Just starting to learn o-scope, F5 1k square looks funny - diyAudio
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Old 29th February 2012, 01:14 AM   #1
JZatopa is offline JZatopa  United States
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Default Just starting to learn o-scope, F5 1k square looks funny

So I bought an O-scope years ago to work on amps and it has been collecting dust. Today I pulled it out and decided to play with it a bit. I don't have a signal generator so I decided to use the 1khz square wave generator that is built into the scope. First I tested the scope itself and it seems to be working fine. Next I tested my aleph 1,7P that I bought off the site and the square seemed close enough to the input for me. The last thing I decided to test was my F5. Now I don't have a large 8ohm resistor so I used a 8ohm tweeter. I know this may not be the best load but it was what I had on hand. Now the amp sounds pretty good to me but I have had a sneaking suspicion that something is wrong somewhere in my system somewhere. I am just not sure if it is my source, preamp or my amp. Attached is what the output of the scope looked like. I had to kind of rig everything up so there may be something throwing everything off right now but I plan to buy some extra cables tomorrow and retest everything. Does this look like a problem with the amp, my setup or something else?



* There is a good chance I am doing something wrong here. I will post more pics tomorrow after I retest.
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Old 29th February 2012, 01:18 AM   #2
JZatopa is offline JZatopa  United States
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Here is the input
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Old 29th February 2012, 06:27 AM   #3
h_a is offline h_a  Europe
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Quote:
Now I don't have a large 8ohm resistor so I used a 8ohm tweeter.
Then use whatever high power (3W) resistor you have at hand; a tweeter will not give comparable information due to its large inductance. Also you risc to blow your tweeter as these little fellows are not intended to take significant power.

If you don't have 3 W resistors, parallel 0.25W types and observe the output voltage as to not overload them. For a first toying around, any resistor value will do.

Hannes
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Old 29th February 2012, 08:13 PM   #4
Boofers is offline Boofers  Canada
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You could also check the output with no load connected. Have the input on CH1 and output on CH2, displayed at the same time.
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Old 1st March 2012, 04:56 AM   #5
JZatopa is offline JZatopa  United States
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I didn't realize that you could check the outputs without a load. I just did and everything looks good. Now I just have to figgure out the P3 mod.
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Old 1st March 2012, 03:48 PM   #6
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Are you certain that the average DC value of the test pulse is 0V? Routinely
a 0v-+5v sigal is provided and the F5 is DC coupled.

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Old 1st March 2012, 04:58 PM   #7
Eric is offline Eric  United States
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If you foresee more testing in your future, check out the big green 120w resistors here: Resistors & Pots-MPJA, Inc.

I grabbed 4 of the 16 ohm ones and strung them in parallel with a switch. Two at a time for an 8ohm load and 4 at a time for a 4ohm load. They worked great for testing my Aleph-X amps.
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Old 1st March 2012, 05:16 PM   #8
JZatopa is offline JZatopa  United States
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I am new to using a signal generator and and o-scope. What you see above was just me doing things wrong. Its actually very cool that I can now put a signal into an amp and measure what is coming out. I am finally starting to understand how to test a few things. I think DIYaudio needs someone to do a tutorial on how to test all the aspects of an amplifier correctly, for those of us who still have more to learn. I know you can find the information online but it is usually spread accross many different sites and doesn't always deal with the test you may want to use on an amplifier.

I now have some large power reistors, when I get a chance I will hook it up and post a pic.

Last edited by JZatopa; 1st March 2012 at 05:20 PM.
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Old 1st March 2012, 09:24 PM   #9
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Quote:
Originally Posted by Nelson Pass View Post
Are you certain that the average DC value of the test pulse is 0V? Routinely
a 0v-+5v sigal is provided and the F5 is DC coupled.

DJNUBZ..... Important point from Mentor Pass, above. Put you scope probes on your scope's 1 KHz output. Make certain your scope is set to DC inputs. Adjust your gain and trace centering so that your squarewave fills your CRT display, with the lower and upper traces of the square wave an equal distance from the center horizontal gractile of your CRT (i.e., the middle horizontal line).

Now, disconnect your scope probe from your scope's 1 KHz input, and connect that probe to ground (at the scope inputs). The resulting trace should lie directly over that center horizontal gractile. THIS MEANS YOU HAVE NO "DC" in your scope's 1 KHz output. (Some scopes will provide a TTL (transistor-transistor logic) 5V square wave, varying from 0 VDC to about 5 VDC). Make certain you are NOT using a TTL signal into your amp--your square wave must be symmetrical on both sides of zero volts.

If you want to get serious about testing, you will want to use an audio generator with variable output frequencies and amplitudes. Either a stand along audio generator (good ones avaiable via surplus stores), or a PC sound-card based unit is good.
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Old 1st March 2012, 10:09 PM   #10
JZatopa is offline JZatopa  United States
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OK so I am getting the hang of it. The square wave pics are using the 1khz calibration on the o-scope. The sine waves are from the eico signal generator I just picked up. The Eico is far from perfect but it's better then nothing. I know I could always use a cd player or computer but I like that I can just twist a dial and see what happens with the signal generator.

All of these were taken with a 4ohm load on the speaker terminals.

1st pic is from my aleph p1.7
2nd - F5
3rd- F5
4th - F5 - just starting to clip from what I see. Is this how an F5 should look when it clips?

I zeroed out the DC before all of this.


To me it looks like the F5 handels a square wave better then the aleph P 1.7.
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