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#11 |
R.I.P.
Join Date: Jul 2004
Location: Scottish Borders
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calculate the current through each resistor, using I = V/R, write your results on your sch for testing checks.
Then calculate the resistor dissipation using Pq = I^2 R I suggest no resistor is asked to dissipate more than 50% of it's maximum power rating, i.e. for 600mW 1% metal film resistors the Pq<300mW The NFB pair (upper and lower) need to be very accurate/non variable. To help maintain this non variablity arrange for the Pq <<50% of rating. The lower NFB usually has no problem. It's the upper that can be asked to dissipate a lot of instantaneous peak power when the output gets near clipping. I usually suggest this upper NFB resistor has a Pq <10% of maximum rating. Max Voutpk = sqrt{Power *2 * Rload} For a 50W into 8ohms amplifier, the max Poutpk = Sqrt{50*2*8} = 28.3Vpk A 20k NFB upper resistor will pass an instantaneous peak current of 28.3V/20k = 1.4mApk Peak instantaneous power = 0.0014*0.0014*20000= 0.04W = 40mW = 6.7% of the 600mW maximum rating. No need for a 1W resistor.
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#12 | |
diyAudio Member
Join Date: Jun 2011
Location: Connecticut
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Quote:
In 22.0K, Mouser has on offer 23 resistors, 1% or better, 1/4W or more. You might take a second and gain and little more facility with Mouser's site. ![]() |
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#13 | |
diyAudio Member
Join Date: May 2016
Location: Bournemouth UK
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Quote:
I think all of that has already been said by others in this thread, hopefully this makes it easier to understand. I don't know the schematic but I used 1/4W all round apart from a 1W from the LM3886 output pin as part of the Zobel network (pin 3 to low value resitor to 0.1uf to ground). Last edited by simonra; 16th October 2017 at 07:09 PM. |
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#14 |
diyAudio Member
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Take it from us kind folks then - there's absolutely no need for 1W rating in these resistors. 1/4W is absolutely fine.
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'The total potential here must be nothing less than astronomical.' 'Nothing less. The number 10 raised almost literally to the power of infinity.' |
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#15 | |
diyAudio Member
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Quote:
diyaudioprojects.com/Chip/LM3886_CA/lm3886-manual.pdf Page 6 has a parts list. The amplifier section Zobel resistor is shown as a 2.7 ohm 2 watt. The power supply section has a couple of 2 watt's also. Last edited by Fusor; 17th October 2017 at 02:55 AM. |
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#16 |
diyAudio Member
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The OP has confined the resistor question to R2, Rf and R3 on this particular thread. None of those need to be higher than 1/4W.
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'The total potential here must be nothing less than astronomical.' 'Nothing less. The number 10 raised almost literally to the power of infinity.' |
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#17 | |
diyAudio Member
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Quote:
![]() 22k 1watt resistor is not necessary, you should use 1/4 or 4/10 or 5/10 or 6/10 watts , and 22k instead of 22.1k 1% resistors will make no difference in that schematic. Last edited by asuslover; 17th October 2017 at 03:24 AM. |
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#18 |
diyAudio Member
Join Date: Feb 2013
Location: Netherlands
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A little help from Google.
http://datasheet.octopart.com/5033ED...eet-118557.pdf SFR16S0002212FR500 Vishay / BC Components | Mouser Nederland
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Regards Mark. |
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#19 |
diyAudio Member
Join Date: Feb 2015
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1% tolerance is a must, wont cost that much compared to 5% carbon composites... And the feedback resistor has some serious influence on the performance on your amp. D.Self had published an article over thermal induced distortion, and in my own experience the difference between resistors could be easily heard when used in feedback loop. So go for the ones with low current noise and parasitic inductance.
Two of these high grade resistors per amp wont make you bankrupt anyway ![]() At the moment I`m using Vishay ptf56 (or 65? not sure) for their low tempco, next step will be the Z-foils lol Last edited by ApolloZhang; 17th October 2017 at 08:56 AM. |
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#20 |
diyAudio Member
Join Date: Feb 2009
Location: Calgary
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Usually ±1 % resistors are the same cost as ±5 % types. For surface mount resistors, you can even get ±0.5 % resistors for the same cost as ±5 % types. I see no reason not to use ±1 % types.
Definitely use metal film resistors (regardless of tolerance) for the lowest THD. The resistors in the Zobel and Thiele networks are generally 2.7 Ω, 2 W and 10 Ω, 2 W, respectively, though I'd make the Thiele one of a lower resistance for better transient response. Metal-oxide (MOX) resistors are fine for this application. Tom
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