Emitter Resistors

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Hi,
for your range of power it looks like a 1pair or 2pair output stage is suitable.
I suggest about 3W wire wound.

I have seen on this forum a suggestion that the emitter resistors should be non inductive but I cannot confirm this.
If non inductive is important then wirewound is not suitable unless they are specially wound to eliminate inductance. I believe a few manufacturers can supply this specification.
 
It is neither expensive nor particularly complicated to make them non-inductive if one does that this way as I am doing last 20+ years. If you use metal-film 1% 1/2W resistors, this way you have practically non-inductive 5W resistor that occupy les space and is much cheaper than some exotic "audiophile-grade" types. Please note that there are TWO emitter resistors of 0,1 ohm on the picture! This might be useful idea for someone.
 

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Hi Bora,
that piggy back layout on the unetched board looks really clever.
Well done.
It's one I may copy, since I bought 1000 1r0 some months ago but put off using them due to board space issues.

Does the back to back layout help cancel the small inductance?
Is it the parallel combination that reduces the small inductance?
 
Hi AndrewT,

>Does the back to back layout help cancel the small inductance?<
No, they are positioned that way mostly for saving space. In most cases this arrangement occupy less space than standard single resistor of comparable wattage.
>Is it the parallel combination that reduces the small inductance?<
Exactly! As you know, inductances are behaving like resistors for that matter, hence 10 equal inductances in parallel results in total inductance of 1/10 of any single one. Good quality MF 1% resistors already have very low inductance and with this arrangement total will be 10 times lower! I tried many times to compare this arrangement with commercial Hi-End resistors, but neither instruments nor my ears were able to detect any difference in amplifier performances or characteristics worth mentioning (and spending money).
 
What would be a good choice for emitter resistors in a 50W-100W design?
Kind has been discussed.

As to value 0.1 to 0.47 was suggested - somewhere else on this forum this was discussed (good reading, sorry I do not have the link at my finger tips) and John Curl suggested a voltage of 15 to 25 mV (better double check that number) across the resistor and various references to why (Gilbert I think and Doug Self and others if memory serves, Blomley (sp?) maybe). So rather than ask with a power we should be asking what bias current you will be using, or pick a R value and then set the current.

This value will give the best linearity in a class ab design, use a bigger resistor with more voltage drop and it should give you better temperature stability all other things being equal - but go for the linearity.

Bill
 
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