the capacity of a electrolytic capacitor.

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Pim Driessen

Member

• Feb 4, 2020

• #1

Dear people,
With existing components I want to build a Class A amplifier, if transformer is used Amplimo 68014 - 2 x 18 volts 6.25 A, wants to use double bridge cells 8 x DSEI30, now electrolitic capacitors have to be purchased.
There are two options for not spending too much money: Mallory 25000 uF 75 volts or Kemet 47000 uF 63 volts, what is wisdom in this?
Kind regards,
Pim

 

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Nigel Goodwin

Super Moderator

Most Helpful Member

• Feb 4, 2020

• #2

Basically bigger is best - and don't forget you will need substantial heatsinking, as you'll be wasting a huge amount of heat.

 

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Pim Driessen

Member

• Feb 4, 2020

• #3

Dear Nigel,
The cooling profiles are fine, see photo.
Then I go for the 47000 uF 63 volts.
Thank you.
Kind regards,
Pim

 

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audioguru

Well-Known Member

Most Helpful Member

• Feb 4, 2020

• #4

It looks like a completely symmetrical class-AB amplifier. Maybe its P1 control can be turned up so its heating is 87W in class-A when its maximum audio output is only about 32W into 8 ohms.

 

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Pim Driessen

Member

• Feb 4, 2020

• #5

Dear Audioguru,
That's right, it's an AB amplifier, but I screw the quiescent current up to 1.7 Amps, 32 watts is enough.
Greeting,
Pim

 

audioguru

Well-Known Member

Most Helpful Member

• Feb 4, 2020

• #6

Why heat your home with the amplifier? When the no bias current in class-B is increased a little into class-AB then the crossover distortion is eliminated.
That is why there are very few class-A amplifiers made.

 

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Pim Driessen

Member

• Feb 4, 2020

• #7

Dear Audioguru,
That is a good question why a Class A, I think they just sound good.
But when this amplifier is ready I will put it to the test and let it work in class A / B.
What should the quiescent current be 500 MA ?.
Greeting,
Pim

 

audioguru

Well-Known Member

Most Helpful Member

• Feb 4, 2020

• #8

For only 32W output it does not need 6 output transistors. A pair of output transistors eliminate crossover distortion when they idle at about 25mA.
But since transistors have a wide range of spec's, adjust the idle current while looking at a sinewave output on a 'scope.

 

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Pim Driessen

Member

• Feb 4, 2020

• #9

Dear Audioguru,
I already adjusted the schedule.
And definitely do the experiment with 2 transistors per channel.
I will certainly implement that with the oscilocope.
Greeting,
Pim

 

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unclejed613

Well-Known Member

Most Helpful Member

• Feb 5, 2020

• #10

Pim Driessen said:

What should the quiescent current be 500 MA ?.

Click to expand...

for AB biasing, should be between 15 and 30mA (between 6 and 14 mV measured from emitter on + side output device to emitter of the - side output device). you could probably safely go as high as 50mA (22mV across emitters). the best way to set it properly is using a distortion analyzer and at 1 or 2 Watts at 20khz. adjust the bias to where the crossover distortion disappears.

 

audioguru

Well-Known Member

Most Helpful Member

• Feb 5, 2020

• #11

With a +/-25V supply and an idle current of 500mA the output transistors dissipate 25W all the time.
With 25mA the idle power is only 1.25W, a huge difference.