LP12 Spec Analysis

First, using a pink noise source via CD, I produced this chart of the frequency response of my measuring system, as you can see its about flat (well within a db, which given the distance from the music room to the romm with the computer will do for the moment).

Then using a HiFi News test disk (HFN001) , and the pink noise on that I got this response. This show me that the my TT and cartridge, is not producing a flat response, but that I should look further to see if within the 1 octave curve, if there is more info to be found.

The above results led me to look further, so armed with a copy of CoolEdit2000, which is what I used to create the white and pink noise CD, I first sampled white noise from the CD, this gave this response

Establishing the test system

As this shows the system has a flat response within most of the range of interest to within less that +-0.5 db, and that I am happy to assume that the 3db rolloff at > 290kHz, is due the the sound card and the connecting 30ft of cheap mike cable.

I then repeated this with the CD Pink noise source, this showed the expected 3db per octave slope. It would have been nice to do this with white noise, but I havn't a vinyl white noise track at the moment.

Now to the vinyl

Ok, that gives us a reference to compare the vinyl results. The area of interest is the 20Hz to 1kHz, but I am also interested in the general response over 20Hz to 20kHz. The specs on the Ortofon cartridge, specify a Channel balance of < 1db at 1kHz, and a Frequency response of 20-25,000 +4/-2dB, so lets see how that compares to reality.

This first result was done without the amp on, so its just the output from the phono stage, so there will be no room interaction

I have added lines 1dB apart with the 3dB per octave slope for comparison. First thing that shows is that the channel balance is well within spec, looking like less that 0.5dB at 1kHz. The other thing that seems obvious is there is no noticible peaks and troughs in the lines, and while there is a +2/-3dB range (using 1kHz as 0dB) this is within the quoted specs for the cartridge. Sp no apparent LP12 artifacts there.

Next, given that the LP12 has a reputation for feedback (at least according to some on the list), I repeated the above, but this time with the amp on, and a measured SPL of 80dBa at the turntable. The first graph, is with the lid up

Then again with the lid down

To me this shows that there is not a big (if any) measured effect of either feedback, or resonance. It would have been nice to have done this before I added the marble support under the TT, I do suspect that the results may well have been different, as I am sure that there was a big subjective effect to adding the marble. But I have to admit to not wanting to reduce the quality of the setup just to make a measurement.

Sensitivity to external sound

It has been commented that the LP12 suspension can make it succeptable to feedback and changes in performance because of "pick up" of external sound. So tho investigate this, I have looked at what comes out of my phono stage, with the stylus on a stationary record, with the amp and speakers playing pink noise, at a level of 90dBc measured at the turntable.

First I recorded with the connection to the phono amp disconnected and terminated with 100k resistors, to try and determine the noise floor of my sound card and connecting cable. This image shows this result.

Next the result of the phono amp powered up, with no sound in the room, this should be the noise created by my soundcard and the phono stage together,

Next the pink noise was played in the room. First a sample was done with the LP12 lid down.

This shows that there is sound pickup, but I would have been amaized if there was no trace, it doesn't look that much to me, and I would suspect that there would need to be a hell of a lot more gain involved to get actual feedback.

This final trace shows the result with the lid up, this shows that the lid does reduce the level of "pickup".

If people want to examine the samples that led to these graphs, feel free to drop me a line at nick@lurcher.org and I will put the original .wav files up on the server.

Whats going on at 10kHz ?

One thing the above DOES show me, is that there is something going on at 10kHz, looks like a resonance of some sort, this is a area I will be looking at more, as I have been suspecting there was something going on with the sound at this point. I think a different amp could be the solution, when I get around to this (a RB250 is on the xmas list), I will repeat this and see what we see. Looking at the later traces, the 10kHz peak is still there. I wonder just whats going on...

More to come. Nick