SMPS's / switching PSU's and their somewhat variable specifications.

[Mark S.]

I've been trawling the interwebs for PSU's for a TPA3251 'mini amp' that's on the way, ended up buying a Mean Well GST120A24-P1M 24V/5A supply (a discontinued line but numbers of NOS available) which, purely coincidentally, turns out to have been recommended by one Nelson Pass.

Since it's a Class I supply (with an earth pin on the AC inlet), I also need a 5V supply for my USB DAC (to a USB isolator or possibly the DAC's 'aux' power inlet) because it crackles and buzzes through any amp from a ground loop unless using a Class II PSU (2-pin AC-in, no earth) for the amp.

Happened across another Mean Well, a GS18A05-P1J (a small brick rather than a wallwart), also discontinued, which has a rather impressive ripple/noise spec of 50mV, almost every other 'inexpensive' 5V switcher I've come across is 200mV (a few at 150mV, 250mV or even 300mV).

I'm surprised that there isn't a sort of informal list of general purpose SMPS's known to be good for audio applications among enthusiasts (I was tempted to say audiophiles but I guess this isn't really about 'audiophile' equipment).

 

[Old_School_Brad]

If I were sourcing a power supply, I’d approach it the same way -by checking the ripple and basing my choice on that. So, there is indeed a specific rating to consider.

Perhaps there isn’t a definitive list because many new models are being released?

I’ll keep an eye on this thread. I might try a low-ripple power supply for my miniDSP DDRC24 to see if it reduces the noise.

 

[Mark S.]

This is the problem, there are squillions of cheap, obscurely-named wallwarts and bricks to be had, some of which might be very good. Then there are the 'name brands' which have presumably established themselves over years or even decades. I always dismissed Mean Well because, well, how does that read except as a 'Chinglish' name some wannabe firm in Shanghai came up with - turns out they're generally very good.

A decade or so ago I bought a couple of small 'Mascot' linear supplies (for phono and headphone amps) which were excellent, as their specs suggested, but who's heard of them?

There's a similar situation in other industries. Hankook tyres? Very funny - except they now apparently match (and generally under-cut) the likes of Michelin, Pirelli et al in all but the most extreme track and hypercar tyres.

 

[Chr1]

Indeed. Maxxis tyres are similar in the motorcycle world.

Will watch this thread too.

 

[Mark S.]

Indeed. Maxxis tyres are similar in the motorcycle world.

Will watch this thread too.

Strange to think that even 'Bridgestone' was a silly name once upon a time! ETA >> damn, I'm old - most will be too young to know that the name was pilfered from 'Firestone'.

ETA >> I used to ride. IIRC when I started the only proper tyres were, what, Dunlop, Avon and Continental (the latter generally for 'sensible' BMW riders)? ETA >> Hasten to add I carried on until the last bike I owned, an FZS600 Fazer bought as a 40th B-day present to myself - by then the standard fitment Bridgestones were a frikking revelation.

 

[Wolf11Man]

If you use a simple transformer with a cheap regulator instead of a SMPS, you will have a nice clean output with no ground problem. Your USB DAC should have a second, own power supply, cleaning up the voltage even better.

If you have the DAC inside your amp, a cheap LM317 regulator board, sourced by the 24 Volt and adjusted to 5 Volt, should work just fine. There are some on eBay etc. for about 1$. I usualy buy 10 of them in one order.

 

[Mark S.]

If you use a simple transformer with a cheap regulator instead of a SMPS, you will have a nice clean output with no ground problem. Your USB DAC should have a second, own power supply, cleaning up the voltage even better.

If you have the DAC inside your amp, a cheap LM317 regulator board, sourced by the 24 Volt and adjusted to 5 Volt, should work just fine. There are some on eBay etc. for about 1$. I usualy buy 10 of them in one order.

No doubt, but nowadays I'm only doing plug-n-play.

 

[Wolf11Man]

There are still wall warts around that use a transformer. You only have to find a matching one. You may try a power bank, just to make sure the supply is really the problem.

 

[Mark S.]

There are still wall warts around that use a transformer. You only have to find a matching one. You may try a power bank, just to make sure the supply is really the problem.

As I mentioned, I bought a couple of small, <1.0A 'Mascot' linear supplies back in the mid 2010's.

IIRC one was was 24V for a Creek OBH-11 headphone amp in place of the switcher that came with it - Creek themselves sold an 'upgrade' linear PSU which was several times the price of the Mascot.

