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-   -   the power of myth (http://www.diyaudio.com/forums/everything-else/66788-power-myth.html)

darkfenriz 27th October 2005 08:27 AM

the power of myth
 
I cannot tell, using bad words or not so bad words nor in my native language how angry I become why I read something like this:

http://hyperphysics.phy-astr.gsu.edu...dio/imped.html

What is sociological mechanism of these 'truths' still existing? Big mistery to me.... Am I the only one?

Christer 27th October 2005 10:36 AM

It is hardly socialogical in this case, but a misunderstanding of basic electricity theory. Impedance matching is true and useful and tought in introductory courses, but it is not applicable in this case. The theorem says that if we have a source with a fixed impedance, we should select a load of equal impedance in order to maximize the power transmitted to the load. However, one cannot turn it backwards as they have done in this case. Here the load has a fixed impedance, not the source, so the theorem is not applicable.

peranders 27th October 2005 10:42 AM

A good rule: Don't believe everything you read :idea: There is no subsititute for true knowledge and understanding but it's sometimes hard filter out totally wrong "facts" if the source seems to be trustworthy and your own knowledge is little.

My question to the author is: Can you show your theories in the real word? I'll bet he can't.

Christer 27th October 2005 10:47 AM

Actually, this is hardly the first time this misunderstanding appears in audio. The problem is probably that this is a very basic and valid concept that is tought in all introductory courses, but some people are not so good at realizing what a theorem says and when it is applicable. This is misuse of a perfectly valid theorem.

Edit:

I think the theroem even did have a place in audio once. I am not good at valves, but AFAIK a valve amp typically has a rather high output impedance (without op transformer). For equipment with built in speaker, where amp and speaker were designed and selected together, there was probably a point in selecting a speaker with a matching impedance. I am guessing a bit here about valve equipment, but I suppose it was common to do so in order to both avoid an output transformer and to maximize the power into the speaker. The theorem looses its purpose for audio when we buy speakers separately since they have standardized impedance (usually 4 or 8 Ohms). The low output impedance of SS amps further makes the theorem totally uninteresting.

jan.didden 27th October 2005 10:51 AM

It really is hilarious. If the guy would take the trouble to half the amp internal impedance from 8 to 4 ohms he would immediately see an increase in Pout, which would invalidate his whole case.
This is not just stupidity, but it borders on the criminally insane!

Jan Didden

dhaen 27th October 2005 11:05 AM

Stupid indeed..
 
What he's really trying to say is that if the amplifier had a source impedance of 8 ohms then you would get maximum power (into the speaker) if you chose an 8 ohm speaker.
Taking this principle further, we should be hunting for 0.00.. ohm speakers so we can "maximize" our power output.:D

This is a major weakness of the Internet. Anyone can publish without any scrutiny, whether from peer's or publisher. Have you ever tried to find reliable medical data? It's fightening! At least this is "just audio"......

Edit: added (into the speaker)

Christer 27th October 2005 11:24 AM

It would be really frightening if somebody made a marketing thing of this and started to sell 8 Ohm amplifiers, arguing that is optimal for 8 Ohm speakers. I'm sure many customers would think it sounded logical.

Netlist 27th October 2005 11:44 AM

The author is mailable so why not invite him?
http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html

/Hugo :)

dhaen 27th October 2005 01:21 PM

Quote:

Originally posted by Christer
It would be really frightening if somebody made a marketing thing of this and started to sell 8 Ohm amplifiers, arguing that is optimal for 8 Ohm speakers. I'm sure many customers would think it sounded logical.
Well it would be logical if you are in the "no damping" camp, and don't mind throwing away half your power...... Enter Thorsten?.......
I'll be first in the queue to start selling 8 ohm balanced matched interconnects...

phase_accurate 27th October 2005 01:28 PM

The "no damping camp" is a little more refined than you think: They achieve high output impedance by the use of current feedback.


Regards

Charles


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