AudioFreak said:
True about the fans, but they don't help the transformer much. The fact is the Crown probably has a 13 pound 800 VA or a 15 pound 1000VA transformer for 900 watts out. And it's a safe bet the transformer would survive without a fan (the output transistors certainly would not) especially for less demanding use.
The rule of thumb I've seen most often for sizing transformers is 1 kg (2.2 pounds) per 100 watts output into 8 ohms. Many amps use smaller transformers than that. No fans required
I agree about the 3db. I pointed that out earlier in the thread.
The other thing is, darkwind_j said he wants to drive 2 pairs of speakers with this amp. So, at best, it's likely a 4 ohm load.
What's not clear is what the ultimate application is. Is it for home use? Pro use? Are there 2 channels? Just 1 channel? What's the total budget for parts?
600VA for a 400W class AB amplifier sounds about right for 400W output to me too. It is enough for 1/3 average power program material at least. Now, 1/3 average power is quite heavily compressed stuff and IMO almost unlistenable. It should usually be enough.
It won't do full power sine continuously with such a transformer though. The amplifier will require about 600W from the power supply doing that, and the power factor of a capacitor filtered power supply is about 0.6-0.7 or so, needing about 1000VA from the transformer.
However, transformers contain a lot of metal and take a long time to heat up. The 600VA transformer will probably take that overload for 30 minutes without complaining if starting out cold, leaving lots of time for testing with full power sinewave.
What it is that the original poster needs or wants is a bit unclear though. He says the two speakers are "400W", but not if this is a program or RMS rating. What is the impedance?
I'd select amplifier power based on required SPL and speaker efficency instead.
It won't do full power sine continuously with such a transformer though. The amplifier will require about 600W from the power supply doing that, and the power factor of a capacitor filtered power supply is about 0.6-0.7 or so, needing about 1000VA from the transformer.
However, transformers contain a lot of metal and take a long time to heat up. The 600VA transformer will probably take that overload for 30 minutes without complaining if starting out cold, leaving lots of time for testing with full power sinewave.
What it is that the original poster needs or wants is a bit unclear though. He says the two speakers are "400W", but not if this is a program or RMS rating. What is the impedance?
I'd select amplifier power based on required SPL and speaker efficency instead.
nigelwright7557 said:
Bipolar output transistors don't really have linear "resistance" (although MOSFETs do have a tiny bit). The losses in either type of output stage are mostly related to the waveform used. With a square wave at full power Class B amps are extremely efficient. It gets a bit complex to calculate efficiency using music, with power supply sag, idle current, reactive loads, etc. without making some reasonable assumptions.
In theory, using sine wave signals, Class B output stage efficiency can approach 78%. In practice, with idle current and other losses, it's typically around 70%.
megajocke said:
True. I should have said 600W DC and a 400 watt sine wave output would eventually overheat a 600VA transformer.
Crown does a nice job of publishing amplifier efficiency data for various kinds of program material. They have thermal dissipation datasheets for many of their amps. They use a power factor of 0.83 but that's for a balanced bridged design. Here's an example:
Crown Thermal Data
I typically assume 0.7 in my design work. So 850 VA would be the minimum for a transformer to run forever at full power. For an excellent discussion of the power factor, see megajocke's excellent comments in the PSU Capacitor Design thread.
i am talking E-I here, not torroid, besides, torroids are still scarce in manila, they are not standard of the shelf items in most stores...if i get it right, darkwind will use a costum job traffo, i am merely giving him a tip as to quality of transformer to watch out for...
cores being sold in manila, even if brand new are known to like a lot of turns to get the temperature rise to reasonable levels...besides ambient temperature can reach into the 30's celsius during summer....giving a lot more leeway is imho wise thing to do...
once opun a time i asked one of the "gurus" here as to the optimum size of the power transformer to use......
and the response was, "get the biggest sized transformer that will fit your box"....
hey, this is DIY, not a commercial thing, we are not constrained to follow commercial designs......
darkwind wants an amp, he is unlikely to sell them once he had made them....he will use them for PA work most likely and earn some pizza money......
and the response was, "get the biggest sized transformer that will fit your box"....
hey, this is DIY, not a commercial thing, we are not constrained to follow commercial designs......
darkwind wants an amp, he is unlikely to sell them once he had made them....he will use them for PA work most likely and earn some pizza money......
Tony said:
Yes and he also has said he does not have much money to spend. So, it is bad advice to tell him he needs a much larger transformer than he really needs. A 30 pound transformer--toroidal or EI--would be around 1500 VA - 2000 VA. That is a big waste of money for a 400 watt amp.
I was just trying to save darkwind_j some money if he does decide to buy a new transformer.
to give you an idea, a toroid with this spec:
"transformer,toroidal,625VA,2x40V,7.81A" from RS component Philipines costs, 7,057 pesos, http://philippines.rs-online.com/web/search/searchBrowseAction.html?method=getProduct&R=4209251
now an EI of same rating, will probably cost darkwind some 3000pesos only.....
now talk about saving money eh?
that can't be right.RocketScientist said:
160VA for a 100W single channel weighs more than 1kg.