The other was for a phono amp (Graham Slee? Can't remember now, I tried a few).

The Mascots were very good (dead silent in the analogue 'domain'). I believe they still make them, but not cheap (relatively speaking - they probably are by 'audiophile' standards)..

 

[HaveMeterWillTravel]

If you use a simple transformer with a cheap regulator instead of a SMPS, you will have a nice clean output with no ground problem. Your USB DAC should have a second, own power supply, cleaning up the voltage even better.

If you have the DAC inside your amp, a cheap LM317 regulator board, sourced by the 24 Volt and adjusted to 5 Volt, should work just fine. There are some on eBay etc. for about 1$. I usualy buy 10 of them in one order.

I assume you mean a linear voltage regulator. Good luck in longevity without a full bridge rectifier, filtering and stabilization caps, etc

 

[HaveMeterWillTravel]

One of my “go to” for clean(er) power supplies is Acopian. But quality isn’t cheap.

 

[HaveMeterWillTravel]

I've been trawling the interwebs for PSU's for a TPA3251 'mini amp' that's on the way, ended up buying a Mean Well GST120A24-P1M 24V/5A supply (a discontinued line but numbers of NOS available) which, purely coincidentally, turns out to have been recommended by one Nelson Pass.

Since it's a Class I supply (with an earth pin on the AC inlet), I also need a 5V supply for my USB DAC (to a USB isolator or possibly the DAC's 'aux' power inlet) because it crackles and buzzes through any amp from a ground loop unless using a Class II PSU (2-pin AC-in, no earth) for the amp.

A couple of points:
It has been my experience that that vast majority of "laptop" style power bricks "float" the DC output(s). That is, the DC return of the output is neither tied to the input power's "ground", nor the "neutral".

By their nature, transformers almost always have parasitic capacitance which induces common mode voltage/current between the primary and secondary windings, often called "leakage current" or "touch current".

I bet if you took your DMM on AC voltage, then measured across the DC return and ground, you'll measure AC voltage at 60 Hz. AND if you measured from DC positive and ground, you'll also measure a similar AC voltage. I've seen from mV to ~65 Vac @ 60 Hz. BTW: this common mode voltage is not included in the typical "ripple" measurement.

Now place the DMM in the AC current and connect the leads in a similar fashion. Wait, won't that be a ton of current?

Well no. It's a relatively high impedance across the transformer barrier.

There are various standards on the maximum allowable leakage current, but all are going to be around 3.5 mA. Unless it's a medical grade, those are less than 0.5 mA.

Dealing with the effects of that common mode voltage/current has it's various trade-offs.
One can directly tie the output return to (either) AC neutral or ground. But then it's not floating/isolation.
Input EMI filtering can actually increase the leakage current, specifically where typically Y-Class safety capacitors are placed.
So, there is not really one 'correct' solution.

When you connect a device to the power (and it's not connected to anything else), it's now going to have that same common mode voltage/current. So if you measured 55 Vac before and now power the amp with it (leaving the inputs/outputs not connected to other components), then measured from say, the negative input terminal of your amp to earth ground what do you get? Probably a little bit less.

If you plugged in a pure battery sourced signal into the amp (say a self-powered USB DAQ connected to your phone), they'll all be floating up and down at 60 Hz. If you connect a speaker to the output of the amp, it'll probably sound fine as well.

If you added say a pre-amp or a DSP that is also powered off the same Meanwell, well again, they're all floating up and down together.

However, as soon as you connect any device which has it's own AC/DC power supply, or a path to earth ground .. well, now instead of simply all floating up and down with a (relatively harmless) common mode voltage; it'll now have a much lower impedance path back to ground. So it'll generate the full 60Hz leakage current flowing from one end to another.

Let's say that path flows over the RCA cable into the amp.

Won't it only flow on the shield? Well, no. As it's common mode, not differential.
The current will be semi-split between the two conductors, but uneven.

Now your amp is amplifying it.

Okay, nice story. I wasn't going to leave you hanging without a solution.

Well, there isn't a single solution in all cases.

I would first try typing the power supply's DC return to the DC return of what ever is powering the other devices.

That may or may not be possible.

You could also try tying the DC return to Earth ground. Sometimes that helps, some times it doesn't.

If not, given your signal chain isolating the USB connection may be the best alternative.







btw: simply because the IEC 320-C14 receptacle has the 'ground pin' does not mean it can't be a Class II. It may simply not be connected to anything on the inside. It's one less part they have to deal with.