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In Reply to: Random guess posted by GRH on May 1, 2007 at 05:18:33:
Gary,The effect seems to follow the CD player. Some CD players don't do it with any amp. Is there a similar effect on the output stage due to capacitive loading?
Jerry
Follow Ups:
I spoke to one of my engineering buddies, the "analog circuit wizard". I told him about my experience, and he thinks it sounds like oscillations. The added capacitance of the cables could be creating something like a Colpitz oscillator out of the CD player's output circuitry.I do know that John Dunlavy was a proponent of very low capacitance IC's and so was "Gizmo", but he never expressed it in that way.
Jerry
Jerry, I think your “analog circuit wizard” is on the right track.It’s been a long time since I exercised brain cells in this area, but there are a few things that can lead to waveform distortion or parasitic oscillations. Parasitic oscillations are usually at high frequencies and components / sub-components can cause a high frequency resonant circuit when the device has high gain. Because oscillations can be dampened by spoiling the Q of the resonant circuit, small changes to circuit lead lengths and position can stop a resonance. Occasionally, a few pf can move the oscillation especially if the resonant circuit includes a decoupling capacitor (which may have an internal series inductance). I’ve seen parasitics on a scope / analyzer show themselves as unexplained distortion or jumps in dc offset.
Some old grad school notes also lead me to believe that if an output load has an excessive capacitance, it can also cause a mismatch at high frequencies resulting in standing waves on the output cable.
If there is a weakness with input IC shielding / ground performance / routing, it also increases the probability of picking up RF / EMI which is amplified as non musical distortions and can create a small potential. If only a small portion flows back via the input, then a frequency boost or oscillation may occur.
It looks like the interactions you saw are more sensitive than I would have previously guessed but it makes sense looking back.
The JPS Labs cables I favor have both low capacitance and inductance resulting in a very low dielectric constant of 1.16.rw
I'm no analog wizard as evidenced by my anecdote so I would go with your buddies theory. My only point was that small impedances can have a much larger effect than anticipated especially if some sort of feedback/resonance (e.g.oscillation)occurs which is consistent with your friends explanation. Sounds like the extra capacitance of the NAD interacts with some CD players output stage and the extra capacitance of the longer cable is the proverbial straw.
Gary
That's kind of what we pieced together as the likely scenario. But without some experimental data, it's just speculation.It does bring up an old "thing" of mine. That is, the "interface" errors are often larger than the internal errors in components. For example, the just mentioned aberration caused by extra cable capacitance at the interface between the Cd player and the preamp. Further, it doesn't show up in the bench tests on either component.
Amp and cable to speaker is one I've hammered on for years here and other places. Speaker to room, of course, generates big errors. And phono cartidge to phono preamp can generate frequency response aberrations larger than the specs for either the cartridge or phono preamp.
There are lots of others, some fairly obscure. Like having a tape deck connected to the tape out jacks, and the tape deck not powered up. Turns out some tape decks had a very low input impedance in that condition and it loads down the selected input.
This is all caused by the industry having a component orientation, instead of a systems orientation. So there are no good systems interfacing specifications, just industry standards for measuring the components on the bench.
When are you coming to Oberlin? I didn't save the e mail.
Jerry
Hi Jerry,Do you have any recommendations for a good capacitance meter. I think it would be interesting to test some of the cables that I have.
Dave
Parts Express has a small stand alone Meter that has sufficinet resolution for the small capacitances in cables. Don't get the LRC meter, as the capacitance ranges don't go low enough.Jerry
Hi Jerry,Thank you for the info. I had seen and wondered about that specific metter. Parts Express is not always the cheapest game in town but they are very reliable.
Best wishes,
Dave
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