Hi,
If your transformer is 50/60Hz which is probably it since you have also Europeen primary mains, it was designed to work at 50Hz also which is the worst case. The higher the frequency the smaller the tranni gets, just like the one in switching mode power supply. Regardless of the mains you are using, a 96VA tranni is a 96VA tranni.
One easy way to fix this is th use the tranni you have and use the GC to drive efficient speakers 🙂
The Lm3886 is best suited for 4 ohm loads as per the National spec. sheets so your 48Vct may be a little high (+- 35Vdc).
Ideally you would be better off with a smaller voltage and higher secondaries outputs current.
Regarding the 2A fuse, if it is an IEC fuse it will work normally to 150% without blowing (i.e.3A) and it will blow at 210% of the fuse rating. North American fuse have a better tolerance which is from 130% - 150% instead of the Europeen fuse at 150%-210%
Build it with the 48Vct,2A and have FUN, this it what matters.
Regards,
Eric
If your transformer is 50/60Hz which is probably it since you have also Europeen primary mains, it was designed to work at 50Hz also which is the worst case. The higher the frequency the smaller the tranni gets, just like the one in switching mode power supply. Regardless of the mains you are using, a 96VA tranni is a 96VA tranni.
One easy way to fix this is th use the tranni you have and use the GC to drive efficient speakers 🙂
The Lm3886 is best suited for 4 ohm loads as per the National spec. sheets so your 48Vct may be a little high (+- 35Vdc).
Ideally you would be better off with a smaller voltage and higher secondaries outputs current.
Regarding the 2A fuse, if it is an IEC fuse it will work normally to 150% without blowing (i.e.3A) and it will blow at 210% of the fuse rating. North American fuse have a better tolerance which is from 130% - 150% instead of the Europeen fuse at 150%-210%
Build it with the 48Vct,2A and have FUN, this it what matters.
Regards,
Eric
This transformer can probably not provide 150VA when driven from 120V, even if loaded as designed. On 240V the whole winding takes the power, on 120V the (I^2) R loss will be double (with full load).
If the transformer had dual primaries so you could get 150VA out of it, just using the 24-0-24 would get you more than the nominal 96VA with a loss of regulation.
In this case, you could probably get the 'rated' 96VA out of it for an audio amplifier. One per channel would probably be enough to power a 3886 to 50W/4R.
The two issues on power transformers are heat, and core saturation. In general, EI cores suffer overload better than toroid types. The main reason being that the core saturates more gradually vs the torroid will just experience a colapse in the inductance and the primary current will shoot sky-high. The extra current on an EI type just increases the (I^2) R loss and increases the heat. The time constant on the heat is on the order of one hour or more, so if you can live with the decreased regulation and keep the duty cycle down to keep the average power under the limit; you're fine.
I have some nice EI control transformers rated at 240VA. I have used them in a power amp that will actually deliver 600W! At that power level the regulation is quite poor (about 10%), and will quickly overheat unless the duty cycle is less than about 25%. It works well for a home theater subwoofer amplifier.
If the transformer had dual primaries so you could get 150VA out of it, just using the 24-0-24 would get you more than the nominal 96VA with a loss of regulation.
In this case, you could probably get the 'rated' 96VA out of it for an audio amplifier. One per channel would probably be enough to power a 3886 to 50W/4R.
The two issues on power transformers are heat, and core saturation. In general, EI cores suffer overload better than toroid types. The main reason being that the core saturates more gradually vs the torroid will just experience a colapse in the inductance and the primary current will shoot sky-high. The extra current on an EI type just increases the (I^2) R loss and increases the heat. The time constant on the heat is on the order of one hour or more, so if you can live with the decreased regulation and keep the duty cycle down to keep the average power under the limit; you're fine.
I have some nice EI control transformers rated at 240VA. I have used them in a power amp that will actually deliver 600W! At that power level the regulation is quite poor (about 10%), and will quickly overheat unless the duty cycle is less than about 25%. It works well for a home theater subwoofer amplifier.
I think the spec sheet is saying the 3886 is best suited to 8ohm loads.
It's a pity they don't show the data for 12ohm and 16ohm. That could be used to extrapolate for parallel use into higher impedance loads. The spreadsheet similarly omits the higher resistance data.
I think I would hook up 120VAC to the 240V and 100V taps, that would give about 20.5-0-20.5 (about right for a 4 ohm 3886).
Hi,
there is a possibility that the primary uses a thicker wire from neutral to 120V tapping and thinner wire from 120V to 240V tappings.
Connecting 110/120V across the thinner wire will cause overheating.
there is a possibility that the primary uses a thicker wire from neutral to 120V tapping and thinner wire from 120V to 240V tappings.
Connecting 110/120V across the thinner wire will cause overheating.
one wouldn't want the fuse trip at 2A.Nordic said:
The 2A are the transformers nominal output current on which it could be loaded over a longer period, within a given enviromental temperature, not its maximum current.
Keeping the temperature low is important. The case needs ventilation slots!
regards
I see that the wires/windings do get progressively thinner - COM is thickest and 240V is thinnest.
John what are you going to use this transformer / amplifier for? Are you going to listen to music and movies or are you going to do full continuous power testing? Siegfried Linkwitz uses a very small transformer in his multi-channel dipole speaker systems and they seem to do well playing loud music.
I was planning on using the amp to check preamps and speakers, nothing major. Maybe some background music too...
I have a few of these trannies, but wanted to use as few as possible, that's all.
I have a few of these trannies, but wanted to use as few as possible, that's all.
AndrewT said:
No, you are not completely wrong. 🙂 Yes, you are right in that the primary current increases. But so does the secondary current, and they flow in opposite directions and cancel each other out. For a resitive load, current flows out of the positive secondary end but primary current into the positive primary end. These wires go in the same direction so the currents cancel each other out.
There are some side effects though, the increased voltage drop over the primary resistance causes the magnetizing voltage to drop, decreasing flux a little. This is what prevents catastrophic flux walking in push-pull converters used in for example car amplifiers. I'm not sure in which way leakage inductance changes the flux,but none of these effects should be very big in a normal transformer.
One way of calculating saturation point is to calculate at which current-turn product the core saturates, this is useful for designing chokes and such. The ampere-turn limit of a core increases when you introduce an air gap. The saturation current of a transformer is usually not much bigger than the peak magnetization current. This causes lots of fun stuff, like toroids popping your breakers when power is turned on at the zero crossing as the flux will grow to double the usual value.
Another way of calculating saturation point is to calculate the volt-seconds per turn for saturation. This product does not change when the core is gapped actually. At the same voltage current increases when you gap because of the decreased inductance.
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