Repost: Design Error Time Up.

Hi to all the really nice folk here:


My point is now VERY well proven - it is nigh impossible to identify design errors by just looking at the schem.

What instead happens IS folk see GHOSTS - perfectly good designs are condemned as being flawed in some nonsense way.

Mostly for the simple reason that the arm chair experts have *never seen* one working - necessary to discover how wrong they are.

And even that experience would not convince all here of their dumbest ERRORS !!



The Yamaha P2200 has a badly designed VI limiter - it does a fair job of protecting the output BJTs but prevents the amp from driving even average speaker systems with a 4 ohm nominal impedance to rated power.

A two minute, back of an envelope calculation reveals that fact - IF you know how to do it and what the result needs to be.

This article, by me published 20 years ago, tells the whole, gory story and describes easy to do tests that characterise the issue and give a pass / fail rating without risk to the amplifier concerned.

Fig 4 shows how the P2200 reacts to a 5.6 ohm inductive load with 45 degree phase angle. It is not the worst example of this - the huge, heavy Bose 1800 is twice as bad reacting to a 12 ohm load in the same way.

IMO, only Bill Sloman got even close.

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Who can figure out the simplest, effective fix for the Yamaha amp ?

Must drive 4ohm and 5.6 ohm loads at 45 degrees inductive, no spikes.

Equates to 18amps at peak and 9amps at zero volts, while staying within SOA limits for the BJTs - including time.

Use data for this output device, as used for repairs.

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The P2200:

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..... Phil

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Phil Allison
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