Indeed it is, just so people have an informed choice about whose opinion to go with my figure is 3.
Sorry Typo error, Should have been BR-100.
Well tell me If I have wrongly understood Table 1 Maximum Power Supply Voltages. from AN-1192
No, you haven't wrongly understood it, I think the authors wrote it wrong as they say elsewhere on that page:
National’s Overture power
amplifier series amplifiers will give the most output power
and power dissipation will be kept within limits when each
amplifier sees a load impedance of 4Ω – 8Ω.
So to me, that table one of the many misleading things in the document - they don't seem to have based the table on the effective loading seen by each chip.
With the +/-20V supplies and a bridged configuration, you'd get a swing of about 34V. A 4R load would not exceed the typical current limit of the chip (34/4 = 8.5A) so its really a question of how well the configuration makes use of the thermal capabilities of the chip. According to their own words, a 2R load doesn't get the best from the chip thermally - and they didn't say that based on the distortion (which goes up quite a lot into 2R). National provide no data in the datasheet about 2R performance, so you're going to be sucking it and seeing...
National’s Overture power
amplifier series amplifiers will give the most output power
and power dissipation will be kept within limits when each
amplifier sees a load impedance of 4Ω – 8Ω.
So to me, that table one of the many misleading things in the document - they don't seem to have based the table on the effective loading seen by each chip.
With the +/-20V supplies and a bridged configuration, you'd get a swing of about 34V. A 4R load would not exceed the typical current limit of the chip (34/4 = 8.5A) so its really a question of how well the configuration makes use of the thermal capabilities of the chip. According to their own words, a 2R load doesn't get the best from the chip thermally - and they didn't say that based on the distortion (which goes up quite a lot into 2R). National provide no data in the datasheet about 2R performance, so you're going to be sucking it and seeing...
I have checked their calculations in 4.3 and agree. So with the effective 2R load on each amp, we'll get 40W dissipation in each and a maximum 110W output.
We need to keep the die temperature below 150oC - this rules out the isolated package as that's 2oC/W theta-jc. 40W can only be handled by the non-isolated variant (LM3886T) which is 1oC/W. At 40W, the case will be at 110oC and, assuming you go for a silicone filled washer, 90oC at the heatsink surface underneath. You say your ambient might be 50oC so your heatsink would need to be specified at 0.5oC/W (you're dissipating 2 * 40W).
This specification is not really representative because an amp does not have its maximum dissipation at full power, so its unwise to specify the amp based on the chip dissipation at full power. Look at the curves on p14 of the LM3886 datasheet titled 'Power Dissipation vs Output Power'. Naturally enough, they don't show one for 2R load....
Notice that the efficiency here is 110/190 = 58% whereas theoretically a class B amp can achieve 78%. This loss is in the main due to the fact the LM3886 can't swing closer than 5V from the rails with its punishing 2R load. Adopting a different chip/topology would definitely save money on aluminium for heatsinking.
We need to keep the die temperature below 150oC - this rules out the isolated package as that's 2oC/W theta-jc. 40W can only be handled by the non-isolated variant (LM3886T) which is 1oC/W. At 40W, the case will be at 110oC and, assuming you go for a silicone filled washer, 90oC at the heatsink surface underneath. You say your ambient might be 50oC so your heatsink would need to be specified at 0.5oC/W (you're dissipating 2 * 40W).
This specification is not really representative because an amp does not have its maximum dissipation at full power, so its unwise to specify the amp based on the chip dissipation at full power. Look at the curves on p14 of the LM3886 datasheet titled 'Power Dissipation vs Output Power'. Naturally enough, they don't show one for 2R load....
Notice that the efficiency here is 110/190 = 58% whereas theoretically a class B amp can achieve 78%. This loss is in the main due to the fact the LM3886 can't swing closer than 5V from the rails with its punishing 2R load. Adopting a different chip/topology would definitely save money on aluminium for heatsinking.
is this a different way of saying you agree this time, whereas at first you insisted I was giving bad advice.
