Hello, everyone! I'm planning to build dual mono lm3886tf amplifier to use with my 8 Ohm 70 watt guitar speaker.
I've calculated power and voltage characteristic of transformer for my needs using formulas presented in this web page. A Complete Guide to Design and Build a Hi-Fi LM3886 Amplifier - Circuit Basics
So I've got following values:
P supply= 160 watt. Peak voltage is 24 V. Transformer rating is 240 VA per channel.
As for toroidal tranformers, here I have two options to buy
1) 2х25V, 7.6A, 350-450 VA
2) 2х25V, 4A, 220-260 VA
Which one is better, taking into consideration their amperage and power VA rating? As I understood LM3886 need nearly 5A of current, right? Hence transformer should supply more than 5A, doesn't it?
Will it be OK to use two different transformers for each mono channel or should I buy one but 500 VA transformer?
I've calculated power and voltage characteristic of transformer for my needs using formulas presented in this web page. A Complete Guide to Design and Build a Hi-Fi LM3886 Amplifier - Circuit Basics
So I've got following values:
P supply= 160 watt. Peak voltage is 24 V. Transformer rating is 240 VA per channel.
As for toroidal tranformers, here I have two options to buy
1) 2х25V, 7.6A, 350-450 VA
2) 2х25V, 4A, 220-260 VA
Which one is better, taking into consideration their amperage and power VA rating? As I understood LM3886 need nearly 5A of current, right? Hence transformer should supply more than 5A, doesn't it?
Will it be OK to use two different transformers for each mono channel or should I buy one but 500 VA transformer?
first myth bust:
The transformer does not supply the current going to the speaker.
The main smoothing capacitors plus the MF and HF supply rail decoupling supply all the current going to the speaker/load.
Second myth bust:
The current out of the secondary is not a sinewave when feeding a capacitor input filter.
The current going through the secondary windings and rectifier to charge the smoothing capacitor/s is a series of very high current pulses. This heats the windings and the rectifier more than simple sinewaves.
The average current draw through the power amplifier is probably only 1/6th to 1/10th of the peak current of those charging pulses.
For domestic use a transformer rated from one to two times the maximum output power of the amplifier is about right to give good sound performance and prevent the transformer from overheating.
For live music production that recommendation should probably be increased: probably around 1.5times to 2.5times the maximum power of the amplifier.
Your LM3886 has a maximum of around 65W to 70W into a 8r0 test load, even though the manufacturer says it's only 50W into 8r0.
Assuming the higher figure, one would need a transformer to power one channel of around 100VA to 175VA
Both your suggestions seem to be far too big for both performance capability and for temperature rating.
The transformer does not supply the current going to the speaker.
The main smoothing capacitors plus the MF and HF supply rail decoupling supply all the current going to the speaker/load.
Second myth bust:
The current out of the secondary is not a sinewave when feeding a capacitor input filter.
The current going through the secondary windings and rectifier to charge the smoothing capacitor/s is a series of very high current pulses. This heats the windings and the rectifier more than simple sinewaves.
The average current draw through the power amplifier is probably only 1/6th to 1/10th of the peak current of those charging pulses.
For domestic use a transformer rated from one to two times the maximum output power of the amplifier is about right to give good sound performance and prevent the transformer from overheating.
For live music production that recommendation should probably be increased: probably around 1.5times to 2.5times the maximum power of the amplifier.
Your LM3886 has a maximum of around 65W to 70W into a 8r0 test load, even though the manufacturer says it's only 50W into 8r0.
Assuming the higher figure, one would need a transformer to power one channel of around 100VA to 175VA
Both your suggestions seem to be far too big for both performance capability and for temperature rating.
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AndrewT is correct on the transformer sizing.
Even so, had you illustrated the need of a bit larger volume of entirely useful watts, it would have to come out of a proportionately larger transformer. The need of that exception, was not proposed.
It could be done, with a bit more care though.
Even so, had you illustrated the need of a bit larger volume of entirely useful watts, it would have to come out of a proportionately larger transformer. The need of that exception, was not proposed.
It could be done, with a bit more care though.
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