F5 C source resistors on the MOSFETs - diyAudio
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Old 16th September 2014, 10:40 PM   #1
potepuh is offline potepuh  Slovenia
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Default F5 C source resistors on the MOSFETs

Hi guys


I need some help, why source resistors on the MOSFETs fukushima 0,47ohm are gone in my F5C after 2 months in one chanel?

I am running the amp at 32VDC. all other parts are ok. Any idea, do I need to increase power of source resistors or their value?
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Old 17th September 2014, 03:02 AM   #2
eyoung is offline eyoung  United States
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One BOM specifies 3W wire wound non-inductive if you are using the "TO220" resistor the may need a heatsink to handle the heat if high biased... do you have a voltage drop across the resistor?

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Old 17th September 2014, 04:39 AM   #3
sangram is offline sangram  India
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I wouldn't use eBay resistors on an F5. Get something proper. Caddock, Mills and even Ohmite are better bets. In very desperate situation you could use two or three metal film resistors in parallel.
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Old 18th September 2014, 08:49 PM   #4
potepuh is offline potepuh  Slovenia
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Originally Posted by sangram View Post
I wouldn't use eBay resistors on an F5. Get something proper. Caddock, Mills and even Ohmite are better bets. In very desperate situation you could use two or three metal film resistors in parallel.
The MPC74 were from ebay, I will never buy resistors from e-bay.
can i use Dale .5R 3W CP-3 that i have on stock or should i buy 598-SLPX223M050H9P3 from mouser.

I saw in more occasion that people use two 1R 5W resistor for this job, should I also go this way? My amp is biased at 2A (1A per mosfet) per rail at 32VDC, I must to decrease a litle because off the heat 1,6A per rail will be ok. F5 power amplifier
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Old 18th September 2014, 09:48 PM   #5
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by potepuh View Post
The MPC74 were from ebay, I will never buy resistors from e-bay.
can i use Dale .5R 3W CP-3 that i have on stock or should i buy 598-SLPX223M050H9P3 from mouser.

I saw in more occasion that people use two 1R 5W resistor for this job, should I also go this way? My amp is biased at 2A (1A per mosfet) per rail at 32VDC, I must to decrease a litle because off the heat 1,6A per rail will be ok. F5 power amplifier
1A of bias through each 0r47 Source Resistor is equivalent to 0.47W. That is only 9.4% of rating.

A 5W 0r47 resistor has a current rating of 3.26A. 2pair output stage allows for a continuous total output rating of 6.52A, (9.2Apk)
These resistors will tolerate an overload of +50% to +100% in the short term.
I would expect 5W Source Resistors to easily manage music and audio reproduction at normal listening levels.
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Old 18th September 2014, 10:04 PM   #6
SonyFan is offline SonyFan  United States
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Quote:
Originally Posted by sangram View Post
I wouldn't use eBay resistors on an F5. Get something proper. Caddock, Mills and even Ohmite are better bets. In very desperate situation you could use two or three metal film resistors in parallel.
I have been impressed with the quality of Ohmite parts using them for restoration work.
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Old 19th September 2014, 12:15 AM   #7
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In the meantime, these resistors are important to the resulting sound in the F5, no matter what any empiricist says.

Based on the findings of Simon7000 and others, it's better to use a) paralleled resistors, or B) high quality resistors and in both cases over-rate the wattage considerably. In this case, especially if the amp will run into class B at any point, or get clipped. Simon7000 did a rigorous test for distortion in resistors, fyi.

Imo, 5 watts is nice - there are Caddock and Vishay parts that are rated much higher (with heatsinks) that have been reported as best by some F5 builders.

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