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Old 22nd December 2011, 02:29 PM   #1
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Default Need some help with Class A JLH power parameters

I am going to be building a JLH amplifier. I have read most everything online I can find but I am still confused about a few things. I'll try and list my questions in a way that makes sense but feel free to elaborate with your answer if there is more that should be said.

1) How do you determine the output power of the amp? The way I understand it the available power of the transformer will dictate the output of the amp up to the maximum heat dissipation the output transistors can stand. Is this correct? And if so can you tell me how much transformer is required to safely get the most out of:
A single pair of MJ15003
A dual pair of MJ15003

2) Assuming an 8ohm speaker is being used. What should the voltage and current be set to for the above examples?


Lets start with this and see what happens.

Thanks.


Jeff
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Old 22nd December 2011, 04:46 PM   #2
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Jeff,

You have got it in a nutshell in (1) provided that you understand the Safe Operating Parameters of the transistors in question.

We normally approach a design from a requirement side, in other words what is the requirements for an amplifier not how much can I get from...

I have just completed a stereo JLH amplifier capable of about 12 watt (rms) into 8 ohms resistive load using a pair of MJL4281A transistors per amp. One thing I must emphasise is that you will have to get rid of about 45 watt of thermal energy per channel which is no joke.

For my JLH I use a 30V DC regulated supply and the bias current is set to 1.4 amp. The heat sink is rather large and when running it is hot to touch, in other words you cannot hold it for more than about 10 seconds.

I do not use the fan, because it is too noisy, but with the fan obviously it is ice cold.
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Old 22nd December 2011, 05:09 PM   #3
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In a nutshell, you should consider is that half the supply voltage will be dropped across each output device, so together with the bias current, it should fall inside the DC safe operating area of the device, de-rated by 1.5 watt/deg C temperature rise above 25 degrees centigrade.

Lets assume that you can control the heat to a maximum of 60 degrees centigrade. That would mean 60 - 25 = 35 deg. De-rate the device by 52 watt.

The device at 60 degrees is de-rated to 197 watt. A class A amplifier is about 25% efficient, therefore the peak audio power that you could expect from the device would be one quarter of 197 watt or 49 watt.

The output voltage swing into 8 ohms to realise this power would need to be about 20 volt and current would be 2.5 amp.

Most of the folk here would probably agree that this is very close to the absolute maximum of the devices and two devices would be much safer in the real world with varying ambient and varying mains voltages, etc.

In my humble opinion a 50 watt JLH amp would require a 45V DC supply 2.5 amp bias current and at least two pairs of output devices as well as an enormous heat sink.
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Old 22nd December 2011, 07:08 PM   #4
AndrewT is offline AndrewT  Scotland
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Krell's KSA50 uses 2pair of output devices with large fanned coolers for 50W of ClassA using those transistors and their complementary.
KSA50 has it's bias set to 1.9A (950mA through each output device).
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Old 22nd December 2011, 07:41 PM   #5
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Krell KSA50 is an over biased class AB amp and 1.9^2 x 8 = 28.8 watt. The only real class A would be single ended I guess.
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Last edited by Nico Ras; 22nd December 2011 at 07:43 PM.
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Old 22nd December 2011, 08:44 PM   #6
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The power of the amplifier is defined by the supply voltage and the load.
The transistors should be chosen to deal with supply voltage and the predefined minimum speaker resistance on the output.

The transformer should provide the maximum current the supply voltage will put out into the load plus a bit for losses into the heatsink. For class AB 60% of power goes to load and 40% is lost in heat in the heatsink.
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Old 22nd December 2011, 11:57 PM   #7
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JLH with shunt regulated drive current, and a pair of Schottkys, can shape
square law vs square law (the difference of such curves is a straight line).
Like Class A, no transistor switches fully off. Like AB, dissipates a lot less.

Three or four component shunt can reduce wasted heat a hundred Watts.
Normal JLH class A was never a straight line vs straight line to begin with.
Both curves bent severely by beta droop, and in a heat wasting direction.
My suggestion merely reverses the direction of that curvature...

Last edited by kenpeter; 23rd December 2011 at 12:02 AM.
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Old 23rd December 2011, 04:40 AM   #8
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Kenpeter, give and example of your suggestion it may be interesting to try.
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Old 23rd December 2011, 10:47 AM   #9
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by Nico Ras View Post
Krell KSA50 is an over biased class AB amp and 1.9^2 x 8 = 28.8 watt. The only real class A would be single ended I guess.
I don't know why you stated such.
The Krell KSA50 is a ClassA amplifier that achieves 50W of ClassA output into 8r0.

The maximum output from a single ended ClassA amplifier is equal to the output stage bias current. The JLH is not a ClassA SE output stage. It has some form of modulated bias.

The maximum output from a Push-Pull ClassA amplifier is equal to twice the output stage bias current. Some of the later Krells were not ClassA, they had some form of modulated bias.

If the KSA50 has a bias of 1.9A, then the maximum ClassA output current is <3.8Apk. The maximum ClassA power output is <3.8^2 * 8r0 / 2 < 57W, but not as low as 28.8W
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Old 23rd December 2011, 02:14 PM   #10
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Quote:
Originally Posted by Nico Ras View Post
Kenpeter, give and example of your suggestion it may be interesting to try.
Shunt Regulated JLH
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