Transformers: How big is TO big??

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Due to the rise in copper prices and the coincident rise in transformer prices, I got to thinking: how much of a transformer do we really need for an audio PSU? I have always erred on the side of "bigger is always better" since I have always been lucky enough to find "overkill" transformers surplus (thanks ApexJr!).

Still due to the transient nature of audio signals in conjunction with the often large rail capacitances we use for amp PSU's, how would you go about actually calculating what you really NEED for an amp as opposed to simply using the maximum peak amperage and getting a transformer that can handle that? Google wasn't really any help on this front, and I figured a ton of you EE types probably solved this problem a long time ago as undergrads!

I would love to hear any insight people may have on the subject.
 
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I've gone from 0.8 to 2.5, the sweet spot (price/quality) seems to be in the 1.5-1.8 range. This varies wildly with designer's tastes, application and speakers' abilities/demands, so take this IMO.

Overkill never hurts though, and you'll see recommendations of 300VA for a 60-watt amp. I have no access to toroids at reasonable prices, so cannot verify that such a thing could sound better, but I'll assume that may also be true.
 
In a normal capacitive power supply, conversion between Watts and VA is approx. 0.68:1

A typical real world class AB amplifier has an efficiency of maybe 65%.

Music has a duty cycle that is highly dependent on the type of music you are listening to but for argument sake, let's say 40%.

So....

For a Mono 100W Class AB amplifier, you can figure a sensible minimum transformer rating (assuming sufficient capacitance in the power supply) of:

(100*0.4) / (0.68*0.65) = 90VA

With that said, there are good (technically sound) reasons to use larger than the minimum rating required.
 
AudioFreak said:
With that said, there are good (technically sound) reasons to use larger than the minimum rating required.
Your explanation makes good sense. The missing part for me was the ~40% duty cycle approximation, but that sounds completely reasonable. Thanks!

As for the reasons to have a larger transformer, of course a larger transformer would run cooler at the same current draw, making the chances of failure over time lower (like cracked enamel on the windings causing shorts and such), and a transformer that was too small might hum, but other than that, I can't really think of good reasons to go much beyond what is needed.

Either way, 0.9 * wattage = VA sounds like a completely reasonable (well justified :D) rule of thumb as a minimum for transformer ratings.
 
Hi,
the peak output current is supplied by the smoothing caps.
The transformer recharges the caps.

I usually suggest that the VA is between 1*max power and 2*max power. Most are happy with 1.5times.

I have found that using a slightly smaller transformer can be compensated for by increasing the smoothing cap value.
 
In addition to his webmaster's voice: first Q is which load ?

For Class AB, the 100W/90VA would be for a true 8 Ohm load.
Depending on the intended load, same calculation for either 4 or 2 Ohm, which boils down to a minimum of ~150VA for 4 Ohm operation.

Some manufacturers used to offer their product with powersupply options.
Good example were the Berendsen power amps, Red and Blue edition, only differed in the toroid and capacitor bank size.
Or The Cyrus PSX.
 
jacco vermeulen said:
In addition to his webmaster's voice: first Q is which load ?

For Class AB, the 100W/90VA would be for a true 8 Ohm load.
Depending on the intended load, same calculation for either 4 or 2 Ohm, which boils down to a minimum of ~150VA for 4 Ohm operation.

My calculations were based on having rails specific to the load hence why I didn't specify a load in my calculations. It certainly is true that losses will be higher into lower loads so increasing the minimum transformer rating in such situations is certainly to be well advised.
 