I am going to be headache for you guys. ( I owe you pain killers. ) 
Actual problem is, I am trying for amp and its power supply that would fit in sub-woofer enclosure as like a plate amp. So to keep size and cost of power supply small (Savings of transformer cost and costly filter caps), without compromising its required power rating,I was trying all this acrobats.
Further I have plans to ask you if you know how to calculate heat sink requirement with forced air cooled.
Sorry If I am really that bad.
Actual problem is, I am trying for amp and its power supply that would fit in sub-woofer enclosure as like a plate amp. So to keep size and cost of power supply small (Savings of transformer cost and costly filter caps), without compromising its required power rating,I was trying all this acrobats.
Further I have plans to ask you if you know how to calculate heat sink requirement with forced air cooled.
Sorry If I am really that bad.
No, its not agreeing with you about '14A - 22A peak' rather agreeing with the calculations for the power output and dissipation for the bridged amp into 4R in AN-1192.
I don't say your advice is bad, rather its ill-founded in this particular instance - that of peak current requirements into a woofer.
Further I have plans to ask you if you know how to calculate heat sink requirement with forced air cooled.
Sorry If I am really that bad.
I have no experience of forced air cooling amps, so I'll leave that calculation to others
Are you really going to put a fan in a subwoofer?Yes, Unfortunately if I had to make choice. fan will get down size of heatsink. While I googled I found the size of heatsink drops by 60% to 70%. I wonder how market Plate amps fit continues 100W in to such a small Plate amp modulewithout using ClassD solution.
I would mount fan ( like that found in computer smps) on heatsink. and the heatsink sill be popped out from back side like that of any other plate amp.
also if i get a suggestion about complete design, that would fit like Plate amp in my subwoofer enclosure. I would built it. But the component has to be easy available. low cost and small form factor. I would be happy with that.
I would mount fan ( like that found in computer smps) on heatsink. and the heatsink sill be popped out from back side like that of any other plate amp.
also if i get a suggestion about complete design, that would fit like Plate amp in my subwoofer enclosure. I would built it. But the component has to be easy available. low cost and small form factor.
I would be happy with that.
Let's see, you have access to a few laptop supplies (say, Rs. 250/$5 each) and a 4 ohm subwoofer load. For the cost of those supplies you could build a linear supply with a transformer, two honkingly large capacitors and a bridge rectifier so I don't see how your way solves any cost issues. The transformer would be an EI and the caps would be of second0rung brands, but it would still be a heap better than those noisy little laptop supplies with no ability for any kind of current delivery.
Remember music has a very high peak to continuous ratio, PCs and laptops are more or less continuous loads and ramp up is relatively slow whereas audio needs very high instantaneous peak current delivery. The supplies you have in mind may be suitable for small desktop speakers or suchlike, definitely not a subwoofer of any mettle.
Now as to the load itself, a couple of LM3886 or 7294s in parallel mode will be the way to go. Resistor matching is an issue you have to take care of and idle dissipation will be a little high if not perfectly matched but once the system is running the differences are swamped by the total dissipation anyway...
The fan is an unfortunate choice. Two 3886 chips work fine on a small 8-10" x 5" profile without needing forced cooling, and it will cost less than a fan and heatsink combination like you're thinking. A quiet fan is not cheap, whereas extrusions are easily available and cheap as dirt. The whole thing with a power supply should be well within the Rs. 1500/$30 mark including supplies and a sink, I'm not sure how you're planning to optimise the cost further.
Remember music has a very high peak to continuous ratio, PCs and laptops are more or less continuous loads and ramp up is relatively slow whereas audio needs very high instantaneous peak current delivery. The supplies you have in mind may be suitable for small desktop speakers or suchlike, definitely not a subwoofer of any mettle.
