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Old 30th June 2003, 06:15 PM   #1
Borc is offline Borc  Slovenia
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Lightbulb Leach amp bias diodes

Hello!

Anyone tried to replace bias diodes which are hard to mount with TO-220 transistors in B-E connection? That will be easier to use with "normal" heatsink.
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Old 30th June 2003, 06:20 PM   #2
mlloyd1 is offline mlloyd1  United States
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I've done that for other amps in the past (not the Leach). Works well. It is definately more convenient...
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Old 30th June 2003, 06:32 PM   #3
Borc is offline Borc  Slovenia
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You mean use of standard Vbe; one transistor mounted on heatsink. I was thinking to change diodes in series with transistors connected b-e in series.
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Old 30th June 2003, 06:38 PM   #4
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Quote:
Originally posted by Borc
You mean use of standard Vbe;
better yet, you could use amplified diodes: tie the base with the collector. it is said to generate sharper cut-off than a traditional diode.
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Old 1st July 2003, 05:21 AM   #5
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Default Re: Leach amp bias diodes

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Originally posted by Borc
Hello!

Anyone tried to replace bias diodes which are hard to mount with TO-220 transistors in B-E connection? That will be easier to use with "normal" heatsink.
You need only a PN-junction so a normal transistor will do a "diode". You could also use a "normal" Vbe multiplier, two resistors and one transistor. I suppose Prof. Leach has chosen a more complicated Vbe multiplier in order to get a more accurate compensation but the three part solution will also work. Maybe you will get lower currents when the amp is warm, not constant, but this isn't a big disadvantage.
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Old 1st July 2003, 05:46 AM   #6
andy_c is offline andy_c  United States
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Default Re: Re: Leach amp bias diodes

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Originally posted by peranders

I suppose Prof. Leach has chosen a more complicated Vbe multiplier in order to get a more accurate compensation but the three part solution will also work. Maybe you will get lower currents when the amp is warm, not constant, but this isn't a big disadvantage.
Here is the explanation from Dr. Leach's website of why he chose the diode approach:

I like the idea of having the Vbe multiplier on the circuit board to minimize stray capacitance to ground from the output of the second stage. This leads to better stability. Remember, the second stage sets the dominant pole in the open-loop transfer function. I don't want that transfer function to be a function of unpredictable wiring capacitance to a Vbe multiplier on the heat sink. Therefore, I opted for the diodes on the heat sink. The stray capacitance of the leads to the diodes can be isolated with series resistors. You can't use the series resistors if you put the Vbe multiplier on the heat sink.
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Old 1st July 2003, 06:21 AM   #7
BrianGT is offline BrianGT  United States
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The diodes aren't that difficult to deal with. Just put them on the heatsink near your devices. Here is a picture of the latest leach amp that I helped a friend build, using newer MJL1302A/3281A devices.

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Old 1st July 2003, 06:52 AM   #8
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Default Re: Re: Re: Leach amp bias diodes

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Originally posted by andy_c


Here is the explanation from Dr. Leach's website of why he chose the diode approach:

I like the idea of having the Vbe multiplier on the circuit board to minimize stray capacitance to ground from the output of the second stage. This leads to better stability. Remember, the second stage sets the dominant pole in the open-loop transfer function. I don't want that transfer function to be a function of unpredictable wiring capacitance to a Vbe multiplier on the heat sink. Therefore, I opted for the diodes on the heat sink. The stray capacitance of the leads to the diodes can be isolated with series resistors. You can't use the series resistors if you put the Vbe multiplier on the heat sink.
It's correct that stray capacitance is a no good thing, therefore keep the wires short, valid in most parts in the amp.
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Old 1st July 2003, 09:30 AM   #9
sajti is offline sajti  Hungary
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Quote:
Originally posted by BrianGT
The diodes aren't that difficult to deal with. Just put them on the heatsink near your devices. Here is a picture of the latest leach amp that I helped a friend build, using newer MJL1302A/3281A devices.

--
Brian

Douglas Self recommended to put the diodes to the top of the output devices. This gives better sensitivity, and less capacity, due the large distance from the heatsink.

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Old 2nd July 2003, 03:47 AM   #10
AJT is offline AJT  Philippines
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'Anyone tried to replace bias diodes which are hard to mount with TO-220 transistors in B-E connection? That will be easier to use with "normal" heatsink.'


i actually use the base collector junction, yes it can be done, i find them to be more siensitive due to the nature of construction of the transistor, i use case 77 types mostly....
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