DS it may sink 12mA) - 50mA on -12V line should be enough unless there is another sink on this line.

[guybrush]

Thank you all for your replies; I was able to narrow down the issue to the actual PicoPSU.

Ironically, it was a legit PicoPSU, not a clone; and the clone I purchased works like a champ. Audio is good, overall the system is stable and I get the right voltage out of the pins.
@Daedalus my -5 was the -12V pin; clearly that PSU was shot because it was not giving me the -12V at the connector

After the swap of the PSU, everything is working fine now; I am very happy. Thanks again!