300VA for a 100+100W two channel weighs more than 2kg.
The rule I have seen and used is
VA = 100% to 200% of total watts.
Weight does not come into the equation.
I tend towards the upper end of that equation 150 to 200VA for each 100W.
An old rusted out microwave oven on the large size has a fairly heavy chunk of tranny iron in it. I know you can salvage one of these for almost nothing. Then rewind it. I have at least ten of them in my junk pile waiting for a good project. I got all of them for free on trash pickup day at the curb. There are some other goodies that can be salvaged like fans and mosfets and small heatsinks, relays and on/off switches.
PLEASE: Do not use it like it is. Some have 2000 volts on the secondaries.
A really good secondary result of salvaging is the knowledge you gain in the process. At lot of members here will help out as you have found.
I alway thought that the Phillipines close proximity to China, Malaysia made for cheaper and more abundant parts. I for one living in the States wish I had access to more of the Chinese diy items.
Tad
PLEASE: Do not use it like it is. Some have 2000 volts on the secondaries.
A really good secondary result of salvaging is the knowledge you gain in the process. At lot of members here will help out as you have found.
I alway thought that the Phillipines close proximity to China, Malaysia made for cheaper and more abundant parts. I for one living in the States wish I had access to more of the Chinese diy items.
Tad
38+38Vac @ 8Aac is 608VA.
That is the recommended transformer for amplifiers of 300W to 600W total maximum output.
But 38+38Vac does not easily suit 400W into 8r0.
you need a 60+60Vac or 65+65Vac transformer to get sufficient DC supply voltage (about +-90Vdc to +-95Vdc) to give the 80Vpk output that your 8r0 load will need for 400W.
This is a very expensive target to meet.
Reduce your ambitions until you have accumulated the knowledge to make all these decisions for your self.
That is the recommended transformer for amplifiers of 300W to 600W total maximum output.
But 38+38Vac does not easily suit 400W into 8r0.
you need a 60+60Vac or 65+65Vac transformer to get sufficient DC supply voltage (about +-90Vdc to +-95Vdc) to give the 80Vpk output that your 8r0 load will need for 400W.
This is a very expensive target to meet.
Reduce your ambitions until you have accumulated the knowledge to make all these decisions for your self.
AndrewT said:
If you stay towards the lower end of your guideline it does work although not quite as well for the smaller transformers. 100W amp, 120 VA transformer, 1.2 Kg. 200W amp, 230 VA transformer, 2.1 Kg. It works better with larger transformers. 400W amp, 500 VA transformer, 4.0 Kg. Those are real numbers from the Avel Lindberg Y23 series.
But yeah, if you're designing a higher-end amp, you probably want to go bigger. But darkwind_j is clearly looking to save money so I was trying to keep things more affordable as the transformer is usually the most expensive component in an amp. The 1 Kg per 100 watts is supposedly a guideline for what's used commercially in mainstream products.
I agree you should pick a transformer by VA not by weight. But that only works when you *know* (and trust) the ratings. If you're using a transformer found in an old amp, buy one surplus, have one laying around, etc. they're often not marked in any useful way. So weight is generally a reasonable guideline.
AndrewT said:
That is the key point debated in this thread. 38 + 38 will get close to 400 watts bridged. But darkwind_j wants to drive 2 pairs of speakers (i.e. 4 ohms or lower) and also wants to keep the cost down. So the issue is should he try to use a bridged amp, or should he try and get a different transformer?
This thread is partly why I started the "bridged vs conventional" thread. At some power level bridging has some important benefits. Most commercial amps over about 400W/ch use some form of bridging these days. So one can argue that's a good solution here--especially when you already have a suitable transformer.
Darkwind_j, you still have not told us if this is for home use or professional use (PA, DJ, etc.)? And do you need 1 channel or 2 channels?
darkwind_j said:
Tony said:
Don't we live in a global economy? I buy parts from all over the world. People buy and sell things on eBay from all over the world. PayPal handles just about every currency.
The days of the local electronics shop next to the grocery store are mostly gone. Even Radio Shack here in the USA hardly stocks any electronic components anymore. Almost everything has moved online.
So I'm not sure it really matters that much where you live unless you live in a country with very difficult customs policies for small personal shipments or somewhere suppliers will not ship to?
we are playing with words here.....of course you pick transformer by va.....there are no transformer vendors selling their traffos on mere weight specs....
i was merely imparting to darkwind what to look for, i should know this, because i make my own transformers for my projects.....and in my hative country which is the Philippines, there are a lot of shenanigans about this....
so don't dicredit my posts until you know all the facts.....
iron has a density of 0.27 pounds per cubic inch, copper at 0.33 pounds per cubic inch.....
now a 100va transformer will be physically bigger than a 200 va one.....
so much so that the latter will be heavier than the former, so now how can weight not be in the equation....
pray tell me.....oh, and i am talking about E-I here, although torroids also follow this trend....
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