Uranium, some of the advice you are getting is obviously guesswork. The circuit details are not being referred to, as few people have them. The optimum EF bias point is, I believe, 26mA current, measured as around 6mV across each output emitter resistor of 0R22, if they are used. However, that is not specified here.
A DMM is fine for DC bias setting but the advice to get professional service for a bias circuit fault will be worth the cost. These are not cheap, new or used and one more assumption here may result in just a piece of junk.
A DMM is fine for DC bias setting but the advice to get professional service for a bias circuit fault will be worth the cost. These are not cheap, new or used and one more assumption here may result in just a piece of junk.
I usually run 50-75mA PER pair of output transistors, which is around 25-30mV across 0.44 ohms (emitter to emitter)
Ian, for class AB amplifers (and I am assuming this is what we have here) the optimum bias setting is indeed 26mV across the the emitter dgeneration resistors. So, for a 0.33 Ohm resistor, you get about 78mA and for 0.22 Ohm about 118mA etc. The 26mV figure comes from a paper written by an HP engineer Barney Oliver in the 1970's and this figure quoted by Self, Cordell et al.
Some designers don't follow this too much to the letter and bias higher (more class A) or lower (cooler heatsinks) or just because they prefer the sound with a particular bias setting. Separately, the dynamic bias conditions in EF2's and especially EF3's move around significantly depending on the output stage thermal design and the short term dynamic dissipation, so in fact a class AB amplifer spends a good deal of its time in either over or under bias.
Some designers don't follow this too much to the letter and bias higher (more class A) or lower (cooler heatsinks) or just because they prefer the sound with a particular bias setting. Separately, the dynamic bias conditions in EF2's and especially EF3's move around significantly depending on the output stage thermal design and the short term dynamic dissipation, so in fact a class AB amplifer spends a good deal of its time in either over or under bias.
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BTW, with TMC, you can bias up much lower and still get respectable distortion performance - not that I advocate that personally.
Thanks Bonsai, you are absolutely right as Mr Self describes and I have erred by scaling directly from Vbias.
Odd, because I must have read the chapter 50 times. Hmm....got a few readjustments to do now 😱
Odd, because I must have read the chapter 50 times. Hmm....got a few readjustments to do now 😱
John Curl is correct gentleman, 15 mv is enough, seems all forget that there is a intrinsic resistance on the emitters and probably degeneration on the bases too.
Homemodder you are right. You have to consider 're as part of emitter degen resistor for the 26mV setting.
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