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-   -   Output stage emitter resistors (https://www.diyaudio.com/forums/solid-state/341208-output-stage-emitter-resistors.html)

alibear 8th August 2019 08:19 PM

Output stage emitter resistors
 
Hi, can either metal oxide or metal film resistors be used in place of wirewound resistors as emitter resistors in a BJT output stage?
Regards
Alan

rayma 8th August 2019 09:01 PM

Quote:

Originally Posted by alibear (https://www.diyaudio.com/forums/solid-state/341208-output-stage-emitter-resistors-post5877482.html#post5877482)
Hi, can either metal oxide or metal film resistors be used in place of wirewound resistors as emitter resistors in a BJT output stage?

Wire wound resistors are more rugged. Metal film or metal oxide types will open circuit more readily due to overloads. This could put a high DC voltage on the speaker.

nigelwright7557 8th August 2019 09:44 PM

Wire wound resistors are wound back on themselves to cancel out any inductance.

anatech 8th August 2019 10:07 PM

Hi alibear,
Nigel is right. I've been using non-inductive wire wound resistors for years.

-Chris

wayne 8th August 2019 10:10 PM

Metal oxide film is a good output stage resistor. Wire wound are inductive unless wound properly and then are a bit pricey. Panasonic has them in 3 Watt size.

JMFahey 8th August 2019 10:38 PM

You can get them non inductive but 99% emitter resistors out there are wirewound for robustness and take no special precautions against inductance since it´usually way too low to matter anyway.

anatech 9th August 2019 02:21 AM

Hi JMFahey,
Actually, I've seen plenty of amplifiers where the inductance of the emitter resistor made the difference between oscillation and stability. Even in my own prototypes, this can make a difference. I was used to designing with the flat Japanese "plate" resistors.

-Chris

Ian Finch 9th August 2019 03:30 AM

It's a question of available power ratings as much as specific amplifier designs where stability is doubtful. There are plenty of low-medium power commercial amplifiers (Naim Audio comes to mind) where standard 2-3W metal film resistors are used, sometimes interchangeably with single layer, wire wound types.

Some designs even use multiple parallel resistors but current sharing during fault conditions needs to be considered carefully. Close-wound wire will usually withstand overloads longer than film types but "Z folded" flat metal ribbon, as used in the Japanese low inductance "cement" resistors, is variable. Original types are usually robust but more recent copies seem to burn out fast during overload, probably because they are unsealed and have little but air to conduct heat away from the wire.

Kay Pirinha 9th August 2019 08:40 AM

Quote:

Originally Posted by anatech (https://www.diyaudio.com/forums/solid-state/341208-output-stage-emitter-resistors-post5877768.html#post5877768)
Actually, I've seen plenty of amplifiers where the inductance of the emitter resistor made the difference between oscillation and stability. Even in my own prototypes, this can make a difference. I was used to designing with the flat Japanese "plate" resistors.

Oh yeah! WW source resistors yet were known as a possible cause for oscillations in the original (four device) ELEKTOR Crescendo amplifier from December 1982. Some months later a workaround was published: A small capacitor between both source legs should solve the issue. The later Mini Crescendo and also the Gigant (close relationship to the Crescendo) provided five 1 ohm 1 watt carbon resistors in parallel instead of one 0.22 ohms 5 watt WW.
Best regards!

nigelwright7557 9th August 2019 08:46 AM

Quote:

Originally Posted by JMFahey (https://www.diyaudio.com/forums/solid-state/341208-output-stage-emitter-resistors-post5877647.html#post5877647)
You can get them non inductive but 99% emitter resistors out there are wirewound for robustness and take no special precautions against inductance since it´usually way too low to matter anyway.

If the emitter resistor inductance is bad why put an inductor on the output of the amp ? In fact the speaker is pretty inductive too.

I have been building amps for 40 years and have yet to see an amp have compensation for emitter resistor inductance.

I have been designing amps for 10 years and have never compensated for emitter resistor inductance. In every case any oscillation found has been due to other factors.


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