"Speaker resistor?"

[teemuk]

I have noticed that many designs have a resistor with a very small resistance (about 0.1 - 0.33 ohms) in series with the speaker load and ground. What is the purpose of this resistor? I know it's there to prevent something but what? Oscillation, power loss on wire, ground loop, RF?

Where should this resistor be located circuit wise? Close to the signal ground point, close to the power ground point, close to the chassis ground point or close to the speaker load?

I know several people have opinions about this. Thanks in advance.

Teemu K

[janneman]

This resistor is not there on its own, rather is will normally carry a small inductor. The purpose of the inductor is to isolate the amp from capacitive loads for high frequencies and thus help stability. The resistor is there as a physical support and also to lower the lf resistance of the inductor wire.

BTW, if several people have an "opinion" on this, how are you going to chose the right one ?

Jan Didden

[teemuk]

Hi Jan,

Originally posted by janneman
This resistor is not there on its own, rather is will normally carry a small inductor.

No. I'm not meaning that resistor. If you look some schematics you'll find a small resistor (or lot's of them in parallel) from the point where the speaker load would "normally" connect to ground. This resistor will have a very small resistance, something like 0.1 ohms while the one in parallel with an inductor is usually 10 ohms or so.

Originally posted by janneman
BTW, if several people have an "opinion" on this, how are you going to chose the right one ?

Well, i guess i choose the opinion that sounds most logical and test whether it is true or not. I haven't seen any discussion about this resistor anywhere and i honestly have no clue why to put it in there in the first place. I was hoping someone could present me a theory. Details like this seem to be overlooked in common audio theory.

Teemu K

[Christer]

It doesn't quite fit your description of connection, but the values you mention makes me think you mean the emitter resistors (or source resistors) of the output transistors. If the output stage has emitter followers (or source followers), the transistors are usually connected to the output via small resistors on the order of 0.1 - 1 Ohm.

Do you have a link to some specific schematic with the resistors you mean?

[janneman]

What you describe could be the sense resistor in a current feedback amplifier. But these are relatively rare. As Christer says, do you have a diagram?

Jan Didden

[darkfenriz]

I guess I saw one in a grouitar amplifier, probably to lower damping as tubes have (pathetic isn't it). Otherwise it could be just a 'model' of cable resistance or something? Or current sensing like Jan mentioned.
just guessing
regards

[teemuk]

Here's few of them...

0.22 ohms in this one:
http://www.mif.pg.gda.pl/homepages/tom/files/65W.gif

0.1 ohms in this:
http://www.mif.pg.gda.pl/homepages/t...es/8240-61.pdf

This one has a network of resistors:
http://www.stlouismusic.com/download...CA/24701C4.PDF

0.1 ohms again:
http://www.stlouismusic.com/download...+/20101H7_.PDF

Teemu K

[sss]

Originally posted by Christer
It doesn't quite fit your description of connection, but the values you mention makes me think you mean the emitter resistors (or source resistors) of the output transistors. If the output stage has emitter followers (or source followers), the transistors are usually connected to the output via small resistors on the order of 0.1 - 1 Ohm.

Do you have a link to some specific schematic with the resistors you mean?

i also think so



if this is the case those transistors are added to insure thermal stability , as u should know , when transistors get hot the hfe increases also , without those resistors the current will rise and nothing will stop it .....eventually causing

[janneman]

Originally posted by teemuk
Here's few of them...

0.22 ohms in this one:
http://www.mif.pg.gda.pl/homepages/tom/files/65W.gif

0.1 ohms in this:
http://www.mif.pg.gda.pl/homepages/t...es/8240-61.pdf

This one has a network of resistors:
http://www.stlouismusic.com/download...CA/24701C4.PDF

0.1 ohms again:
http://www.stlouismusic.com/download...+/20101H7_.PDF

Teemu K

Yep, all are sense resistors for current feedback. The current through the speaker is sampled with the resistors and a voltage representing this current is used for feedback.
Current feedback by its nature tends to keep the output current through the speaker constant even if the load impedance changes, so the amp works as a current source with a very high Zout. Damping is virtually non-existing, therefore sometimes this is combined with voltage feedback to get at least some form of damping.

The last three using a combination of voltage and current feedback. The two last ones seem functionally equivalent.

But, as I said, these are relatively rare. the vast majority of amps don't have this and use voltaedback from the output node.

Jan Didden

[darkfenriz]

yes, all are guitar amps, because guitar speakers are preferably driven by tubes. Forget about it. It is not for Hi-fi, neither make your SS guitar amp sound tubey. I think so..