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Old 18th June 2004, 03:35 PM   #1
angel is offline angel  Norway
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Default FET/BJT hybrid amplifiers

Have any of you tried using bipolars for the output stages of an Aleph, X or SoZ?

I'm looking to build a low power amp, like 5-10W into 4 ohm, for example using the Aleph topology. However, at these power ratings, the Vgs losses of MOSFET transistors get to be really noticeable. So, I'm wondering if it might make sense to use a N-channel MOSFET input, and a PNP output to cut the losses a bit, or whether this would be too hard to do without impacting sound quality.
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Old 19th June 2004, 01:20 AM   #2
MikeW is offline MikeW  United States
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Build a Mini-A (mosfets), JLH (bipolars), or Le Monstre (Fet input Bipolar output). They are all nice amps. Or build all three.
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Old 19th June 2004, 01:37 AM   #3
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Default COMPLEMENTARY JFET FRONT-END, BIPOLAR OUTPUT

Hi Angel,

There is a thread "25 watt class A into 1 ohm" active that uses complementary JFET front-end and bipolar output for modest class A power into low impedances. You may want to simulate and give your comments on this design.
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Old 19th June 2004, 01:42 AM   #4
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You could do all this stuff with bipolar, but since the current
gain of the average bipolar is limited, you usually find yourself
using a Bipolar Darlington arrangement instead of a MosFet.
Once you get the gain up, it works fine.
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Old 19th June 2004, 01:46 AM   #5
angel is offline angel  Norway
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Default Re: COMPLEMENTARY JFET FRONT-END, BIPOLAR OUTPUT

Quote:
Originally posted by LineSource
There is a thread "25 watt class A into 1 ohm" active that uses complementary JFET front-end and bipolar output for modest class A power into low impedances. You may want to simulate and give your comments on this design.
Had a look at it, and my initial thought was "is all this necessary".. It looks like it should measure pretty well, but I'm wondering if anyone has actually built it yet. Complicated circuits tend to yield complicated sonics.

I'll think about it anyway =)
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Old 19th June 2004, 01:52 AM   #6
angel is offline angel  Norway
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Quote:
Originally posted by Nelson Pass
You could do all this stuff with bipolar, but since the current
gain of the average bipolar is limited, you usually find yourself
using a Bipolar Darlington arrangement instead of a MosFet.
Once you get the gain up, it works fine.
I was kind of thinking about using the 2SK389 for input, and a darlington with constant current loading, or a compound follower.

The darlington sort of defeats the entire point, though, which was to reduce losses due to Vgs/Vbe.
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Old 19th June 2004, 02:37 AM   #7
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Vgs will be about 4.5V, and Vbe will be about 1.4V
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