Now as to the load itself, a couple of LM3886 or 7294s in parallel mode will be the way to go. Resistor matching is an issue you have to take care of and idle dissipation will be a little high if not perfectly matched but once the system is running the differences are swamped by the total dissipation anyway...
The fan is an unfortunate choice. Two 3886 chips work fine on a small 8-10" x 5" profile without needing forced cooling, and it will cost less than a fan and heatsink combination like you're thinking. A quiet fan is not cheap, whereas extrusions are easily available and cheap as dirt. The whole thing with a power supply should be well within the Rs. 1500/$30 mark including supplies and a sink, I'm not sure how you're planning to optimise the cost further.
Laptop Supply costs me here @ Pune 300/- each so 600/- for 2. filter cap 470uF x 8 = 56/-
70VA transformer CRGO costs me 800/- ( without taxes) filter caps 4700/50V = 100/- each
I use 4. so the total cost is 1200/- on power supply. but on laptop smps its say 660/-
well forget the cost for some time. transformer size is a constrain too. Actually this is the first time I am using any smps for audio. I too believe in transformer supply.
Friend, I remind you again. think this as a whole unit that would go as a plate amp. so the size of transformer.
BTW you are from India, SO do u know any local plate amp mfg. or source to buy.
70VA transformer CRGO costs me 800/- ( without taxes) filter caps 4700/50V = 100/- each
I use 4. so the total cost is 1200/- on power supply. but on laptop smps its say 660/-
well forget the cost for some time. transformer size is a constrain too. Actually this is the first time I am using any smps for audio. I too believe in transformer supply.
Friend, I remind you again. think this as a whole unit that would go as a plate amp. so the size of transformer.
BTW you are from India, SO do u know any local plate amp mfg. or source to buy.
Nope, I
1) don't use a subwoofer
2) build all my amps on my own
3) think that you're paying about 600 rupees too much for the transformer, and about twice of what you should on the caps.
I buy a 250VA EI (real capacity is a little less, say 150VA) for about Rs 300 and 10kuF/50V caps for under Rs. 100 each. Both in Calcutta and Mumbai. It just takes a bit of patience to hunt the right stores. You could put together an entire power supply for around the same price as those power adapters, and the 4x4700uF caps in your example (or two 10kuF that I use) will be capable of a lot more peak current than those puny little laptop supplies.
You need a trip to Mumbai, however, that will cost extra. I can't help you about the size. It is a plate amp, and inside a subwoofer, so I don't really know how size is relevant. A 3" section is enough to accommodate a regular EI. The difference between a 2" toroid or laptop supply and a EI is just an inch or so. I don't know what's more important - one or two inches, or a seriously compromised electronic device. The heatsink can be mounted outside the plate, with the devices mounted flat to it through an appropriate cutout.
Anyway, it's your call. Good Luck, I do hope you find what you're looking for.
1) don't use a subwoofer
2) build all my amps on my own
3) think that you're paying about 600 rupees too much for the transformer, and about twice of what you should on the caps.
I buy a 250VA EI (real capacity is a little less, say 150VA) for about Rs 300 and 10kuF/50V caps for under Rs. 100 each. Both in Calcutta and Mumbai. It just takes a bit of patience to hunt the right stores. You could put together an entire power supply for around the same price as those power adapters, and the 4x4700uF caps in your example (or two 10kuF that I use) will be capable of a lot more peak current than those puny little laptop supplies.
You need a trip to Mumbai, however, that will cost extra. I can't help you about the size. It is a plate amp, and inside a subwoofer, so I don't really know how size is relevant. A 3" section is enough to accommodate a regular EI. The difference between a 2" toroid or laptop supply and a EI is just an inch or so. I don't know what's more important - one or two inches, or a seriously compromised electronic device. The heatsink can be mounted outside the plate, with the devices mounted flat to it through an appropriate cutout.
Anyway, it's your call. Good Luck, I do hope you find what you're looking for.
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