Xref: utzoo rec.audio:31334 sci.electronics:19891 Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!zaphod.mps.ohio-state.edu!mips!cs.uoregon.edu!milton!sumax!polari!miker From: miker@polari.UUCP (Mike Ranta) Newsgroups: rec.audio,sci.electronics Subject: Re: Transitor deterioration Message-ID: <3961@polari.UUCP> Date: 5 May 91 01:55:12 GMT References: <18030315@hpfcdj.HP.COM> Distribution: usa Organization: Seattle Online Public Unix (206) 328-4944 Lines: 31 In article <18030315@hpfcdj.HP.COM> myers@hpfcdj.HP.COM (Bob Myers) writes: >>Can anyone tell me if the power transistors used in ampifiers degrade >>over time? What I'm getting at is: will it be necessary, after x number >>of years, to replace the transistors to get that "like new sound". I >>know that with tube amps, you must eventually retube. > >I would expect other components in the amplifier to degrade far more quickly >than the power transistors; compared to some parts, the power transistors >might as well be immortal (assuming they don't fail catastrophically). This is an interesting question. Supposedly, bipolar transistors are prone to a phenomenon known as "secondary breakdown". This occures when they're driven near the limits of their "safe operating area" as the output devices in audio power amps often are if you like it loud. Secondary breakdown is where the current flow becomes localized within the silicon "die" instead of being uniform. Bipolar devices conduct "harder" as they get hotter and hence you end up with local "hot spots" on the die. Supposedly each time this happens it alters the gain and other key properties of the device. This is one of the big "claims to fame" of MOSFETS--they're immune to secondary breakdown and hence will not degrade. If the amp manufacture did their homework, the current limiting circuitry should keep the output devices within their "safe area" and this shouldn't be much of a problem. For amps that are rarely driven hard and driving relatively "easy" loads, it shouldn't be a problem regardless of the design. Also, only the output devices would be prone to this problem. Next time I'm working on an older amplifier, I'll test some of the output transistors and compare them to a new example of the same part. This is only relative, however, as you really need to test the SAME part when it's new and after several years of hard use... Comments?