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help!ab class mosfet amp
help!ab class mosfet amp
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Old 19th September 2005, 11:31 AM   #31
mbonus is offline mbonus  Croatia
diyAudio Member
Join Date: Jun 2005
Location: Osijek
it's me again...
i finally finished this amp
and i had a small problem
when i tested my amp..
I didn't make original power supply yet
so i tested it on +/-12v 200VA (only this
i've found at my home..)
problem was that I heard a very small(but it's here)
buzz from the speaker and I don't know what is wrong.
The signal was from my soundcard so i thought this may
be the problem but when i put av400 on my soundcard,there
are no problem with buzz.
Is the power supply of +/-12v maybe a problem?
everything else works great. pretty clear and dynamic sound
except this buzz when i turn down the volume.. .
and another thing-it is very loud to me for this power supply.
(much louder then my old 18W hi-fi..)
is this ok?
thanks everybody!

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Old 19th September 2005, 04:14 PM   #32
ilimzn is offline ilimzn  Croatia
diyAudio Member
Join Date: Feb 2005
Location: Zagreb
The buzz (50Hz or 100Hz) is likely hum pickup from th emains line or the transformer stray field, and probably has to do with your wiring. People often assume that wires can be routed anywhere, but this is not true, though the subject where they should go, is quite complex if expressed as a set of do's and don'ts. The idea is to recognise the loops where current flows and understand that current through them will induce a magnetic field, as well as a magnetic field will induce current in them. To avoid this, areas described by the loop must be kept as small as possible, and of course, non-overlapping if we want no interaction. Secondly, things like ground lines DO have resistance and voltage drops - large currents through the ground will make potential differences on the ground wire, which may be picked up by amplifier inputs, feedback loops etc.

Regarding class of operation, in this amplifier it depends solely on the bias current of the output stage, so it's a matter of how you adjust it, given the required output into the given load. That being said, it is NOT the same what current you adjust. For one, increasing it, increases heat dissipation. At +-40V power supply, class A operation for a 4 ohm load would require idle current equal to maximum load current, about 8A, therefore 640W of heat dissipated, clearly well beyond what a pair of IRFPs can do, and indeed even if they could, the heatsink would be enormous (there is no escape from physics...). The same is true for the original design since it uses even lower rated transistors. Therefore, you can chose various settings in class AB. That being said, there is an 'optimum' idle current for the given load, but in the case of driving a speaker, which is a complex load, the value is mostly a compromise arrived at by lots of listening tests, and heat generation/reliability considerations.
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