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Old 31st March 2010, 02:46 AM   #1
jt1 is offline jt1  United States
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Default Transformer Ratings

Is there a general rule of thumb when selecting power and output xformers for a specific application?

If a given circuit calls for 90ma power and 3watts output, how much overhead is recommended for reserve power draw and peak output...is there a general %'age or does it vary, and can you over do it?

thanks

jt
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Old 31st March 2010, 05:07 AM   #2
AJT is offline AJT  Philippines
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afaik, there are no transformers that go that low power wise, my rule of thumb is "as big as you can afford or as your box can fit"
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Old 31st March 2010, 06:54 AM   #3
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If you choose a power transformer just big enough, it will run at its maximum load. This means voltage regulation will be poor, the core will become quite hot, and the transformer may buzz. If you add, for example, a 50% margin, it will run cooler, will not buzz, and voltage regulation will be better.

At power-on, you may find your circuit draws more than 90ma because of capacitor charging etc. It also depends on topology. Class A amps are a near constant load for the power transformer while class AB or B amps draw little current at idle and significantly more on signal peaks.

OPTs is a different story. The necessary core size depends not only on power but also on the desired LF cutoff frequency.

Kenneth
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Old 31st March 2010, 07:05 AM   #4
aandy is offline aandy  United Kingdom
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You can do what i did.
To get Good quality toridal transformers i made my own.
this is because most transformer manufactures have no idea how to make a Good transformer for Audio.
Junk Junk door stops
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Old 31st March 2010, 07:11 AM   #5
AJT is offline AJT  Philippines
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but they sure know how to make profits.....
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Old 31st March 2010, 07:48 AM   #6
jt1 is offline jt1  United States
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kavermei - Thanks, that makes sense to me about the power xformer...as far as the OPT goes just how critical is the input impedance? if a circuit calls for 3300 ohms and you have only 2500 or 5000 ohms as an option which way to go...I understand you will loose power by using the 5000 ohms, but is it the correct choice, what is the effect either way?

thanks
jt
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Old 31st March 2010, 09:10 AM   #7
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Using an OPT with higher impedance than called for gives you less power, less distortion, and, in some cases, a bit more bass extension. Too low will not sound very good.

Ideally you could get an OPT with multiple taps and experiment a bit, see what sounds best to your taste.

Kenneth
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Old 31st March 2010, 09:36 AM   #8
jt1 is offline jt1  United States
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kavermei - Thank you, exactly what I needed to know.

jt
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