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Remember also, that using oversized reservoir caps causes the bridge rectifier to conduct for a very small phase angle. This increases the effect of "copper losses" in the transformer and wiring, and makes the peak charging currents very large. This can also increase the radiated magnetic field from tranny. As always it's a compromise, designing a nominal 100 +100 watt amp intended for domestic duty with music is VERY different to an amp intended to work into low Z loads for extended periods with continuos signal. A 300 VA rating should be more than enough for a 2 channel amp for domestic use with perhaps 6800mfd as a maximum reservoir value. I know many will disagree on this, but in practice this is what I have found works well. Any noise and ripple on PSU should be rejected by the amp itself.
Regards Karl
 
Absolutely. Light listening home use is not remotely comparable to Pro Audio sound reinforcement use. The latter has to be able to operate reliably delivering often close to maximum power output with often highly compressed content into difficult loads for hours on end. I would never recommend using a transformer rated as low as my calculations suggested. I simply wished to show the aspects that are considered to determine a minimal rating requirement for a Class AB amplifier used in a typical home "hifi" use. I personally would probably use about double the minimum for Class AB home use (much bigger for Class A). For Pro use, switching supplies pretty much reign supreme these days.
 
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Besides the tighter voltage regulation under load a larger
transformer using larger wire and more iron will be more efficient creating less heat. Say we have something drawing 100w on
24 hours a day 365 days a year and electricy is $.10 per kw/h.
that's $87.60 a year. So a 150va transformer may be ok but
if a 300va is 1% more efficient it might be cheaper in the long run. Then there is the weight issue. I have 2 transformers
perfict for the Pass F4 one is 15lbs and 400va the other is 1.2kva
and ~60 lbs guess which is more likely to be used if I build something.
 
dfdye said:
Wait, so you are saying a 100W amp * 1.5 = 150VA transformer? That doesn't really sound too insane, but is it really that simple? Am I just obtuse for never seeing this before??
If the smoothing is much you can take out only half the VA rating, mind that.

A normal size is 100 VA for 100 W for home use but 150-200 VA is considered good. The way of know if the transformer is enough is to measure the temperature . 105 deg C inside is the limit but maybe it would feel better if the transformer is not hot.
 
I would be a little concerned if the transformer got to 105 deg C! :eek:

I was thinking that the temp issue can easily be addressed by a thermal circuit breaker strapped to the transformer to ensure that it doesn't over heat. The problem I am quickly encountering is how to get one physically small enough with a large enough current rating that will be able to be mounted to the transformer. . . .

Just thinking out loud here if anybody has any suggestions. In the mean time, I'll keep flipping through digikey to see if something wll work. Again, I am sure somebody has solved this problem before!
 
You could use a temp sensor along with a simple ciruit and a relay to cut off power should temp reach unsafe levels.

Personally I'd just pick a transformer of sufficient rating that it is not in danger of overheating even under extreme conditions, and then fuse it such that any fault that could potentially overheat the transformer would blow the fuse first.
 
TheMG said:
You could use a temp sensor along with a simple ciruit and a relay to cut off power should temp reach unsafe levels.
What would the benefit of this approach be over a thermal circuit breaker?
Personally I'd just pick a transformer of sufficient rating that it is not in danger of overheating even under extreme conditions. . . .
Certainly a possibility, and an approach I have taken many times in the past, but it seems quite wasteful to me. And yes, I agree that appropriate fuses on the transformer windings are an appropriate method of protection, but a secondary form of protection never hurts!
 
Question: So, what scenerio provides that an amplifier's transformer could overheat without it being noticed?

I think that if your amplifier's transformer gets swamped, that the sound is so terrible that it just won't be operated that way for more than a few seconds.

I tried this on a 24va for an experiment (bad sort). It didn't get warm. No harm came to any components. However, the output quality was useless.

In my opinion, its better to use a large EI than an overly small toroid. That can save some cash. Compare $36 new EI, a thrift salvage find price, and the price of a new toroid. Wow.

Also, extra capacitors can run smaller transformers. For instance if a particular designer recommended 200va, yet provided only 1 Nichicon FG 1500uF per rail. . . to run a 100va, just add more of the exact same model capacitor. Well, keeping a design mostly intact is the easy way.
Otherwise, about $8 worth of Mallory caps could do a lot for budget power needs.
 
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