I got it from eBay. Cheap and simple but works as intended. AC 150V-280V 100A 4000W Power Supply Delay Soft Start Board Fr Class A Amplifier | eBay
That's a good question. I don't have a firm answer.
I don't think there's a need for a soft start. If there was, Mean Well would have built one in. That said, the inrush can be substantial, so the life of your power switch may be affected if you switch the mains directly.
If you use the low-voltage power-on function of the Mean Well switchers, you will not have any issues with the inrush current. You'll get some when you plug the amp in the first time, just as you do with your laptop charger, but after that everything is handled electronically.
Tom
I don't think there's a need for a soft start. If there was, Mean Well would have built one in. That said, the inrush can be substantial, so the life of your power switch may be affected if you switch the mains directly.
If you use the low-voltage power-on function of the Mean Well switchers, you will not have any issues with the inrush current. You'll get some when you plug the amp in the first time, just as you do with your laptop charger, but after that everything is handled electronically.
Tom
Be careful with those eBay soft starts. There are two things that pop out at me immediately when viewing the eBay listing linked to above:
E = 0.5*C*V2
For the Power-686, I'm using four 22000 uF capacitors in total. Assuming ±36 V rails, that's two 2x22000 uF capacitor banks charged to 36 V:
E = 0.5*2*2*22000E-6*362 = 57 Joule.
Thus, if the inrush limiter should survive startup, it must be able to handle this energy. Something like the Amphenol CL-90 commonly mentioned on this forum can handle 36 J. As an exercise in data sheet critique, I suggest reading the Chinese "data sheet" attached and compare & contrast with the Amphenol data sheet for the CL-90 (and others).
Now, nobody says the inrush limiter has to be along for the entire charge cycle. One could short it out with a relay once the supply voltage exceeds a certain value. That would limit the 'V' in the equations above. With the square term, reducing 'V' drastically reduces 'E'.
Regarding the second point. I understand that driving inrush limiters directly from the mains is a common practice. Rod Elliott does the same in his inrush limiter circuit. I'm not a fan of that practice. I'm just not. I much prefer to use low-voltage control.
So why does the inrush limiter in the eBay circuit survive (at least so far)? Probably because the relay kicks in pretty quickly and shorts out the inrush limiter. The inrush current limiter is mostly needed for the first few mains cycles until the magnetizing current has been established in the power transformer and the supply caps have started to collect some charge.
One could probably extend the life of the supply caps by allowing the inrush limiter to work until the capacitors have reached, say, 2/3 of the full rail voltage. That'd limit the inrush current into the capacitors.
Tom
- The NTC inrush limiter has no specification regarding its maximum energy handling capability. I've attached the spec sheet for the inrush limiter.
- The circuit is driven directly from the mains. No isolation transformer.
E = 0.5*C*V2
For the Power-686, I'm using four 22000 uF capacitors in total. Assuming ±36 V rails, that's two 2x22000 uF capacitor banks charged to 36 V:
E = 0.5*2*2*22000E-6*362 = 57 Joule.
Thus, if the inrush limiter should survive startup, it must be able to handle this energy. Something like the Amphenol CL-90 commonly mentioned on this forum can handle 36 J. As an exercise in data sheet critique, I suggest reading the Chinese "data sheet" attached and compare & contrast with the Amphenol data sheet for the CL-90 (and others).
Now, nobody says the inrush limiter has to be along for the entire charge cycle. One could short it out with a relay once the supply voltage exceeds a certain value. That would limit the 'V' in the equations above. With the square term, reducing 'V' drastically reduces 'E'.
Regarding the second point. I understand that driving inrush limiters directly from the mains is a common practice. Rod Elliott does the same in his inrush limiter circuit. I'm not a fan of that practice. I'm just not. I much prefer to use low-voltage control.
So why does the inrush limiter in the eBay circuit survive (at least so far)? Probably because the relay kicks in pretty quickly and shorts out the inrush limiter. The inrush current limiter is mostly needed for the first few mains cycles until the magnetizing current has been established in the power transformer and the supply caps have started to collect some charge.
One could probably extend the life of the supply caps by allowing the inrush limiter to work until the capacitors have reached, say, 2/3 of the full rail voltage. That'd limit the inrush current into the capacitors.
Tom
Tom, thanks for the information re soft starts and NTC inrush limiters. I agree its not an ideal solution and also share your concerns. you are right that the only reason it survives is that the relay kicks in quickly (After about a second).
In its defence it has so far worked fine, and was a quick and easy solution to get the amp up and running, but I feel it would benefit from a slightly longer delay and reading the info you've provided a more suitable NTC. I've just found this web site with lots of info and on line calculators that i'm about to have a play with, and which also might be useful to other readers. How To Select an Inrush Current Limiter | Ametherm
I will probably have a look round for a safer more suitable solution in the near future.
