Transformer Power Rating

[atheos]

Hello guys,

In the Projects->audio section
(http://www.electronics-lab.com/projects/audio/017/)
there is this amplifier project to be built.Somewhere in the middle there is the power supply schematics link (marked bright red).The supply to be constructed gives output +-40V/8A.The transformer should give 2x28V/8A.

My question is when ordering the transformer of 2x28V 8A,8A should be for each winding or for total?

Thank you in advance
atheos

 

[audioguru2]

Hi Atheos,
I don't see how that amplifier can provide 100W continuously into 8 ohms with low distortion when using only a dual 40V supply. 100W into 8 ohms requires a sine-wave of 80V peak-to-peak. The amplifier's Q10, output emitter followers and emitter resistors all create a voltage loss so the output certainly won't be 80V p-p.
The amp will probably provide 85W into 8 ohms or 140W into 4 ohms.

Even a dual 28V transformer won't produce a total of 80VDC at full power after rectifying and filtering. It will produce about 72V to 76V. Maybe that's part of why an underpowered 8A transformer is specified. Only 3.5A will be drawn from the transformer when the amp is providing 140W into 4 ohms and heating itself 70W. With only a 3.5A load on an 8A dual 28V transformer, its output voltage will probably rise to the dual 30VAC required to make plus and minus 40VDC. But don't use a dual 30V transformer or a transformer with a lower current rating, its output voltage will rise too high at idle. The extra power from the 8A transformer can be used to power a second amp for a stereo system.

Instead of using a transformer with two separate 28V windings, a transformer having a center-tapped 56V/8A output will work the same but will probably be less expensive.

The 4700uF filter capacitors are too small for good bass response, low hum/buzz at full output or for a stereo amp. Use 10,000uF caps.

I hope you are not building the amp from its schematic. It has a serious error where Q5 is shown with its base and emitter shorted. The PCB is correct. I have corrected the schematic for you:

View attachment 36492

 

[Kevin Weddle]

The 8Amps is the maximum RMS current that the wire of the transformer can withstand. This holds true for both the primary and the secondary. You might be tempted to use the average value but I think it is the RMS.

 

[audioguru2]

Hi Kevin,
8A is the transformer's maximum continuous RMS rating for its output at "normal" ambient temperature and not totally enclosed.
Transformers are not rated for "average" current but for your information, the average voltage or current of a sine-wave is 90.099 percent of its RMS value.

If an 8A transformer was used in this project to power a stereo amplifier, at full continuous output into two 4 ohm speakers the transformer will be providing 7A. Playing music or speech with the loudest peaks causing the amps to reach clipping, the average RMS current for a stereo amp will be only 1.41A to 4.44A so the transformer will be warm but certainly not hot.

 

[atheos]

Hello again,

I would like to thank audioguru and kevin for taking a look in my problem.To be honest I didnt get the answer I was looking for but you solved a problem I never saw (thanks to audioguru for the schematics)
Since english is not my mother language

a)I will assume that a "center-tapped 56V/8A" is a transformer with a primary and secondary windings ( that means no dual voltage)
b)To my first question again.When I see
e.g 2x28V/8a,the 8A is for the total current or for each winding?

Thank you for your patience
atheos

 

[audioguru2]

Hi Atheos,
Since each amp will provide 140W into 4 ohms and 70W to heat itself, a stereo amp will require 420VA from the transformer.
Therefore the transformer must be also rated for at least 420VA, which when divided by 56V equals 7.5A.
If you used a transformer with two 28V secondary windings each rated at only 4A, the transformer will provide only 224VA which is not enough.
The secondary windings are in series, when two 28V windings are connected together with a center-tap, so each must pass 7.5A. ;D

 

[crflorin]

Hello guys! (I'm new around here )
I've built this amp and after adjusting the current to 50mA I've went further with the signal. Everything was great... the sound was verry verry good but in less then a minute the power transistors broke down! this happened twice!!
what should I do!!!
What could be the problem!!!! :'(

 

[audioguru2]

Hi Crflorin,
Welcome to our forum. ;D
I think your amp experienced "thermal runaway", where the transistors heated causing them to conduct more, causing more heating which caused them to conduct even more, causing even more heating etc.

The project says, "The transistor Q7 is used to balance the circuit at different temperatures and must be mounted on the heatsink between the out put transistors." That's your problem, Q7 must be mounted on the heatsink. ;D

 

[ljmnunes]

I hope you are not building the amp from its schematic. It has a serious error where Q5 is shown with its base and emitter shorted. The PCB is correct. I have corrected the schematic for you:

 

[manblood]

Hi everyone
I'm kinda new at this kind of stuff. Could u please reply my questions?

- I can't understand the L1 part. should ibuild itor buy it Iasked a few placec but i couldn't fand How can i make it?

- And I couldn't find the part BDW84D but i found BDW84C. Can i Use instead of it.

Thanks...

 

[audioguru2]

- I can't understand the L1 part. should i build it or buy it? I asked a few places but i couldn't find. How can i make it?

The project doesn't say its value so you could calculate it, then buy one and wire it in parallel with its resistor. The poor translation messes-up its simple construcion instructions.

- And I couldn't find the part BDW84D but i found BDW84C. Can i Use instead of it.

Look at the difference in their voltage ratings, and what happened in the other guy's amplifier that tried it.

 

[Mickey1]

Huh...guys...I'm having a dilemma here...I'm building a 8Ohm version, so I was wondering what about the R3,4,17 and 23 - I don't put them, right? Insead of them, do I connect the pins where they shoud be? I short them?

And what is that about BDW83 & 84"C" instead of "D"? What will happen? Because I've got the same problem... I bought the "C" version.

Please reply and...thanks!!!

 

[manblood]

I still can't understand  :-[

now i'm gonna buy 1w resistor and wire which is 0.5mm and wrap it around the resistor and wire them paralell.
is it true?

But what is the resistor's value(how many ohms)? and must there be a space between turns?

 

[audioguru2]

I still can't understand

 

[manblood]

Thank you audioguru

i think i'm gonna try it with 1ohm 1watt resistor

I'm looking pictures to figure out what is placed on place of L1 part  ;D.
this is a different picture:

 

[audioguru2]

The coil is not wound around R14. It is wound on something smaller like a 1W resistor but that resistor isn't connected to the circuit.

 

[manblood]

if it is not connect to the resistor (it makes sense to me) i think it's wound on resistor because its core must be carbon.
I asked my friends about it and they say it hasn't got value(henry) because you can not buy that value

Then I'm gonna buy a resistor (i think its value doesn't change anything maybe important thing is its size, maybe itmust be 1w resistor size) and wound 0.5mm wire on it and try it.
But i will do it one week later because i have exams.

Thank you very much audiguruyou are very helpfull
(And sorry if there is wrong sentences English isn't my primitive language)

 

[faizanbrohi]

Hello , Everyone i build this project but because of the two wrong darlingtons , i bought BDW84A and BDW83A and a little inconsistance i am building it again.
i want to just confirm that i have wound the inductor 10 turns on 1W resistor of 1.5Kohm i found from my electronics junk and the wire is 24SWG copper wire enamel coated (24SWG = 0.55mm). i will send some pictures of it so please see audioguru if i have done the right thing .

 

[faizanbrohi]

Here are the pictures (only of inductor ) . you might see that the project is not fully assembled.  but just see the inductor wound around 10 turns of 24SWG(0.55mm) copper wire.

View attachment 38359

View attachment 38360