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oscilloscope, setting gains


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kamberkash 
Copper - Posts: 89
Copper spacespace
Joined: March 11, 2007
Location: Kentucky, United States
Posted: November 26, 2007 at 1:17 PM / IP Logged  

would there be any drawbacks to purchasing this o-scope? http://www.partsexpress.com/pe/showdetl.cfm?&Partnumber=320-290  the main reason i like it is because it is so freggin cheap compared to some other desktop type scopes. 

the only thing i really need to use it for is setting the gains on my amps, properly.  also i have never used an oscope before, so if i decide to buy it i would like to know how to use it, anyone have any pointer or resources?

boogeyman 
Silver - Posts: 359
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Joined: October 18, 2007
Location: Tennessee, United States
Posted: November 26, 2007 at 1:27 PM / IP Logged  
  go to   www.csie/courses/baict/bac/jf/labs/scope/oscilloscope.html
haemphyst 
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Platinum spaceThis member has been recognized as an authority in Electrical Theory. Click here for more info.spaceThis member has been recognized as an authority in Mobile Audio and Video. Click here for more info.spacespace
Joined: January 19, 2003
Location: Michigan, Bouvet Island
Posted: November 26, 2007 at 1:44 PM / IP Logged  
I have no idea what boogeyman was linking to... I tried .com, .net, .org, .edu. Nothing worked. Wanna try again, boogeyman?
But in answer to your question, no, no problem at all... I have one I use it frequently. One note though, invest also in a set of rechargeable batteries for completely portable use, and the DC adapter for desktop use. The 20 hours for a set of batteries is VERY optimistic!
It all reminds me of something that Molière once said to Guy de Maupassant at a café in Vienna: "That's nice. You should write it down."
boogeyman 
Silver - Posts: 359
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Joined: October 18, 2007
Location: Tennessee, United States
Posted: November 26, 2007 at 3:33 PM / IP Logged  
   sorry about that kamberkash andhaemphyst....try searching the web " using an oscilloscope" .  besiides yor scope should come with detailed instructions...      
kamberkash 
Copper - Posts: 89
Copper spacespace
Joined: March 11, 2007
Location: Kentucky, United States
Posted: November 26, 2007 at 3:53 PM / IP Logged  
yeah, i already tried searching "hot to use an oscilloscope" and came up with quite a few replies.  the only thing i could find is how to use the desktop type scopes, but ill keep searching.  im sure they will have similar functions.  as long as the scope comes with directions, i'll probably be able to figure it out.
i am an idiot 
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Joined: September 21, 2006
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Posted: November 26, 2007 at 4:27 PM / IP Logged  
Alpine Guy 
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Location: Canada
Posted: November 26, 2007 at 4:43 PM / IP Logged  
I have found some pretty sweet deals on desktop units on e-bay before..used of course, but if you plan to use it around the shop and be in and out of your car a lot, the hand held unit will be your best option.
2003 Chevy Avalanche,Eclipse CD7000,Morel Elate 5,Adire Extremis,Alpine PDX-4.150, 15" TC-3000, 2 Alpine PDX-1.1000, 470Amp HO Alt.
kamberkash 
Copper - Posts: 89
Copper spacespace
Joined: March 11, 2007
Location: Kentucky, United States
Posted: November 26, 2007 at 5:21 PM / IP Logged  
http://www.subwoofertools.com/forum/setgain.asp   i found this just looking around the internet.  does anyone see a problem with this method for setting the gains on an amp?
stevdart 
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Posted: November 26, 2007 at 7:40 PM / IP Logged  

It is a correct procedure and that link is a nice little tool for the first timer to save and refer to.  There is, however, the possiblility of error whenever you assume that what is rated is actually what it is.  You can't "know" what the output of an amplifier is, only what it's rating is.  And you can't know what impedance the driver averages over the target frequency band, only what it is close to.  Because a DVC 4 ohm rated speaker is paralleled to make a rated 2 ohm impedance doesn't mean that it is actually a 2 ohm impedance.  It may average 1.7 or 2.4, as examples. 

Use the Ohm's Law calculator in that link and work out a scenario:  put in 1.7 for impedance instead of the rated 2 ohms.  And instead of a, let's say, 500 watts into 2 ohms amplifier rating, put in 466 watts.  These are very real possiblities, and you should be looking to set the amp gain at 28 volts but you are assuming all data as given is flawless and are setting it at 31.6 volts.  That is a situation where you have set the gain so that clipping occurs when volume is turned up loud.  In this case, instead of setting gains correctly after going through all the trouble of making (or buying) a test CD and purchasing a DMM, you have set the gains too high.

Finding actual amplifier output or measuring actual nominal impedance in the target range are difficult procedures, (and I'm not even sure of how the impedance could be measured).  These tests can't be done with a digital multimeter and a test tone CD.

This is why the listening procedure should also be used.  The test CD is employed here, as well, but instead of silently looking at voltage you are listening for the change in the tone (use hearing protection).  The onset of clipping noticeably changes the pitch of the tone.  You would slightly turn the gain back until it sounds clear again.

At this point, do the voltage measurement as a means of double checking yourself.  You might find it's possible that you were hearing something that wasn't there, so by using both tests you should be able to arrive at the highest level of clean tone sound.  If, using the case above, you consistently hear the tone change (clipping) at 31 volts level but not at 28.5 volts level, set it to the lower 28.5 volt level.

Build the box so that it performs well in the worst case scenario and, in return, it will reward you at all times.
kamberkash 
Copper - Posts: 89
Copper spacespace
Joined: March 11, 2007
Location: Kentucky, United States
Posted: November 27, 2007 at 10:43 AM / IP Logged  

thanks for an explanation and not a simple yes or no. 

say i use an oscope to set the gains, and i use a sine wave at 60hz. i would set the gains so there is no clipping, but would this change with different tones?  or does the frequency even matter?

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