What Voltage Rails for 50w in 8 Ohm?

[chipwelder]

Hi, just wanto check if I need 50w in 8 ohm, will double into 4 and who knows into 2...

Do I need a xxxVA transformer with 30V taps, for 36VDC rails?

[Crafty]

I think you will need slightly less. I'm no pro in this area, but I think you will end up with 37,38(ish)V on the +/- rails. Tangmonster seems to be the uber pro on these things

What IC/Chipamp are you going to use, or are you building with transistors only?

[chipwelder]

This is for the GEM amp...

http://gmweb2.net/the_gem.htm

Way down under construction notes, they mention 30V rails for 35W/8ohm, and on the drawings they mention 36V rails...

[undefined.za]

Well with 30Vac taps, you will end up with 42Vdc rails in theory, with actual rail voltages (after losses) being closer to 41Vdc.

If you want approx 36Vdc rails, you need a transformer with 25Vac taps, which will give you about 35Vdc.

Regarding transformer VA rating, the more the better! The problem with small VA transformers is bad regulation and low current capability, this means that when you drive the amp hard, the voltage it supplies drops and it struggles to accommodate the current demand which leads to early clipping.

[Crafty]

Well with 30Vac taps, you will end up with 42Vdc rails in theory, with actual rail voltages (after losses) being closer to 41Vdc.

If you want approx 36Vdc rails, you need a transformer with 25Vac taps, which will give you about 35Vdc.

Regarding transformer VA rating, the more the better! The problem with small VA transformers is bad regulation and low current capability, this means that when you drive the amp hard, the voltage it supplies drops and it struggles to accommodate the current demand which leads to early clipping.

I have a set like that. No Center Tap, but when hooked in serie it gave a very smooth output. (Checked on osciliscope) Seeing as it was too strong for my 40W amp, I bought a weaker one from RS, which suited my needs better.

Make me an offer if you like ChipWelder.

[tangmonster]

The rail voltage needed for a specific output will differ from amplifier design to amplifier design.

lets rather concentrate on resistive load and what voltage is needed for what RMS wattage:

ohms law is a lovely thing

V = IR
P = VI

For DC constant current (NOT audio AC signal)

8ohms:

50W = 20V x 2.5A

For AC sine waves we need to use the RMS voltage and RMS current You get this by using the peak voltage and current and dividing by 1.414 for a SINE wave:

50W = (28.28/1.414) x (3.535/1.414)

So for 50W sine wave we need 28.28V peak into 8ohm resistive load.

If you halve the 8ohms to 4ohms the current will double and so will the wattage.

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

But it has become the norm to talk about rms watt when you mean you used the rms voltage and current to get true average constant power.


SO

in short

for a perfect amplifier you will need +-28.28V rails to get 50W into 8ohms or 100W into 4 ohms.

BUT on average you need between around +-35 rails to get 50W into 8ohms.

 

[Atjan]

/\ Thanks for the explanation Tang. Why 2.5A though?
Also, what makes an amp not ideal, ie 28V for 50W? How do you get to 35V from 28V?

[tangmonster]

lets look at this sine wave



lets say 1 = 28.28V
lets say it is measured across a 8ohm resistor.
this resistor is dissipating 50W into heat

peak power: normally peak power is calculated by taking 28.28 x 3.5A = 100W
                   this means nothing. This is the power generated into heat for a very very small amount of time when the voltage is at +28.28V or -28.28V

rms wattage: this is the true constant wattage  28.28/1.414 x 3.5A/1.414

PMPO wattage: utterly useless , just like the contrast rating on monitors and tv's it is a made up thing that everyone uses like they please to put a number on a manual that     means nothing

[tangmonster]

/\ Thanks for the explanation Tang. Why 2.5A though?
Also, what makes an amp not ideal, ie 28V for 50W? How do you get to 35V from 28V?

V=IR

you have an 8ohm resistor and you are putting a 20V battery across it

20 =I  x 8

20/8 = I

I = 2.5A

I just just that first equation to show how DC current and differs from AC current calculations. For DC current you don't need to bother with the 1.414 division of the peak voltage and current to get correct power , you just use the dc voltage and current to get power dissipated.

