Resistor tolerance on Chipamp.com LM3886 boards

[AndrewT]

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.

[GOR3]

Originally Posted by Hogwild

the usual online retailers (DigiKey, Mouser and a local one) do not have anything better than 5% components.

I just looked at Mouser Canada's site. They offer 36 different metal film, through hole 22.1K resistors, 1% or better, in powers from 1/4W on up.
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.

[simonra]

Originally Posted by Hogwild

Uh..that was a lot of info, thanks, but I'm still confused. I don't know what tolerance Chipamp.com people used. Does anyone know? I figure I'll just use whatever they used.

Some of you are saying there are plenty of 19.6k or 20k resistors in lower than 5% tolerance 1W or more. Can you please link, cause I'm just not seeing that at Digikey or Mouser Canada, and I checked about 5 times for each. There are listings for them, but they display as ou tof stock with a lead time of 16 weeks. That doesn't help me.

As others have said here it's more important to get closely matching resistors for each of the boards (Rf on one board close in value to Rf on the other board) than it is to get an exact value against the schematic. So if you have a pair of resistors that both measure 22.1k then you've got a good match, use one in each board. If they are way off and you can't get 1% resistors, buy a handful of 5% ones, measure them and use the ones closest in value to one another.
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).

[abraxalito]

Originally Posted by Hogwild

Chipamp.com didn't specify how many watts any of the resistors were. And since I know nearly zero about electronics, I'm at the mercy of you kind folks.

Take it from us kind folks then - there's absolutely no need for 1W rating in these resistors. 1/4W is absolutely fine.

[Fusor]

Originally Posted by AndrewT

which resistors are shown as 1W in the chipamp circuit?
link to your schematic so that we can see what you are looking at.

Here is the pdf users guide that was once on the Chipamp.com site :
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.

[abraxalito]

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.

[asuslover]

Originally Posted by Hogwild

None. Chipamp.com didn't specify how many watts any of the resistors were. And since I know nearly zero about electronics, I'm at the mercy of you kind folks.

My friend , you will find help here, but some(NOT ME ) will jump and ask you : "how come you are a member of diyaudio.com for 7 years but have no clue about the simplest lm3886/75 amp?"
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.

[Mark Whitney]

A little help from Google.


http://datasheet.octopart.com/5033ED...eet-118557.pdf

SFR16S0002212FR500 Vishay / BC Components | Mouser Nederland

[ApolloZhang]

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

[tomchr]

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