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Old 10th July 2011, 07:43 PM   #21
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OK, could you please tell us specifically what you are trying to do? Do you want to control the volume level with an output attenuator instead of the normal way with a volume control in a preamp for example? I'm just trying to get my head around what you goal is.

Mike
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Old 10th July 2011, 07:59 PM   #22
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yldouright: do an A/B test of a system as you suggest compared to a direct connection of the speaker to the amp. If you can not hear the difference, well than...., if you can find a speaker that "does not like" the damping, let us know!
This thread is going in circles and I am getting dizzy! E
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Old 11th July 2011, 01:49 AM   #23
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mickeymoose
I guess moose like to chase their tails around. The only thing circular in what I have stated is the shape of the pot wiper. Seriously, I've read a tighter speaker should be okay with no damping ratio. I've even heard some people say they prefer it because it sounds less compressed, is this incorrect?

Michael Bean
I am investigating the pros and cons of attenuating an amp at its outputs. It seemed to be a way to avoid amplifying noise. I know it has been unpopular and I would like to know if there are new methods or technologies that can make it a practical design choice.

janneman
When I opened this thread, I invited the possibility of any type of attenuator but since the discussion was veered to the common pot, let's examine this first. Doesn't the assertion that current is flowing through the pot to ground assume that the pot is connected as a shunt? Isn't there a way to tie in the pot without it wasting the output to ground?
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Old 11th July 2011, 09:06 PM   #24
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Quote:
This is where the noise of the pot would be of the smallest relative value to the signal, right? What are the issues that make this practice undesirable?
and
When I opened this thread, I invited the possibility of any type of attenuator but since the discussion was veered to the common pot,
I'm certainly willing to disengage my contribution then.
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Old 11th July 2011, 09:53 PM   #25
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OK, putting aside all of the arguements against your idea for being impractical, you cannot improve signal to noise ratio at the back end of the signal chain. As was pointed out in an earlier post, decent quality modern amplifiers don't normally add significant noise, and attenuating the signal at the output won't do anything to reduce noise that has already mixed into the signal, it just lowers signal level and noise together. If you really want to lower noise level (or improve S/N ratio) in you system, you must do it at the front of the signal chain. I highly recommend you take another look at the link that Jan Didden provided in post #15 about Gain Structure, lots of good info there. Frankly, what you want to do, and how you're trying to do it just won't work.

Mike
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Old 11th July 2011, 11:28 PM   #26
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I am with sofaspud (hand me a can of beer and some chips).

Whenever I have one of those Eureka moments I tend to get out the soldering iron, not the keyboard.

Oh, btw, moose don't chase their tails, they are to short (the tails, that is). Dogs do: like in beeing doggedly stubborn.

Ciao, E
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Old 12th July 2011, 06:22 AM   #27
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After my last post, I realized this is in the Software Tools forum. Maybe that is the invite referred to. But a hardware/software solution seems to me even more far-fetched.
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Old 12th July 2011, 11:30 AM   #28
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sofaspud, mickymoose, Michael Bean
This thread was opened to explore amp attenuation. Theoretically, it is better to NOT amplify noise so I started out looking at the rump end of the amp first. You have chimed in your negative comments with only sideswipe justification of your positions. The issue of power dissipation in the attenuator was left unresolved as was the issue of loudspeaker damping and output impedence. If a person cannot accept a blanket dismissal of an idea without logical reasons to do so, does that make him stubborn? It may turn out to be truly impractical to go this route but that does that mean we shouldn't look more closely at it?
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Old 12th July 2011, 02:23 PM   #29
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Quote:
Originally Posted by Michael Bean View Post
[snip] Frankly, what you want to do, and how you're trying to do it just won't work.

Mike
Ohh, it'll work, no reason it shouldn't.
Lets get practical: an 8 ohms speaker connected to the wiper of a pot which is driven from the output of the power amp.
Normally you would chose the pot resistance about 1/10 of the load so that the load doesn't influence the pot curve. So, we need a 0.8 ohms pot. Hmm. That IS impractical...
OK lets fudge the rule of thumb, lets take a pot only 1/2 the load, so we get a 2 ohms pot.

Right. Getting there. Now, we would want say 50W in the speaker with the pot turned all the way up. The amp sees, at max level, a 2 ohms pot parallel to an 8 ohms speaker, which is 1.6 ohms total. For the 50W in the 8 ohms speaker you need 20V RMS, which means there will 10A RMS through the pot and 2.5A through the speaker. The pot will dissipate 200W, and 50W will go to the speaker.

Now turn the pot way down; nothing goes to the speaker but you have still 10A through the pot so the pot still dissipates 200W. At intermediate settings, the pot dissipation will vary but will always be quite high.
But nothing stops you from doing it.
Just get a 2 ohms, 200W pot, mount it such that it doesn't cause a fire hazard, get an amp that can drive 20VRMS into 1.6 ohms continuously, and you'r done. Piece of cake

jan didden
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Old 12th July 2011, 02:50 PM   #30
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Apparently, what we have here is a failure to communicate.

First, you said it yourself,"Theoretically, it is better to NOT amplify noise", but it isn't merely theory, it's a fact. What you're not seeing is that once it's come through the "rump end" of the amp, the noise HAS been amplified and you cannot remove it with an attenuator. Think of it this way, suppose you are in a noisy environment trying to talk on the phone. If the person on the other end can't understand what you're saying, you have two options, either talk louder (improve S/N ratio) or go to a quieter environment (noise reduction), the person on the other end, the "rump end", cannot do anything to improve the situation by turning the volume down on thier end, they can only adjust the level of the unintelligible garble as a whole. Did you revisit the link that was provided about Gain Structure?

Second, we have not chimed in our negative comments with only sideswipe justification of our positions. I provided you with diagrams and simpified calculations to show you how attenuators work, did you read it and do the math? Again, if you put power through a resistor (attenuator) it turns that power into heat, and with more power you'll get more heat, and it will affect loudspeaker damping. It's physics, not a blanket dismissal of an idea without logical reasons.

Third, it has been looked at more closely already and it turns out to be truly impractical to go this route. I recommend you go back and re-read what has been already posted here and study the info more carefully, you might gain some insite as to why what trying to tell you is true.

Fourth, nobody here is trying to beat you up, we're trying to help you understand, but until you're ready to let go of your misguided preconceptions, this will go nowhere. It seems the old saying that "you can lead a horse to water, but you can't make him drink" applies here.


Mike
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