if using single pair lateral output for balanced amp..... are you saying they could also be the obvious choice for the tougher low impedance load on a balanced amp ?
meaning it would be relatively easy and simple to make a quite strong 100watt(balanced)
Or from another angle :
70W/8 continuous power comes to about 100W/4 continuous.
Peak output in 4 Ohm is then 200W.
Picking 200W total Pd for the output stage enables it to handle full output in 8 and 4 Ohm.
Instantaneous output will never exceed the total Pd number, and the output stage can handle bursts in lower impedance loads.
(do not last long, and as such do not heavily load the output stage in a thermal sense)
Pd of an output device does not directly correpond to output power, but to dissipation. (wouldn't make much sense to place a d after the P otherwise
) But as an approximation, Max dissipation level is taken at half output power level : half the rail voltage is across the output device, half Vrail across the load.
But what did I mention about average/continuous Die temperature : at 50% between 25C ambient and 150C kill zone.
At 87.5C Die temperature, see the datasheet Pd/Tc gaph, allowable dissipation is 50% : a 100W device reduces to a 50W one.
So 200W datasheet Pd in devices reduce to a 100W dissipation capable output stage.
Tada : Max dissipation of an output stage that is capable of handling a continuous 4 Ohm load !
So an easy&quick way to thumb-rule check whether a 4 ohm capable Lateral Mosfet class AB output stage is properly dimensioned :
Just total the nominal Pd number, divide it by three, and compare that to the continuous output power figure in 8 ohm.
Examples ;
250W/8 Mimesis 9 : 6x125W=750W. Divided by 3 makes 250W. OK !
140W/8 Crescendo Elektor : 4x100W=400W. Divided by 3 makes 133.3W OK!
350W/8 Mimesis with 3-pair J56/K176 : Not OK.
Don't need no stinkin calculator.
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to get the best audio performance from this amplifier, you should use a separate transformer for each channel and power consumption as large as possible.The capacitor need a small amount, maximum 2 capacitor 10.000uF for a channel.
for example: 45V - 0 - 45V/10A for the left channel
45V - 0 - 45V/10A channel.
thanks
I didnt understand well your question , to give an exemple
the original Hitachi commercial amp circa 1978 used a single
T03 100W TDP output pair for about 75W RMS/4R and the amp
was measured by an audio magazine as 112W RMS/4R , wich
is in the hedge of the devices max current and well above
what is exctractible from a single BJTs pair.
I did some sims of the GM as a comparison with Gaborbela s
project that use a similar topology although a little more complexe,
there s some significant simmed tests in the last pages.
Simulations are reliable enough but final results depend
on components choice and physical implementation.
Jacco , i agree with all your calculations set apart that i was talking
of 70W whatever the load....
Mr Wahab,
you know very well that they're not "calculations", but again a (very crude) simplification.
A complete description would include more parameters, some general, others that apply to a specific layout.
But all those just distract from the point at hand : that it's both extensive experience from many years of practice and can be evidenced on paper.
Source resistors at the Mosfet outputs would take a small part of the output stage dissipation.
Not using separate rails with higher voltage for the drivers would imply that the output can be driven less closer to the rails, leading to a higher relative dissipation of the power devices.
Phase shift between output current and voltage makes the calculation even more complex.
Then there's charging ripple on the rails, which' influence works in two directions.
(I'm an attacker if I copy a few lines. When I write a sentence, I'm a bad joker. When I post six pages, I'm bound to be carried out of this thread in tar & feathers. )
you know very well that they're not "calculations", but again a (very crude) simplification.
A complete description would include more parameters, some general, others that apply to a specific layout.
But all those just distract from the point at hand : that it's both extensive experience from many years of practice and can be evidenced on paper.
Source resistors at the Mosfet outputs would take a small part of the output stage dissipation.
Not using separate rails with higher voltage for the drivers would imply that the output can be driven less closer to the rails, leading to a higher relative dissipation of the power devices.
Phase shift between output current and voltage makes the calculation even more complex.
Then there's charging ripple on the rails, which' influence works in two directions.
(I'm an attacker if I copy a few lines. When I write a sentence, I'm a bad joker. When I post six pages, I'm bound to be carried out of this thread in tar & feathers.
)
There will be Separate transformers for each channel.
Today there is one 625VA for each, wich is too small.
I have two ways to alter this.
Either buy two 1000VA (will give me 9A for each channel, Or just add two more 625VA, wich will add up to about 11A for each channel.
If I choose the 1000VA transformers, I will have to buy a pair of transformers wich is around 25% more costly than two extra 625VA transformers.
The 625VA transformers will then be in house, but the 1000VA transformers will still be in Poland. Freight from Poland for theese transformers probable would buy me yet another extra pair of 625VA.
My choise: Another pair 625VA transformers. And about 11A available.
The capasitor bank is determined.
Original idea was a pair of 10000uF onboard the amplifiers, in addition a pair og 1000uF + 2X20pcs 470uF. Still for each channel. A total of 60000uF each channel. There was to be extra filtration, either via inductor, or via a form of PTC-reactive element. (Bulbe)
The 470uF is dropped out of the planning, but the first 10000uF is replaced by 2X22000. Theese will be closer to the rectifier.
Va some resitive or inductive devices the rails will be fed to the amplifier boards, where the onboard 2X10000uF sits, and provide the peak pulse capability.
This arrangement I have used for tears and years on PA Amplifiers (Up to 500W/channel) with good results.
