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Old 9th July 2005, 11:18 PM   #1
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Default Emitter Resistors

I need to order some emitter resistors for an amp. 0.33 ohm, 5W. I planned on getting some Yageo axial resistors from Digikey, but for the 0.33 ohm 5W resistors, there is a choice between wirewound and metal oxide film.

Shouldn't emitter resistors be wirewound? Or does it matter?
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Old 9th July 2005, 11:27 PM   #2
jaycee is offline jaycee  United Kingdom
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I have seen people argue the non-inductive/metal film type are better. I doubt it makes any big difference. Either would be fine, and the wirewound ones are more common mainly due to price.
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Old 9th July 2005, 11:29 PM   #3
Mr Evil is offline Mr Evil  United Kingdom
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Avoid wirewound ones. Inductance in the emitters can be a source of instability.
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Old 10th July 2005, 12:08 AM   #4
pooge is offline pooge  United States
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Mills makes non-inductive wire wounds in 1% tolerance.
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Old 10th July 2005, 12:12 AM   #5
Eva is offline Eva  Spain
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Wirewound is good for most audio amplifiers, but some circuits require low inductance emitter resistors for stability.

Circuits with local feedback loops closed around the output devices (CFP), with MOSFETs, or with very high unity-gain bandwidth products (>4Mhz) are likely to oscillate when wire-wound resistors are used.

Note that the oscillation may show only in certain circumstances, for example near clipping, so careful testing is recommended.
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Old 10th July 2005, 12:27 AM   #6
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hi
wirewounds are better powerwise
however for presicion hifi i recommend metal oxide
because we want to minimise the effects of wirewounds
at high frequensies

cheers
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Old 10th July 2005, 04:42 AM   #7
Eva is offline Eva  Spain
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Sometimes I've solved stability issues by paralelling a 1 ohm 1/2 watt low-cost resistor with each wirewound 5W resistor. That small resistor appears to prevent the inductive part of the main resistor to dominate at high frequencies >1Mhz, thus improving phase margin.
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Old 10th July 2005, 06:41 AM   #8
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I just wanna match the stock ones. Since this is a late-70's contraption, I'd assume they are wirewound.

If I was changing them all, I'd go for the metal film. As hard as these are to get to, I think I stick a few wirewound resistors in there and call it a day.

Appreciate the input guys...
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Old 16th July 2005, 06:48 AM   #9
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Quote:
Originally posted by Eva
Sometimes I've solved stability issues by paralelling a 1 ohm 1/2 watt low-cost resistor with each wirewound 5W resistor. That small resistor appears to prevent the inductive part of the main resistor to dominate at high frequencies >1Mhz, thus improving phase margin.
That sounds like a good idea, I'll try that. Also, what about a capacitor across it?

I have a question, being there's a lot of mixed views in here about the inductance of the wirewounds, and also you mentioned CFP.

My subwoofer amp I just built is CFP, but I'm using ceramic wirewounds because they are what I had, and are 0.15 ohm 10W which is perfect for a big amp with a 4 or 2 ohm load. Now while building this amp, I noticed I had stability issues when I didn't use resistors at all, but I changed that along with a few other things, and the amp seems really stable with the resistors. Haven't had any extra heat, or any biasing problems, and have run it to +/- 52V so for just testing.

Should I worry about having a CFP with ceramic wirewound output resistors, or is the low 0.15 ohm rating not going to make much inductance to worry about? Also, for the 10 outputs, they each have a compact, low inductance, wirewound 0.13 ohm 3W resistor, that's the size of a 1W film resistor.
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Old 16th July 2005, 08:38 AM   #10
AndrewT is online now AndrewT  Scotland
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Hi Eworkshop,
Why the extra RC leg in the Thiel network?
The parallel Vbe multipliers should not be nescesary unless you are monitoring two heatsinks. The voltage and therefore the power across the CE legs is very small and heating here is never a problem.
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