Emitter Resistors, Please Help a Noob Understand!

[50 Green]

The emitter resistor leads on my Sony STR-7055 are corroded. I tried cleaning them off in place, but that didn't work.

My next thought was to pull them, clean the leads, and flow some solder over the leads to prevent further corrosion... but if I pull them, plate resistors are cheap, so I might as well just replace them.

So here's my question: I've read several threads here (and other sources) that say there's no benefit in using "audio grade" (expensive) resistors for emitter resistors. What I don't understand is that the emitter resistors seem to be directly in the signal path. Please correct me if I don't understand, but doesn't the signal flow from the outputs, through the emitter resistors (and protection) directly to the speakers?

I'm happy to avoid the expense of fancy-schmancy resistors... I just don't understand why every other component in the signal path is critical, then it comes to the final one and suddenly "anything will do."

Can anyone help with an explanation?

Thanks in advance!

 

[mbz]

Generally, I'd agree with what your saying, the properties(inductance, linearity,,,) of the emitter resistors are important. I've not seen those posts, I'm
guessing what they are saying is that the sonic difference between a "good" emitter resistor and a "basic" one does not justify the boutique prices.
However, if I needed a replacement and these boutique items were maybe 50% more expensive then I'd take a closer look at the specs. If they were
5-10 times the prices...

 

[avionic]

Emitter resistors are just to force current sharing.Nothing fancy is required. Name brand wirewound's work and sound fine.

 

[50 Green]

Thank you both for your quick replies.
KOA .47 ohm plate resistors (like replacement) are 50 cents. Mills "audio grade" non-inductive wirewound are something like 8x that!

Emitter resistors are just to force current sharing.Nothing fancy is required. Name brand wirewound's work and sound fine.

Thank you. I understand (somewhat) about forcing current sharing.... The part that confuses me is that (if I'm not incorrect) the signal passes directly through the emitter resistors on its way to the speakers... Is it the case then that whatever part of the signal isn't resisted, passes "unmolested" through the resistor, as though through a straight wire?

Again, I have no qualms about saving money on resistors... Just trying to wrap my noob mind around the concept.

Thanks again!

 

[mbz]

KOA .47 ohm plate resistors (like replacement) are 50 cents. Mills "audio grade" non-inductive wirewound are something like 8x that!

How much inductive is the standard 0.47 ohms? Not enough to make a difference, maybe in an ultra high end amp...

The part that confuses me is that (if I'm not incorrect) the signal passes directly through the emitter resistors on its way to the speakers

Correct, the positive part of the audio signal/half cycle is combined with the negative half cycle at the junction of the emitter resistors. I think the emitter
resistors also provide a load to help keep thermal runaway in tact.

Is it the case then that whatever part of the signal isn't resisted, passes "unmolested" through the resistor,

I disagree with this, all parts of the signal are "affected" by resistors. Ideally only the amplitude is reduced, however due to a small amount of inductance
there is a very small phase shift and additinal impedance, questionable if these are noticable

 

[elnaldo]

I also think that audio grade resistors or components will be more noticeable at the 1st stages of the amplifier circuit, where small differences will be amplified hundred of times.

I don't think that 0,47 ohm at the power amp output will make an audible difference. I know somebody who fitted 22 ohm resistors at that position (misreaded the 0.22 ) and never noticed anything.

I'm talking about home audio quality. I know in this kind of forums you can find very high standars from very demanding people with hi end equipment. I can't tell the difference in that evironment.

 

[gadget73]

At the frequencies involved, the inductance won't hurt anything. Its actually very vaguely possible that removing the inductance could contribute to making the amp unstable. Thats a big stretch, but the inductance will tend to slightly reduce the maximum bandwidth of the amp. We're talking way above the audible frequency range at this point. Extending it further could allow some goofy behavior that wouldn't have existed otherwise.

That said, is this a functional or a cosmetic thing? If they are working, honestly I would vote for leaving it alone.

 

[50 Green]

Thank you all for your replies.

How much inductive is the standard 0.47 ohms?

I should have mentioned that the 0.47 KOA plate resistors are listed as being low-inductance in the data sheet

I also think that audio grade resistors or components will be more noticeable at the 1st stages of the amplifier circuit, where small differences will be amplified hundred of times.

That seems perfectly logical.

That said, is this a functional or a cosmetic thing? If they are working, honestly I would vote for leaving it alone.

They are working at the moment (or at least the receiver plays), however there is a lot of green crystalline corrosion on the leads of at least 2 of the 4 emitter resistors. (They are on very long exposed leads, I presume to bring them off the PCB for cooling purposes.) I don't know whether the corrosion will continue, nor whether it might affect performance in any way.

I tried to remove the corrosion with the resistors in place, but wasn't able to get in closely enough with anything to clean it off... thus my initial idea of pulling them, removing the corrosion, and flowing some solder on the leads to protect against further corrosion..... but at 67 cents apiece for the replacement plate resistors, if I'm putting an order in anyhow (which I am) it seems easier and more economical to just replace them once they're out.

My question about "Why not 'audio' grade' since they're in the signal path?" came up partly out of curiosity, and partly from wondering whether resistor technology might have evolved since the 1970s in such a way that there was a better replacement.
I'm happy to stay with the 67 cent version, if that's as good as any!

Thank you all again!