Dave.
In its defence it has so far worked fine, and was a quick and easy solution to get the amp up and running, but I feel it would benefit from a slightly longer delay and reading the info you've provided a more suitable NTC. I've just found this web site with lots of info and on line calculators that i'm about to have a play with, and which also might be useful to other readers. How To Select an Inrush Current Limiter | Ametherm
I will probably have a look round for a safer more suitable solution in the near future.
Dave.
That's an excellent reference. In particular the app note on transformer inrush current is applicable to the Modulus-686 and Power-686.
It's worth noting that if the power transformer is turned on at the peak of the AC waveform, the transformer inrush current is zero (due to the phase relationship between voltage and current in an inductor). I suspect the smoothing caps will still cause a significant inrush current, but at least the inrush from the transformer can be avoided by timing the turn-on right.
This also means that if you are using a solid state relay to turn on the power, you'll want one that does NOT feature zero-crossing detection.
Tom
It's worth noting that if the power transformer is turned on at the peak of the AC waveform, the transformer inrush current is zero (due to the phase relationship between voltage and current in an inductor). I suspect the smoothing caps will still cause a significant inrush current, but at least the inrush from the transformer can be avoided by timing the turn-on right.
This also means that if you are using a solid state relay to turn on the power, you'll want one that does NOT feature zero-crossing detection.
Tom
No matter when you turn on the transformer, the INITIAL current is zero. Yes, the inrush current -- I.E., the peak current you reach as it 'gets going' -- is different, but the whole point of the NTC is to limit that peak. If you want a defined turn-on point, then zero is your best choice without a lot of added circuitry. With relays, you shouldn't even try to time the turn-on as you likely won't succeed. Triacs/SSRs can be made to reliably and predictably turn off at the zero voltage crossing and they will always turn off at the zero current crossing.
I do agree that the NTC can't be avoided if you want to control the inrush current. I also agree that trying to do anything in the direction of precision timing using a relay will be an exercise in futility.
In an inductor, the current leads the voltage by 90º, i.e. the current is 0 when the voltage is at its peak. So if the mains switch closes exactly when the voltage hits the peak, you should only see the no-load current (magnetizing current). That's also what Ametherm says.
The remaining question is what happens when the transformer is loaded with the supply capacitor bank. That charge will have to come from somewhere. Sadly, Ametherm doesn't cover that in their guide. It may be worth a SPICE simulation or two to tease that apart.
The main take-away here is: Don't switch at the zero crossing.
Tom
In an inductor, the current leads the voltage by 90º, i.e. the current is 0 when the voltage is at its peak. So if the mains switch closes exactly when the voltage hits the peak, you should only see the no-load current (magnetizing current). That's also what Ametherm says.
The remaining question is what happens when the transformer is loaded with the supply capacitor bank. That charge will have to come from somewhere. Sadly, Ametherm doesn't cover that in their guide. It may be worth a SPICE simulation or two to tease that apart.
The main take-away here is: Don't switch at the zero crossing.
Tom
Tradeoffs, tradeoffs. TRIACs and other forms of solid state relays are great for handling the inrush current. Relays tend to wear from the inrush but their longevity doesn't seem impaired by the steady state current flow through the contacts. I'll find a good solution, I'm sure. I just find it interesting how much one can choose to geek out over a switch.
Tom
Tom
Rofl, I had enough inrush to dim the neighbors lights when I lit my old stack up at once.
It was annoying. I'm glad to have moved onto amps that behave much better.
From what I understand the transformer growl is caused by the mechanical vibration from the transformer primary winding. Over time, it will cause the primary to wear through the insulation and short to the core. When that happens, the transformer is done and will need replacement. It might take a decade or two for this to happen in residential hifi setups, but it'll happen eventually.
Limiting the transformer inrush is generally a good idea.
Tom
Limiting the transformer inrush is generally a good idea.
Tom
Mounting brackets
I chose to buy Tom's mounting brackets. I've previously built the Parallel 86 amp so I know that the trickiest part of the build is mounting the ICs to the heatsink and getting the PCB stand offs in exactly the right position. The brackets may seem expensive for what they are but believe me they make mounting the ICs an absolute breeze. Worth every penny for that reason.
I chose to buy Tom's mounting brackets. I've previously built the Parallel 86 amp so I know that the trickiest part of the build is mounting the ICs to the heatsink and getting the PCB stand offs in exactly the right position. The brackets may seem expensive for what they are but believe me they make mounting the ICs an absolute breeze. Worth every penny for that reason.
I've made my own enclosures for my 686 monoblocks. The black anodised heatsinks are in tight physical contact with the aluminium back panel but my DMM does not detect any electrical continuity between them - the black anodised finish has obviously created an electrical insulation layer. I presume I should defeat this by sanding a small area and putting a jumper across to the back panel?
