What Voltage Rails for 50w in 8 Ohm?

[Ampdog]

A few further hints,

One also has to include the equivalent transformer winding resistance in the mix ... which then brings us to the filter input capacitor, the (peak voltage) charge on which will depend not only on the transformer winding resistance but also the load at any moment. (At worst loading, the filter capacitor might not charge to much higher than 80% of the Vpeak of the voltage) ... etc. Designing for worst case, one can end up with a no-load rail voltage of 20% higher than under demand - is the amplifier designed to withstand that including mains variation ....?  Etc. etc.

Trying to say, this exercise is as dependant on the signal demand as anything else. I seem to find that a transformer rated even at 70% of full amplifier output capability does not get overly hot under fairly loud signal conditions. (Of course, if you constantly overload as with guitar music ....) It is often cost-effective to include a rather high value filter input capacitor with a somewhat smaller transformer, than using a rediculously large/expensive transformer. (Even with winding resistance = 0 output power is governed by the capacitor/load value.)

(None of this disparaging your advice, Tang.)

.... or one can chicken out, use an eight times the necessary watts transformer under the "bigger-the-better" banner (not saying that is what you meant, Tang).... like the 800VA power transformer I once found powering a 30W consumption pre-amplifier.   

[Ampdog]

...... technically it is very wrong to talk about RMS wattage. you cannot RMS watt. Watt is the amount of constant power used.

AH! GIVE THAT MAN A 'BELLS'!

But we are talking in vain, dear Tang .......

One thing more, if I may:

This business of providing for low impedance dips in certain loudspeaker designs. One must remember that those dips are normally at one or two frequencies only, so it is a moot point how important it really is. Sadly, again forget about promotional claims (e.g. the beaming glossies never state for what duration that 'twice-the-watt ' can be maintained, only that it can be reached).

[tangmonster]

google for a simple program called psud2

It takes all the mission out of calculating ripple voltages under high load currents with specific capacitor sizes and transformer internal resistance.
Really fun to play with various CRC and CLC filtering.

Chipwelder

To be perfectly honest it is my firm belief that everything above 300VA should have a slow start.

My first 5 channel lm3886 that is using a 500va does however NOT have slow start , but I leave it on 24/7.
The lights really does dim when you switch it on , and the stress on the transformer and the filter capacitors will lower lifespan.

You can use a thermistor which is the simple easy solution and works way too good for how simple it is. OR you could do it thoroughly and get a relay and series resistor.

for 500va i would use 2 x 10W cement 100ohm resistors in parallel. Mechanically simpler but more expensive route is a 56ohm 25W alu case resistor.

around 100 to 200ms should be perfect.

But the thermistor route is perfectly adequate for a simpler solution. Just make sure no-one switches it of and on again too much because when that thermistor is hot it is useless as protection it needs time to cool down again


Another thing to remember above all.

because our ears works logarithmic you need roughly 10 times the volume for your ears to perceive it as 2 x the volume.

The difference between 50w and 100W really is not all that much.

So don't spend days worrying about the 5 or 6 watts you will have less because your voltage rails saggs a bit under high load.

one more thing

with more current comes more heat and more losses. So most amplifiers will not deliver 2x the wattage into 4ohms rather than 8 ohms because more energy is lost with wire resistances and all the other areas where losses happens.

In my personal opinion 2 ohm loads are a joke. Yes i know every car amplifier you pick up has a 2ohm spec , the math and losses just becomes scary at those loads.

[chipwelder]

Very interesting... So actually 2x 300VA should be fine, more than enough considering that I haven't clipped the lm3886's yet. Six of them runs off one 300VA...... (have bottomed out midwoofers though) Can use 25 to 28 V taps...

This will cost predictably roughly 60% of the price of two 500VA's saving ZAR 800...

[tangmonster]

Another thing to remember is that with multi channel amplifiers (i guess you want to use it for a movie surround amplifier? )

The rear channels only does duty every now and then. center and fronts does duty all the time.

So if you are going to use 8 ohm speakers I would say 600VA in total will be just fine.

if you are using two powersupplies.

I would put the rears with the front center on one 300vA and the front 2 speakers and the possible rear center of your 6.1 on the other 300 VA.

[chipwelder]

Nope is for active rig, no xo, just one driver per channel...  Ja I know the 25w version would be fine too... there is always a but whatif one day?