not good news really but here we go,The Lampy cdp is up and running well and sounds (suspiciously) good on some recordings and a bit strange on others. The highs are the issue, something unrealistic and not right there, the bass is superb.
It is running in NOS mode and has no reconstruction filter (low pass brickwall filter at less than 1/2 sampling rate) after the dac.
Nyquist theorem says that a bandlimited analog signal that has been sampled can be perfectly reconstructed from an infinite sequence of samples if the sampling rate exceeds 2B samples per second, where B is the highest frequency in the original signal. That it is bandwidth limited is very important.
We speculated it may be that the equipment and our ears would have the effect of a reconstruction filter and not allow much in the way of f after 22KHz and so a deliberate reconstruction filter may not be needed.
Today I hooked the scope in again but this time to the output of the power amp with full system connected. I played the test signals from the cd, listening to the tones produced at low level and monitoring the signal on the scope.
The idea was to see if the jumbled multiple traces seen previously at higher frequencies were improved by the hf losses in the system. They were indeed, though not by much. I then put a small cap (various 220pF to 22nF) across the cdp output and saw that the traces cleaned up more but that by the time they had been cleaned up at 20KHz there was little amplitude left. So clearly a filter is needed but it must be very steep. We (I) can now see how filtering instead at a higher f from an OS signal is more practical.
Anyway, back to the action, it was hooked up and playing each test tone burst for 10 secs, then the next higher one, etc. The traces became ragged and the tones increased in pitch. I was pleased to hear 5KHz, 10KHz but then amazed to hear 15KHz even louder sounding like the old BBC test tone, but then 18KHz just as clearly though a different tone, and finally 20KHz loud and clear! Wow, this is a rejuvenating cd player! Heard 18KHz before, just, on a good quiet day at high level, but never 20KHz
Popped the disc in the Pioneer and played them again. All traces clean and level. All tones rising in pitch and disappearing at 15KHz as usual
Clearly in the NOS player without reconstruction filter the higher tones were aliased with lower ones and that accounts for me being able to hear "them" and the strangley interesting treble with added details in the music!
This is such a simple test to do. You don't even need the scope, just play the test disc in a stock cd player, then again in your modded player, and if you can't hear the highest tones in the stock player but can in the modded one the answer is aliasing not rejuvenation I think.