Until anyone can convince me this is a bad idea, I will build it this way. But pleaswe come forward i anyone have some well funded reasons I should not.
Agree.
GM9.2 is thew reference here.
For those of You who realy know: Is the output transistors on the 9.2 isolated from the heatsink, or have the just made this arrangement for attaching the heatsink to make it possible to mount the transistors directly to the heatsink, no mica between?
I have used such configuration to several PA-amps where the heatsink has been totally covered by the cabinet, no danger of having it shortcirquited to anyting on the outside, neither any danger for being in touch with potensially dangerous voltages on outside areas.
Would further improve the C/W, wich can be a good thing.
When I post six pages, I'm bound to be carried out of this thread in tar & feathers.
Some of us would the clean You up, and gently throw You in again.
Undoubtly you are right but solving constrained equations , although
not difficult , is quite boring on a DIY perspective , best is to oversize
the things reasonably and it will spare yourself quite some maths hassle
and unreliability issues , after all only commercials designs are "optimized"
to save costs as much as possible , squeezing the last (perhaps) available cent..
Unless you need a PA amplifier an option is to stick with what you
have at hand , 625VA/channel is good enough for 450W RMS/channel
on a permanent output power , 500W RMS/channel can be reached
by a musical program without being as power consuming seen from
the power supply.
Mr Wahab,
not just DIY.
Long long time ago I had to design a platform of a navy divers vessel, for the scuba guys to embark/disembark from with their gear and/or inflatables & RIBs.
As a 2nd year engineering student, I fiercely started setting up all kinds of differential equations, for an accurate calculation to the last digit.
After half a day, the fulltime construction design guy (with a entry level degree) handed me a book with approximation formulas.
Told me to multiply that by a factor 2 or 3, as the marines wouldn't mount the platform if it looked too shabby.
And the exact size material wouldn't be in store anyway, he said, ordering it through the official government channel would take too long and be more costly. (and that was a multi-million conversion project, me also handled the cost estimate)
Commercial audio appears as one of rare few industries where undersizing or making the mark by a hair's breadth is Not unusual.
I strongly disagree with You. Or do I really?
Try look at the simplest of figures. If I want a continious 250W in 8 Ohms, I need a continious currentflow from the transformer of 5,59A, Wich by a fraction is excactly what the 625VA transformers I have is capable of delivering.
Yes, I will probable find the 450W RMS somewhere in an way, but they tell me nothing of what I can expect
I have no particular interrest in making any Watt-War here, but I stick to the FTC-norm when calculate and meassure watts.
Funfact:
And by using one of SONYs ways of determining output power, I probably a couple of kilowatts too.
But still I stick to my thumb-rule, saying approx 3 times the estimated continious poweroutput. 625 is a bit too small, 800 would have been perfect.
I guess I will get my 258W as the maximum continious power.
All in all I guess I will be happy with that.
On the other hand, I am not quite so sure my wife will be.
2,58W would be hers preferance. Anybody heard similar?
Thanks. I am actually relieved.
One never should try that experiment without making all precautions to avoid severe damage.
The 450W figure wasnt pulled exactly from thin air...
The 5.59A are drained from each rail ALTERNATIVELY , so the
average current from each rail is half this value at 250W/8R ,
in fact a single 625VA xformer is enough for two channels
granted the amp is not used at full power for extensive
timings.
I am afraid You have the maths some wrong here.
To produce the 44V AC needed to produce 250W one need the full power from both rails.
This means alternatively 5,59A from each rail. And there is not room for too much more.
A full curve of 44VAC measures 125V peak to peak.
Actually one already at this point can expect some distortion as the peak level, 62,5V ends up in a peak current of nearly 8 Amps. Luckily there is some capasitors present to aid this to happen.
To produce the 44V AC needed to produce 250W one need the full power from both rails.
This means alternatively 5,59A from each rail. And there is not room for too much more.
A full curve of 44VAC measures 125V peak to peak.
Actually one already at this point can expect some distortion as the peak level, 62,5V ends up in a peak current of nearly 8 Amps. Luckily there is some capasitors present to aid this to happen.
Actualy the maths is right providing we are talking of a single channel...
The upper side of the push pull will suplly the 5.6A half of the time ,
during positive going of the signal while the bottom side will conduct
only on negative cycles of the signal , hence the power supply rails
will be sollicited alternatively, the average current drained by the amp
on each rail will be 2,3A.
On a class AB/B biaised push pull each side conduct only 50% of the time ,
the non active side conduction being negligible.
The upper side of the push pull will suplly the 5.6A half of the time ,
during positive going of the signal while the bottom side will conduct
only on negative cycles of the signal , hence the power supply rails
will be sollicited alternatively, the average current drained by the amp
on each rail will be 2,3A.
On a class AB/B biaised push pull each side conduct only 50% of the time ,
the non active side conduction being negligible.
According to profusions application note: The power supply should be capable of supplying the module with D.C. equivalent to 150% of the RMS output power. The mains transformer should be sized according to expected usage. Hi-fi amplifiers use a transformer rated at approx 1.5 x the
total output power (in VA). For a stage or P.A. amplifier the factor should be 2 - 2.5 times the output power. Oversize power transformers will
also enhance the sound quality.
total output power (in VA). For a stage or P.A. amplifier the factor should be 2 - 2.5 times the output power. Oversize power transformers will
also enhance the sound quality.
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