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Old 17th July 2011, 08:20 PM   #1
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Default Mac book pro ringing?

I connected an macbook pro minijack out to my tube amp under test and got this ringing response on the scope when feeding it a 1k Hz tone from a software signal generator. This is the signal from the mac on the input. Not the output from the amp. Any ideas why?
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Old 17th July 2011, 08:43 PM   #2
imix500 is offline imix500  United States
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Does the ringing go away when the Mac is run off the battery and not the ac adapter?
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Old 17th July 2011, 09:08 PM   #3
tomchr is offline tomchr  United States
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To me it looks like impedance mismatch. Connect the sound card output to the circuit using a piece of coax cable. Between the sound card output and the coax center conductor should be a resistor with a resistance equal to the characteristic impedance of the coax. On the output of the coax (from center conductor to ground - across amp input) should be another resistor of the same resistance. If it still rings under those conditions, I'd blame the sound card or software.

Audio frequencies don't normally require transmission lines in order to preserve signal fidelity. But square waves contain a lot of harmonics - in fact, in theory, an infinite number of them. Hence, a square wave is a high-bandwidth signal and should be treated as such.

~Tom
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Old 17th July 2011, 09:25 PM   #4
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The analog output? Limited by the internal DAC?

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Old 17th July 2011, 09:31 PM   #5
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imix500, I tried running on batteries and also tried another tone generator, and played with amplification. No difference unfortunately.

tomchr, I am using an ordinary cheap, mini jack to phono/RCA plugs cable. Ill try your ideas later tonight, when the good wife has had some company. I have 75ohm antenna coax somewhere, would I use 75 ohm resistors, then?

I also worry that the problem may be with the olde tube scope. I am using a new 1x/10x coax probe, though.
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Old 17th July 2011, 09:33 PM   #6
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Yup, analog out and the MacBook Pros internal dac. I dont have an external dac -yet
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Old 17th July 2011, 10:01 PM   #7
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Quote:
Originally Posted by trondareo View Post
I dont have an external dac -yet
Make's a big difference. I use an Edirol FA66 on my music server (iMac G5)

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Old 17th July 2011, 10:26 PM   #8
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Its the filters on the DAC. Try burning a sinewave to a cd and look at the output from a cd player. Its not pretty.
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Old 17th July 2011, 10:43 PM   #9
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So should I put my cash in a Dac or a used bench type signal generator?
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Old 17th July 2011, 11:06 PM   #10
SY is offline SY  United States
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If you want good square waves, build or buy a square wave generator. It's pretty easy to get right, so even cheap ones can work well. You could do it better than you are now digitally with a 192kHz sample rate and some digital filtering, but still well short of about a dollar's worth of parts and a couple of batteries.

A function generator is a good investment. Sine waves are high distortion, but perfect useful for getting power measurements, triangle and square waves are of excellent quality in surplus units or inexpensive kits.
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