Toroidal transformer for L30 / ATOM amps - what to keep in mind?

[salmo]

Hi everyone! In this week I tried to use toroidal 15V (20VA) transformer in place of huge brick from Topping L30. To be hones I'm really surprised with results! Traditional power supply for Topping or Atom amp sips ~3W in total idle, they also very hot after some time when amp is in use. Mentioned toroidal on contrary sips only 0,5W in idle and even after few hours of use with amp is only sliiiiightly warm or almost not.

I know that with toroid I have some disadvantages - coupling capacitance between primary & secondary is higher and also it is not that forgiving for DC components from mains - and here are my questions:
1. Should I use some filtration or DC blocker board on mains input to transformer?
2. Could it somehow degrade audio quality in comarison to traditional power supply?
3. What is... the worst case scenario if I use toroid in place of included power supply for mentioned amps? Only audible hums if something went wrong with mains?

 

[AnalogSteph]

1. On a 20 VA transformer, primary resistance may not be low enough for DC to be a real problem yet. Keep an eye (and ear) on it in use, I guess. If you need a DC offset generator, try a hairdryer with adjustable heat output on the same circuit. (Obvious problem, they're kind of noisy.) If you do notice transformer buzz issues, a DC blocker circuit should get it sorted, and you obviously don't need anywhere near as much capacitance in that as for a 500-1000 VA transformer.
2. Best test for parasitic leakage: Observe hum / buzz levels observed when touching the plug on an audio cable plugged into the amp. That is likely to increase a bit, if nowhere near the level of what you tend to get with a SMPS.
If you want to keep this issue to a minimum, use a 3-conductor means lead and connect secondary-side ground to a ca. 4.7-10 nF Y class (!) capacitor and that to PE (which otherwise is not used for anything else, unless your power supply is built in a metal enclosure obviously - with this little heat to get rid of, plastic would be clearly preferred). Then parasitic leakage and the Y capacitor will form a capacitive voltage divider, and since the former tends to be maybe in the hundreds of pF the offending voltage will be substantially attenuated. (That is assuming you're not in "ungrounded outlet" territory.)

BTW, there is no fundamental reason why a conventional transformer EI couldn't be more efficient, and the stock transformers in question do perform rather below average. (There may not be too many viable options for OEMs left as linear supplies are not too common any more.) Typical reasons for excessive losses tend to be (a) substandard core construction that allows for substantial eddy currents, along with (b) slightly running the transformer into saturation at nominal operating voltage by underwinding it, so that winding resistance is reduced and some more power can be squeezed out.

Either way, your findings are quite encouraging. 0.5 W sounds a lot more in line with EU ecodesign guidelines. Would you mind sharing what brand / model transformer you used?

 

[Old_School_Brad]

Hi everyone! In this week I tried to use toroidal 15V (20VA) transformer in place of huge brick from Topping L30. To be hones I'm really surprised with results! Traditional power supply for Topping or Atom amp sips ~3W in total idle, they also very hot after some time when amp is in use. Mentioned toroidal on contrary sips only 0,5W in idle and even after few hours of use with amp is only sliiiiightly warm or almost not.

I know that with toroid I have some disadvantages - coupling capacitance between primary & secondary is higher and also it is not that forgiving for DC components from mains - and here are my questions:
1. Should I use some filtration or DC blocker board on mains input to transformer?
2. Could it somehow degrade audio quality in comarison to traditional power supply?
3. What is... the worst case scenario if I use toroid in place of included power supply for mentioned amps? Only audible hums if something went wrong with mains?

1. No -not unless you're experiencing issues.
2. No. Not unless there's a problem.
3. If everything's working fine, there's really no worst-case scenario to worry about.

 

[salmo]

Thanks guys!

Would you mind sharing what brand / model transformer you used?

It's talema transformer, probably produced in Germany? But primary/secondary colors are more like polish? ;D

 

[AnalogSteph]

So unsurprisingly, a somewhat renowned / quality manufacturer.

From the catalog:

56xxx seems to be their series for halogen lamps. Must have been a custom job.
Manufaturing locations seem to be in the Czech Republic and India (so your guess with Poland wasn't too far off).

I still don't get why idle losses are scaling so poorly with power rating (it seems to be a general thing with transformers), but I'll take 0.46 VA for a 15 VA unit. With numbers like that, I'm not surprised that power amplifier manufacturers like toroidals. If you have a transformer of a few hundred VA with reasonably high secondaries (and Schottky rectifier diodes on the lower ones), you can get SMPS-grade efficiency with linear power supply complexity and robustness.