oscilloscope, setting gains

would there be any drawbacks to purchasing this o-scope? https://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?

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.

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?

haemphyst wrote:

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Huh? Can you elaborate on that, just a little bit? Your question really makes no sense to me, whatsoever...

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who are you directing this comment to, Haem?

kamberkash wrote:

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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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i am an idiot wrote:

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Hey Aleckxs if you look really close at the "In Use View" picture that I found on the partsexpress page, you will notice that they do have the ground of the probe connected to the circuit board. The probe of the unit is a typical scope probe with the ground wire that clips to the ground of the probe. My Fluke 123 also requires a ground but it is a seperate wire. This one is built into the probe.

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