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Ferrite beads on output transistor base leg

istari_knight

pfm Member
Why ? Something to do with oscillations :confused:

I've read about it in the past on diyaudio but cant for the life in me remember what was said or indeed find the relavent thread.

NAD do it....

ntw_may_2010_ctxt-2.jpg


Just curious :)
 
They buffer the previous stage of the amp against the capacitance of the transistor. Resistors are usually used as gate/base stoppers. Beads in this application are discussed in the Fetzilla thread on DIYA.
 
Yes, those are on the base inputs of the transisors.

Emitter-followers like to oscillate at VHF given half a chance especially with 'fast' transistors like those Sankens. Adding a little bit of resistive loss in at least one of the leads is a good way to control that tendency. Sometimes you see small-value resistors used here but ferrites do the same thing nicely.
 
I think the discussion of beads as gate/base stoppers is well intot he thread, around page 100 or so, so you'll have to do some hunting!

There's some interesting discussion on how varying the value of the stopper resistor affects the bandwidth of the amp, in conjunction with the output impedance of the VAS. Someone suggested trying beads instead of resistors, but I can't remember the outcome. I'm pretty sure the parameters of the bead are critical though to both eliminating oscillation and optimizing the bandwidth of the amp.
 


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