The 35V is an example.

the lm3386 chip needs 35V rails to be able to drive 50W RMS wattage into a resistive 8 ohm load.

http://www.ti.com/lit/ds/symlink/lm3886.pdf

check on page 3:

For 28V rails and 8ohm load the typical output power is 38W but National instruments promises the minimum a chip you buy from them will deliver is 30W.
(this is measured with the chip at 25C)


What makes an amp not ideal. Transistors are not 0ohm switches , they have internal resistance when ON , this resistance goes up when they are hot.
The input of the amplifier when powered from the same voltage rails as main output transistors(most amplifiers works like this) when rails dips for high volumes and low impedances might not be able to switch on the transistor fully.

look at the naim amplifier



note the two 0R22 resistors on the outputs of the transistors to keep things stable.

Technically you have a 0.22ohm resistor in series with your speaker.

lets say you have 28.28V , This is divided by this 0R22 and your 4ohm speaker.

This means that on your speaker you will only have 26.806V and 1.474V across the 0R22 resistor.

This calculates to 89.82W and not the full 100W you are hoping for. just over 10W is wasted in the 0R22 resistor

[tangmonster]

chipwelder to get back to your question:

checking on the website:

"For 70W/4R=2x35W/8R use 30V rails.  For 50W/4R=2x25W-8R use 25V rails with just a single pair of class-AB power transistors."

8ohm

35Wrms =  23.66432V/1.414  x 2.95804/1.414

so for 30V rails according to the website you can get 23.66V on the output driver.

that means a 6.44V loss. this will change a bit for 4 ohms loads and higher rails but for simplicity lets say you want 50W into 8ohms taking these numbers.

You will need 28.28 + 6.44 = 34.72V

34.72Vdc / 1.414 = 24.55Vac


sensible values for transformers:

If you are going to use 8 ohm speakers I would use a 160 - 220VA   25VAC  0 25VAC transformer

If you are planning to use 4 ohm loads I would use a 300va toroidal minimum 400va to 500va will have slightly less voltage drop for high volumes.

there is no rule when calculating transformer sizes. commercial product mostly get away with 0.7 x to 1.5 x the transformer size needed.
But in diy the rule of thumb has always been 1.5 - 2x desired output wattage fro class AB (you can go crazy and use 10x if you want too)

For class A I would start at around 7 to 8 times the output wattage because of the constant current load.

[chipwelder]

Cool thanks!

And these are the same, just the nylon case is different??? seems a bit pricey for that, unless it is hot-dipped etc, so's it doesn't hum etc...
can't find the ones with an earth... 25 -0-25

http://za.rs-online.com/web/p/toroid-transformers/4209172/
http://za.rs-online.com/web/p/toroid-transformers/2238897/

Was thinking 2x 500VA for 6 channels, no need to double into 2 ohm, as it is for an active rig... So there is room to breathe into 4 ohm and I could even squeeze two more channels into it one day when I feel flush. Some think that 500VA is where good regulation starts taking place, that's why two 500VA's even though 2x320VA's should be fine...
 

[Crafty]

Ons moet vir daai 2 sê "Oom Toroidal"

Those 2 toroidals of mine pushes exactly 25v 0 -25v if linked , but doubt it will be close to 500VA. 3 or 4A at most which will make them linked around 160VA, which is used by most stereo setups.
Lemme know if you are interested.
I have bought the new one from RS as well, and they are fairly fast. Took about 6 days to bring it from the UK.

[chipwelder]

That'll be good for one channel, maybe as testing transformer, mmm... will think about it...

[tangmonster]

The encapsulated one will technically be better if using higher constant load.
The standard one will probably be just fine.

Fyi

I have 3 of those plain 500va's doing various duties in my living room plus 1 x 800va for my subwoofer duties. i can't fualt them.
2 of them are used in a Krell KSA 50 Class A clone.

I have another 2 that I will be using for a replacement 5 channel amplifier.

[chipwelder]

Does one need slow starts for 500VA? providing only one tfo is switched on at a time?