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Old 24th March 2008, 10:52 AM   #11
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Quote:
Originally posted by jacco vermeulen
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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Old 24th March 2008, 11:38 AM   #12
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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.
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Old 24th March 2008, 12:07 PM   #13
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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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Old 24th March 2008, 12:36 PM   #14
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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.
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Old 29th March 2008, 05:35 PM   #15
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Question:
The power supply seems to have a large effect on the operating speed of the loudspeaker drivers. How does one figure this out? Are there any general guidelines?
Thanks.
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Old 29th March 2008, 06:03 PM   #16
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Quote:
Originally posted by dfdye
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.
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Old 29th March 2008, 07:45 PM   #17
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I would be a little concerned if the transformer got to 105 deg C!

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!
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Old 29th March 2008, 07:52 PM   #18
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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.
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Old 29th March 2008, 08:20 PM   #19
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Quote:
Originally posted by TheMG
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?
Quote:
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!
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Old 29th March 2008, 10:15 PM   #20
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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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