If an amplifier is any good, it will play small stuff or large stuff just fine. 20 to 60 Watts output is fine. In stereo counting losses, the PT could be 80VA to 240VA. In a commercial design it would be just big enough to suit the price.
If the user wants, and will pay for, LARGE LOUD, then we want hundreds of Watts of output (and appropriate speakers, thousand VA PT, and it becomes a different design. BUT if the amp is *good*, it can still play the small stuff. I have used a 400W amp into efficient theater speakers for background music. Not a cheap way to get a whisper, but there were other days I used the system to call the police (from the next town
). So the plan *absolutely* starts with a Specification of Watts and Impedance. Same as designing a house: bed and bath, or 47 bedrooms, 3 sunrooms, 2 gyms, walk-in fur vault. One can be founded on small boulders, the other needs massive concrete support.
A general plan for *today's* project is around 50 Watts in 8 Ohms. The large chips will do this, simply, well. Next you have to work out the voltage, and the power supply factors.
50 Watts in 8 Ohms is 20 Volts RMS. To do 20Vrms the amplifier output must swing zero to +28V to zero to -28V and back. 56V total, 28V each side of zero. The power supply sags and amplifier transistors lose voltage. We might allow another 25% for this, 35V each side of zero. This is the DC voltage but we buy transformers on AC voltage. The rectifier catches the peak of the sine wave. So 35V DC means about 24V AC. So two 24VAC windings.
If you need Steady FULL Power, as for lab-tests, on paper you need a PT with VA about 1.48 times the audio output. In practice it is never that good and I would design 2X for Steady Test; however even your headbanger heavy-metal is not that severe and 1.5X works fine. So two channels of 50W 8r wants a 150VA 24V+24V which is about 3 Amps.
Ah, but you have 4 Ohm speakers. If the power were SOLID, the power would double. 100W/ch and a 300VA transformer.
Generally we split the difference and shop for a Good Buy in the 150VA-300VA range. (A Real Good Buy in the 600VA range will work, though will be much heavier, which is bad for portable work or even humping it from cellar workshop to attic listening room.)
LOL heeelp one more time
I have tried to calculate but I need your help.
My amplifier shall not play loud at all.
So my dream is to make the amplifier play max 85db (know that it depends on the speakers db). i want the amplifier to grip the speaker with a steel fist and force the speaker to do as it get told, no sloppiness here!
It shall be a true dual mono amplifier 50-100w 8 ohm (isn't 8 ohm still most common today?) Thinking on using two 500VA transformers. (can properly get them cheaper if I am lucky
I have tried to calculate but I need your help.
My amplifier shall not play loud at all.
So my dream is to make the amplifier play max 85db (know that it depends on the speakers db). i want the amplifier to grip the speaker with a steel fist and force the speaker to do as it get told, no sloppiness here!
It shall be a true dual mono amplifier 50-100w 8 ohm (isn't 8 ohm still most common today?) Thinking on using two 500VA transformers. (can properly get them cheaper if I am lucky
A quick way to get an idea is using efficiencies. Class AB has got around 70% at max., class D around 90%. So, with 100W you could do with 160VA if you know the speaker impedance. But, you want it to work with any impedance you throw at it (don't go below 4 Ohm) therefore you need good current margins.
If you can have 100W with an 8 Ohm speaker, you will get 200W with a 4 Ohm speaker using the same power supply, IF the power supply can deliver double the current and the amplifier as well.
Your 100W/8Ohm then turns into 200W/4Ohm and the transformer should be 320VA. A more powerful transformer leaves margins for heating and different amplifiers.
Look for two 320VA-500VA transformers and find a good deal.
If you can have 100W with an 8 Ohm speaker, you will get 200W with a 4 Ohm speaker using the same power supply, IF the power supply can deliver double the current and the amplifier as well.
Your 100W/8Ohm then turns into 200W/4Ohm and the transformer should be 320VA. A more powerful transformer leaves margins for heating and different amplifiers.
Look for two 320VA-500VA transformers and find a good deal.
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Thanks to FauxFrench, JMFahey and PRR It the impossible is done!
I think you are succeeded to get me to understand and feel informed enough to select the specification of the transformer!
My conclusion:
A perfect 28VAC+28VAC should be about 50W RMS, but since nothing is perfect, ad a bit, now 35VAC+35VAC. But then the rectifier comes and disrupt everything. 35VAC * 0,707 = 25VDC that is below the 28VDC or 50W RMS!
So lets start the other way: 28VDC / 0,707 = 39,6VAC so 40VAC
Am I correct or is my understanding totally wasted energy from your part?
I think you are succeeded to get me to understand and feel informed enough to select the specification of the transformer!
My conclusion:
A perfect 28VAC+28VAC should be about 50W RMS, but since nothing is perfect, ad a bit, now 35VAC+35VAC. But then the rectifier comes and disrupt everything. 35VAC * 0,707 = 25VDC that is below the 28VDC or 50W RMS!
So lets start the other way: 28VDC / 0,707 = 39,6VAC so 40VAC
Am I correct or is my understanding totally wasted energy from your part?
Thanks to FauxFrench, JMFahey and PRR It the impossible is done!
I think you are succeeded to get me to understand and feel informed enough to select the specification of the transformer!
My conclusion:
A perfect 28VAC+28VAC should be about 50W RMS, but since nothing is perfect, ad a bit, now 35VAC+35VAC. But then the rectifier comes and disrupt everything. 35VAC * 0,707 = 25VDC that is below the 28VDC or 50W RMS!
So lets start the other way: 28VDC / 0,707 = 39,6VAC so 40VAC
Am I correct or is my understanding totally wasted energy from your part?
No, no, no - the other way around: AC to DC you multiply with 1.41. DC to AC you divide with 1.41.
Example: 26Vac -> 37Vdc; 40Vdc -> 28Vac.
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