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In Reply to: Very interesting posted by GGA on May 4, 2007 at 12:23:32:
I use Sound Forge software, which contains a spectrum analyzer function. First, I copied the output from my phono preamp to 24/96 high-resolution digital using an Alesis ML-9600 Masterlink. Then I transfered the digital files to my computer for analysis in Sound Forge.I never tried this experiment while playing an unmodulated groove, but you are probably correct that the vinyl noise floor would be higher and tend to mask some of the feedback.
Follow Ups:
signal is getting in to your system?
Well, the answer is yes and no!It's obvious that the 60-Hz signal is coming from my line frequency. ;-) However, I don't know if it is introduced into the system by an external source or whether it is simply transmitted through the phono preamplifier's power supply as a result of its design. All I know is that it is totally inaudible and insignificant.
At one point I thought it might be coming from my TV cable because I do get an audible hum through my digital components, which are connected via unbalanced interconnects. When I disconnected the TV cable, the hum went away. Consequently, I made some recent measurements with my TV cable disconnected to see if it made any difference in my vinyl front-end. Unfortunately, it didn't.
Here are these more recent measurements. The first is the noise floor with my tonearm sitting in its rest.
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The next is with the tonearm sitting on a stationary vinyl LP.
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There is virtually no difference between these two above as can be seen from the following graph of the two superimposed upon each other.
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Now, let's play a record. This is the zero-dB reference level of vinyl at 1000-Hz from a professional Denon test record. Notice the arm/cartridge resonance just below 10-Hz and also notice the 50-Hz spike from the 50-Hz line frequency in the country where the test record was made.
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Okay, the finale! Here are all three curves superimposed upon each other.
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nt
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with a CD playing to measure the effect?
To measure what effect? If you are talking about acoustic feedback on a CD player, there is none. ;-)
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No, I did not re-run the acoustic feedback tests like the one below. I'm sure it would look similar, thought, except in color. ;-)
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