I'm pretty new at this and am building a headphone amplifier using a HA-PRO2 kit I bought from eBay. It calls for a 12-0-12 transformer and I've been looking at toroidal transformers. How do I know what VA rating I need? Would a 30VA transformer like this one be sufficient? This is the HA-PRO2 kit.
30 va is many many times more than is needed for a headphone amp like this one although of course it will work OK.
Something like a 6VA would be more than adequate for a headphone amp of this type.
This kind of thing:
https://cpc.farnell.com/multicomp/m...dkey=https:en-CPC/CPC_United_Kingdom/w/search
Something like a 6VA would be more than adequate for a headphone amp of this type.
This kind of thing:
https://cpc.farnell.com/multicomp/m...dkey=https:en-CPC/CPC_United_Kingdom/w/search
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Your average low impedance headphones (of the kind this circuit is intended to drive) need only a few milliamps of current for typical listening levels.
I did some tests on this some years ago using some Sony headphones (MDRV-7) and found that well under 1 volt of drive was louder than I would ever want and typically around 200 to 400 millivolts covered all normal listening.
I did some tests on this some years ago using some Sony headphones (MDRV-7) and found that well under 1 volt of drive was louder than I would ever want and typically around 200 to 400 millivolts covered all normal listening.
The smallest Antek is 10VA and a 25VA costs hardly any more. What I have noticed is that if the design is a Class A amp, err on side of next size up. Talema sealed case toroidals are also very good for this application but about double the price.
10VA $10:
AN-0112 - 10VA 12V Transformer - AnTek Products Corp
25VA $11:
AN-0212 - 25VA 12V Transformer - AnTek Products Corp
I personally would get the 25VA.
10VA $10:
AN-0112 - 10VA 12V Transformer - AnTek Products Corp
25VA $11:
AN-0212 - 25VA 12V Transformer - AnTek Products Corp
I personally would get the 25VA.
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Also:
Small transformers have the poorest regulation so they can have pretty severe voltage fluctuations with load and whilst this maybe fine for one circuit you might just not want the hassle with another circuit where unloaded voltages exceed capacitor voltage ratings for example.
Sorry guys, I have to disagree 
Look at the kit it is to power. There are four 4580 opamps and what actually looks like a small relay. A small 12 volt toroid would have a regulation figure of around 20% meaning the 12 volt AC becomes 14.4 volts under no load. That would generate raw rails of no more than -/+20 volts and the kit clearly shows 25 volt caps and 7812/7912 regulators.
Look at the kit it is to power. There are four 4580 opamps and what actually looks like a small relay. A small 12 volt toroid would have a regulation figure of around 20% meaning the 12 volt AC becomes 14.4 volts under no load. That would generate raw rails of no more than -/+20 volts and the kit clearly shows 25 volt caps and 7812/7912 regulators.
My calcs and experience using a 12v dual secondary trafo like Antek says it’s about 17v rails, no load. A 5v drop through a 7812/7912 is about right. In practice, the 10VA will be a maybe 2V under from sag with any appreciable load. That would put the drop across the VR kind of close to the recommended minimum 3.5v drop.
Have a look at this:
The Signal Transfer Company: Power Output
Don't be put off by the maths, just take each section in turn and try and follow it. Although this was written for designing power amplifiers and supplies, the basic theory is sound and applicable.
Remember that for your headphone amp, you won't be putting more than a couple of volts across the headphone load and so the current is pretty minimal.
The Signal Transfer Company: Power Output
Don't be put off by the maths, just take each section in turn and try and follow it. Although this was written for designing power amplifiers and supplies, the basic theory is sound and applicable.
Remember that for your headphone amp, you won't be putting more than a couple of volts across the headphone load and so the current is pretty minimal.
Something that isn't mentioned in the above link as far as I know is the question of 'regulation factor' of the transformer.
Transformers are normally specified when delivering their full rated load current into a resistive load. At lighter loadings the secondary voltage will rise by some percentage and that is often quoted in the data sheet. For the 7VA transformer I linked to it is 19% meaning the voltage will be closer to 14.28 volts off load.
Transformers are normally specified when delivering their full rated load current into a resistive load. At lighter loadings the secondary voltage will rise by some percentage and that is often quoted in the data sheet. For the 7VA transformer I linked to it is 19% meaning the voltage will be closer to 14.28 volts off load.
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