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ostie01 24th June 2006 12:12 PM

Metal Oxide vs flamme proof vs metal film
Hi, in amplifier, what is the difference between metal oxide vs flamme proof vs metal film resistors, thanks for any help.

BrianDonegan 24th June 2006 01:14 PM

Metal oxide and metal film refer to the material providing the conduction (with a certain resistance) inside the resistor. Metal oxide type generally have higher power handling than metal film, but at the expense of higher noise (generalization).

Flame proof refers to the material encasing the resistive element (the part you see). Not directly related to the resistor type.

poobah 24th June 2006 01:22 PM

Metal oxides are popular because they are less costly than metal films. Also because the resistivity of the bulk material is higher, less spiralling is required, and this would usually mean less inductance. Oxides can also be made physically smaller. They do not have the best temperature coefficient... around 200 ppm.

Metal films are considered to have sonic virtues by some, and in general will have slightly higher inductance. They can be made more accurately and will exhibit higher long-term stability when operated near rated power. Temperature coefficients are better with 50 ppm being about worst case.

Keep in mind, that the issue of inductance is usually blown out of proportion. There are certain applications, here and there, where low inductance is required though. Physical styles of either type (non-axial, non round) are available for special applications, these can be virtually inductance free. These types are usually flat in form.

Fusibles and flameproofs are for rare applications where overload (failure) is likely... it is generally someone like U.L. telling you where to put them. Oxide type resistors often tout their flameproof characteristics as a bonus... they are oxides after all... already burnt in a sense.

Speaking only to round-axial resistors, as long as you stay away from wire-wounds and carbon-comps, sonic effects should be transparent. Many will insist that only cryo-treated powdered rhino horn resistors will bring sonic euphoria.

Surface mounts are a whole 'nother deal.

ostie01 24th June 2006 01:33 PM

OK with that but why Allen Bradley resistor are so expensive , they are carbone and was used in old high end power amplifier. Flamme proof resistor are often found in the bias and at the base of power transistor. Why they are used in such critical place if they are less sonic than metal film. Just asking and thanks for great reply.

poobah 24th June 2006 01:49 PM

The old Allens Bradleys are sought after by people doing restorations, by guitar amp guys for their gritty sound, by audiophools for their "warmer" sound. They are expensive because they are no longer made and supplies are dwindling, or at least that's what they would want you to believe.

Remember the flame proof is a "bonus"... like "mountain grown coffee"... all coffee is grown in the mountains. The oxides do have lower inductance... this can be critical in base/gate/grid "stopper" resistors.

The sonic difference (there ain't one) between oxides and metal film is a contentious issue and the golden-eared will have strong opinions.

Stick with the cryo-ed rhino horn and you can avoid all the bickering.


anatech 24th June 2006 04:52 PM

Another point to consider. Aside from RF applications, carbon composition have been superseded by other types. There is a reason we went to other types. Noise is a big one, and resistance consistency.

I remember having to buy the next value down and filing a "V" notch in the body of the resistor until I got the resistance I needed. Then sealing the notch with lacquer. You had to use a higher wattage part because the notch reduced the power handling capacity in the resistor. The newer parts were closer to the proper value.

At one time they were about all you could get. They were always expensive. IRC, Dale and all the others.


poobah 24th June 2006 05:14 PM

"hand-voiced" resistors... I hope you charged extra for that!

wrenchone 24th June 2006 07:46 PM

Actualy, flameproof resistors are in yet another class from metal oxide resistors. Your garden variety metal oxide resistor will glow orange for quite a while under severe overpower conditions. Carbon comps and carbon films burst into balls of flame. A flameproof resistor by design opens up quietly without glowing or emitting flame. There are flameproof wirewound and metal oxide resistors, though the metal oxide varieties are more common and less expensive.

ostie01 24th June 2006 08:07 PM

HI, I know metal film resistors are very good in audio application but tought that metal oxide was better due to the price so those good old carbon resistor like allen bradley are only good to those want to restore old amp and want to keep them like factory. Thanks

wrenchone 24th June 2006 08:26 PM

The problem with carbon composition resistors is noise and microphonics. They are basically carbon particles and non-conductive binder mixed and scrunched up in a phenolic wrapper. The good resistors like Allen Bradley have nailhead terminals molded in at each end of the resistor to anchor the leads and contact the resistive mix. Inferior brands have the wires simply poked into each end, so the lead to body interface is a lot more delicate. Conduction takes place at the carbon grain interfaces. This conduction is stochastic in nature, hence the potential for more noise, especially in high resistance values. Vibration can change the conduction pattern, causing microphonics. Carbon microphones work on the same principle. This may be a problem only with resistor that haven't been molded properly. I've never bothered to check it for a better brand like AB. Somebody curious enough might want to make a voltage divider with some carbon comps AC couple it into an amplifier, then tap one of the resistors. I usually just avoid the carbon comps, especially seeing as a lot of people charge a premium for them.

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