A downside to a toroid transformer is the possibility of a surge if the power happens to be applied at the zero crossing point of the AC mains.
One solution is to use a thermistor in series with one leg of the primary. This is effective, but it requires that knowing when the amplifier is turned off the thermistor will have to cool before the unit is turned back on. That's easy enough to keep in mind, but what about brownouts? These interruptions are too short to allow a thermistor to cool, so their intended purpose in this application is negated.
One possible solution is to employ an UPS (Uninterruptible Power Supply). Thoughts? Any other solutions? (Yes, i'm aware an EI transformer could be employed instead.)
One solution is to use a thermistor in series with one leg of the primary. This is effective, but it requires that knowing when the amplifier is turned off the thermistor will have to cool before the unit is turned back on. That's easy enough to keep in mind, but what about brownouts? These interruptions are too short to allow a thermistor to cool, so their intended purpose in this application is negated.
One possible solution is to employ an UPS (Uninterruptible Power Supply). Thoughts? Any other solutions? (Yes, i'm aware an EI transformer could be employed instead.)
Submitted for your consideration:
- Soft Start Circuits For High Inrush Loads (Rod Elliot)
- Soft-Start Circuit For Power Amps (Inrush Current Limiter) (Also Rod Elliot)
- The Ultimate Guide to Soft Start Design (@tomchr )
I don't worry or obsess over any possible "surges", or zero crossing issues.
Manufacturers have made products for decades without such issues, and have been fine.
Manufacturers have made products for decades without such issues, and have been fine.
How much of an issue is the zero crossing anyway? It's a core hysteresis that would lag until no more than 0.1T. Hypothetically even if permeability at this point is 20x less resulting into 20x less inductance of power transformer as well, this cannot compare to the instantaneous short-circuit equivalent of torroidal transformers charging high value input capacitor filters, used as classical power supplies for most modern SS amplifiers. This + the combined low Rdc and low leakage compared to high leakage split-bobbin EI trannies is making fuses pop.
I use a low-ohm power resistor in series with the line and the power input connection. The resistor gets shorted out by a time delay relay that quickly resets its timeout in the event of a rapid on-off sequence.
I used to use a resistor on a TD relay for inrush but it was only necessary when trying to get SMPS to start into empty capacitors.
With transformers, I use no inrush limiting whatsoever. A 625VA coil has never blown a 3A slo-blow fuse in start up.
I once has a toroidal powered amplifier that would blow the fuse on power once in 200 times or so. I increased the fuse by ½A and it hasn't blown since.
With transformers, I use no inrush limiting whatsoever. A 625VA coil has never blown a 3A slo-blow fuse in start up.
I once has a toroidal powered amplifier that would blow the fuse on power once in 200 times or so. I increased the fuse by ½A and it hasn't blown since.
No surge possible at zero crossing, "0" as in "zero" volts can´t push any current by definition.
If anything, worry about switching on at peak voltage trying to charge empty capacitors.
Anyone can wind a toroid if you buy the machine 🙂
Personally, I prefer toroidal. They don't hum, they are smaller, they cost less, and they work better for me 🙂
I buy E/I when I can't get a toroid though. Usually Hammond in that case - I have a tube preamp that uses a 252VA 22VCT transformer to make linear regulated 12V to drive DC-DC converters for the HV required and the heaters. I had to glue damping material to it to lower the audible hum/buzz.
Personally, I prefer toroidal. They don't hum, they are smaller, they cost less, and they work better for me 🙂
I buy E/I when I can't get a toroid though. Usually Hammond in that case - I have a tube preamp that uses a 252VA 22VCT transformer to make linear regulated 12V to drive DC-DC converters for the HV required and the heaters. I had to glue damping material to it to lower the audible hum/buzz.
You can wind toroids with a hand shuttle fairly easily. I’ve never would one completely from scratch though - most of my toroid winds are either adding extra secondaries to existing ones, or pulling the secondary I don‘t need and replacing it with one I do. The rest have been the kits from toroid.com that have pre-wound primaries. Many of my EI builds have used the existing primary from a donor core, but often I find I want to rewind an EI with more turns per volt to get the flux density (and the mechanical noise) down from what it was. Then it gets a new primary. Easy as pi when winding on a plastic bobbin.
When buying new power transformers, I go straight to Antek first. EI’s cost more unless they can be found surplus.
When buying new power transformers, I go straight to Antek first. EI’s cost more unless they can be found surplus.
There's a lot of discussion around safety, and rightly so unless you have a Megger/isolation tester. Here's a specification by Toroid International that hints at the detail - this is low power and low voltage secondary as an example:
Source: https://www.farnell.com/datasheets/17046.pdf
Going back to the OP's post.
The MF A220 has as 350VA toroid and no inrush control, yet it's fine and with about 50,000uF of caps in the amp. Only when the caps were old and leaky did to "Buzzz" on switch on. Now after the recap it's silent, even on power on. That's through a 6A line filter too.
I have a Soft Start + DC blocked here
https://hifisonix.com/soft-start-dc-blocker-for-mains/
I use the thermistor + relay bypass technique on all my amps to overcome the inrush current problem. Once you go over 300 or 400 VA you start to run into problems and there is no way you can power a 1 kVA up without some sort of inrush control.
https://hifisonix.com/soft-start-dc-blocker-for-mains/
I use the thermistor + relay bypass technique on all my amps to overcome the inrush current problem. Once you go over 300 or 400 VA you start to run into problems and there is no way you can power a 1 kVA up without some sort of inrush control.
Saturation at turn on is just because momentarily there is a DC component, exponentially decaying with a long time constant (Many cycles at 60 Hz). It can be quite large, but so can the surge from empty caps charging for the first time. The DC-related surge doesn’t go through the rectifier, but the cap-related surge does. It’s more likely to hurt the rectifier than it is the transformer. Especially if you buy into using fast diodes - they generally can’t take anywhere the beating that ordinary ones can.