2.7R zobel Resistor...Watt Rating?

[Wynand]

I've recently bought an Audiosector pcb from Daniel (Thanks man, I't arrived yesterday), but I've got a "problem".

I've got a 3w 2R7 resistor... way too big.
What is the minimum watt rating that can be used?

You can give me the formulas and I'll do the maths myself.
The problem is that I dont know how to calculate it.

Thanks
Wynand

PS I did search but theres too many results (posts per thread)

[Peter Daniel]

I'm using 2W resistors and they fit the board. Also, Zobel components are optional and you don't really have to use them.

If 3W resistors don't fit the board, you may install Zobel circuit across binding posts, p2p (preferrably not too far from amp's circuit)

[peranders]

If you don't plan to test the amp at full power and the highest frequency a plain metal film 0.6 W will do.

In my QRP02 Gainclone I have three 1206 SMD resistors = 3 x 0.25 W

[Wynand]

Thanks. I got some 1W resistors yesterday that'll do for now!

[mrlots2do]

Questions for Peter Daniel, Peranders or any guru.
Q1
I've read about parallel chips having a .22 ohm resistor in series with speaker + or single chip having .1 ohm resistor. Nowhere can I find a watt rating best used for this application.
Q2
Since this resistor is in series with speaker+ I was thinking best to use non-inductive resistors. What do you guys recommend?

Partsexpress only has this value that comes close Dayton 004-.33

[peranders]

Answers from one of the gurus

Q1: take the current at max power P=I*I*R

40 V peak into 4 ohm => 7*7*0.2= 10 W.... but 2-4 Watts will do for "music".

Q2: 0.1 ohms doesn't have particulary much inductance and this is less probably than the inductance of the speaker wire.... so a "inductive" resistor will work as much as a "non-inductive" one.... my opinion.

[jackinnj]

Originally posted by Peter Daniel
Also, Zobel components are optional and you don't really have to use them.

The Overture Series chips with their SPIKE protection will oscillate under certain thermal conditions unrelated to input voltage -- they do a good job of protecting themselves, not your speakers. If you are going to omit this low pass filter then make sure that the device is very securely attached to the heat sink --

PA -- The 2.7R resistor won't really have a lot of VDC across it as it is in series with a 100nF capacitor to ground (if you are following Nat Semi's schematic.) So the current through this resistor is a function of frequency. There is a lot going on in this chip when it is under stress, however.

Complicated answer with the same result though.

[gmilitano]

Sorry to dredge up an older post, but I have the same question. What should the zobel resistors be rated at?

By default, it seems that most are using 2W. However, Nick over at DD has used a LM3886 with 2.7R/0.25W, 0.1uF and reports no problems.

http://myweb.tiscali.co.uk/nuukspot/...lone_3886.html


Also, RJM has used a LM3875 with 1R/0.25W, 0.22uF

http://www.geocities.com/rjm003.geo/...io/diy_gc.html


So having said that, I have the following question.

Can someone cite a reference where you calculate the value of zobel components and power dissipation across the resistor?

Cheers,
Gio.

[AndrewT]

Hi,
the formula was quoted earlier.
P=I^2*R and also P=V^2/R
0.3Aac dissipates 250mW in a 2r7 resistor.
That is equivalent to just 822mV to the speaker.
Sending that 822mV signal to the speaker at very high frequency when the impedance of the cap in the Zobel is low, then the Zobel resistor is at maximum dissipation of 250mW.
That 822mV HF signal is just 84mW of output power.
The only thing stopping the Zobel resistor burning out is the impedance of the 100nF cap. At 20kHz the cap impedance is about 80ohms.

I reckon a 250mW Zobel resistor is maybe just a bit too small.

Four 10r 600mW metal film in parallel give a low inductance 2r5 2.4W resistor. Two mounted above board and two below board would fit easily and fairly neatly.

[Nordic]

K if you want to see pictures of the dead speaker and burnt resistor I can go take it... this resistor's rateing is of no importance, untill it needs to do its job! last time I use a low wattage resistor in a zobel that is for sure... I was lucky to find a replacement driver.