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Output stage emitter resistors
Output stage emitter resistors
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Old 8th August 2019, 08:19 PM   #1
alibear is offline alibear  United Kingdom
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Default 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?
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Alan
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Old 8th August 2019, 09:01 PM   #2
rayma is online now rayma  United States
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Quote:
Originally Posted by alibear View Post
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.
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Old 8th August 2019, 09:44 PM   #3
nigelwright7557 is offline nigelwright7557  United Kingdom
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Wire wound resistors are wound back on themselves to cancel out any inductance.
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Old 8th August 2019, 10:07 PM   #4
anatech is offline anatech  Canada
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Output stage emitter resistors
Hi alibear,
Nigel is right. I've been using non-inductive wire wound resistors for years.

-Chris
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Old 8th August 2019, 10:10 PM   #5
wayne is offline wayne  United States
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Output stage emitter resistors
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.
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Old 8th August 2019, 10:38 PM   #6
JMFahey is offline JMFahey  Argentina
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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.
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Old 9th August 2019, 02:21 AM   #7
anatech is offline anatech  Canada
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Output stage emitter resistors
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.

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Old 9th August 2019, 03:30 AM   #8
Ian Finch is offline Ian Finch  Australia
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Output stage emitter resistors
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.

Last edited by Ian Finch; 9th August 2019 at 03:33 AM.
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Old 9th August 2019, 08:40 AM   #9
Kay Pirinha is offline Kay Pirinha  Germany
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Quote:
Originally Posted by anatech View Post
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!
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Old 9th August 2019, 08:46 AM   #10
nigelwright7557 is offline nigelwright7557  United Kingdom
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Quote:
Originally Posted by JMFahey View Post
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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