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Old 23rd December 2011, 03:06 PM   #11
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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.
These four parts grafted onto your existing JLH...
Turn it into square law vs square law Class A, and a lot less heat.

Afterthoughts of this hurried drawing:

Q1 ought to be on or near output heatsink, so it can feel the heat.
BD139 has a mounting hole for such purposes , 2n2222 does not.
2n2222 probably wasn't the best choice I could have illustrated.

The Schottky diodes should never be bolted to the output sink, but
bent away in a slightly cooler location. This (and Q1 at full heat)
assures the shunting will shape a safe temperature coefficient.
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Old 23rd December 2011, 06:27 PM   #12
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Originally Posted by AndrewT View Post
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
You are right 57 watt peak, but KSA remains just an over biased class AB, you can bias a KSA50 as low as a few mA.
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Old 23rd December 2011, 06:28 PM   #13
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Thanks Kenpeter, I will try your idea tomorrow and report on the practical finding -
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Old 23rd December 2011, 07:55 PM   #14
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by Nico Ras View Post
You are right 57 watt peak,
No the peak power output or instantaneous maximum is ~116W into 8r0, from 3.8Apk and 30.4Vpk

It is not a Krell KSA50, if you choose to ignore the manufacturer's set up information and miss-set the bias to less than 1.9A
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Old 24th December 2011, 12:41 AM   #15
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Originally Posted by Nico Ras View Post
Thanks Kenpeter, I will try your idea tomorrow and report on the practical finding -
You might throw a switch across one or both of those Schottkys.
Any less than 0.65V total drop across here will shut off Q1, and
return the circuit back to normal (extremely warm) JLH operation.

A DPST switch might also provide convenient mounting location
for Schottkys? Since there are tempco benefits for these to be
away from the heat that Q1 is sensing.

Even if you got tempco completely wrong, and Schottkys run away:
Our worst case is merely a return to normal JLH, just as-if shorted
on purpose by the switch. Only of concern if cheating with smaller
heatsink than JLH would normally require.

I don't forsee any such problem. As these diodes are TO-220, and
not dropping enough voltage to make significant heat of their own..
Wire loss to such a switch only makes the current sensing more
stable (slightly resistive) , so a remote diode mounting is fine.

Make sure your Schottkys are not rated in excess of the maximum
current you need them to carry. Else you may not be riding on the
best part of the square law curve.

Last edited by kenpeter; 24th December 2011 at 01:00 AM.
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Old 24th December 2011, 10:36 AM   #16
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Quote:
Originally Posted by AndrewT View Post
No the peak power output or instantaneous maximum is ~116W into 8r0, from 3.8Apk and 30.4Vpk

It is not a Krell KSA50, if you choose to ignore the manufacturer's set up information and miss-set the bias to less than 1.9A.....
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.....
The JLH is not a ClassA SE output stage. It has some form of modulated bias.
Whatever...

BTW. I never said JLH is SE, I said the only real class A is SE. Kindly explain the modulated bias you propose for JLH?
You cannot run this JLH in class B, but you can run a KSA50 in class B so KSA remains an over biased class B
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Last edited by Nico Ras; 24th December 2011 at 10:54 AM.
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Old 24th December 2011, 10:41 AM   #17
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Hi Kenpeter, tried it and your idea works. It runs cold, only thing is the distortion is very much audible now. I tried different values of R which changes both the power dissipation and distortion. The higher the dissipation the less the distortion. When I say audible it is quite obvious, probably 5% or more (subjectively).
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Last edited by Nico Ras; 24th December 2011 at 11:00 AM.
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Old 24th December 2011, 11:07 AM   #18
AndrewT is offline AndrewT  Scotland
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I don't understand the active bias part of the JLH.
What I can confirm is that the output bias varies with through put signal level.
But I don't know how to prove that.

As to your other point.
Your SE to PP comparison is in effect claiming that only SE can be ClassA and that PP must always be ClassB or ClassAB.
In my opinion that claim is completely wrong.
I state and firmly believe that both SE and PP can be ClassA.
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Old 24th December 2011, 11:22 AM   #19
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Hi Nico. Andrew is correct, regardless as to whether it is single ended or pushpull, if the bias is such that the output devices are always conducting (and not any strange sliding bias idea) then it is class A.
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Old 24th December 2011, 11:25 AM   #20
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THe JLH is biassed for half peak output current, and being pushpulll this is a proper class A design.
A single ended design has other considerations depending on whether it is choke/transformer coupled/loaded or resistive or constant current fed.
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