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Old 1st February 2007, 03:00 AM   #1
traw is offline traw  United States
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Default aleph bias questions

I can' t recall the exact resistor number, but the 221k on an aleph 5, or 47k on aleph 3 one for fine tuning the bias. My diy aleph 5 runs with 0.54 volts across the 1 ohm source resistors. In some searching read that the 221k could be removed, infinite. What would aleph 5 dc bias climb to, approxiamately?

Seems could handle some more heat and loved the mini biased hard. In an aleph 3 changed the 47k to 68k and raised bias from 2 to 2.5.. wonder if there is a similar expectation on the aleph 5.

Also... anyone else notice difference in bias when using ztx450 versus mpsa18? In my experience, seemed mpsa18 runs harder. Had a mini biasing at 2amps with mpsa18 in place but fell to 1.6 with a ztx450 in there all other things being same - unless i was imagining things.
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Old 1st February 2007, 04:04 AM   #2
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Assuming that I'm understanding which resistor you're talking about, the voltage will rise to the Vbe of the NPN in the Aleph current source. This will be something on the order of .65V, plus or minus a bit.
Yes, different transistors run at slightly different voltages.
Also, if you take one of the NPNs and play with it as a separate entity, you'll find that Vbe is actually slightly dependent on Vce. If you take the same circuit and run it at different rail voltages, you'll find that Vbe will vary by a hundredth of a volt or two depending on the rail.
Ain't nothing to worry about.

Grey
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Old 1st February 2007, 01:30 PM   #3
traw is offline traw  United States
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also, does lowering the parallel resistance of the grouping of 0.47 resistors increase bias? (sorry, should search and dig some more) for instance, had blindly used 6 0.47's on the output clump of resistors solely becuase didn't have a schematic or knowledge of aleph30 when making and saw it being done on a pic in diyamp aleph gallery.
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Old 1st February 2007, 03:39 PM   #4
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If you mean the parallel resistors at the output, no. However, you can change the bias by adjusting the value of the resistors under the Sources of the output MOSFETs.
Note that this should lead to reconsidering the values of the resistors in the output array--not because of bias, but for the proper functioning of the Aleph current source.

Grey
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Old 2nd February 2007, 02:14 PM   #5
Manu is offline Manu  Europe
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Quote:
Originally posted by GRollins
However, you can change the bias by adjusting the value of the resistors under the Sources of the output MOSFETs.
Note that this should lead to reconsidering the values of the resistors in the output array--not because of bias, but for the proper functioning of the Aleph current source.
Grey
Grey:
Could You tell us how to calculate this values (of the resistors in the output array)?
ThX
Manuel
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