RIAA EQ is +20dB at the bottom, -20dB at the top. SG does not require this so the statement is true.
Dustcover Blues
Most of you probably know that I have always championed the use of dustcovers on turntables even during play, the goal being to protect the record from the environment and shield it from sound. For the first time in my audio career I have stumbled into a problem with this and other than not putting the dustcover down I have not come up with a solution.
Yesterday I was playing Herbie Hancock's Secrets and I cranked it on my favotite song. After about 30 seconds the room started to rumble. My subs were putting out a remarkably clean 20 Hz as if I were playing a test tone. Feedback! Just turn the volume down a little and it disappears. Turn the volume back up and within 30 seconds it starts up again. Did I screw up my cartridge set up? I veiwed the tonearm during the feedback and it was rock solid. Usually with low frequency feedback you can see the tonearm shaking. I played the resonance tracks on an Ortofon test record and both lateral and vertical resonance were centered on 9 Hz With the feedback going and the house shaking I wanted a better look at the cantilever. On lifting the dustcover the feedback stopped! The dust cover is attached to the plinth which is isolated from the sub chassis (tonearm and platter mounted on this) by four springs. The resonance frequency of this suspension is 2 Hz. Nothing above 2 Hz can pass directly through to the platter and tonearm. What is going on here? Any of you scientists out there have a clue? My best guess is that I am dealing with a type of Helmholtz resonation. The dust cover is lowered on four hard rubber pads, one at each corner. There is a 1/16" slot all the way around. This combined with the weight and dimensions of the dust cover creates a resonance at 20 Hz. To get it going I have to turn the volume way up.
Today when I get home I'll play around with it to see if I can figure it out. Any ideas would be appreciated.
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@atmasphere , Have you heard the cartridge? Theoretically it should out track any other cartridge but his MI cartridges track anything I can throw at them. I can correct frequency response problems but I won't get involved if there are not significant advantages in the design. @rauliruegas , I have no experience with the Lino C. I have an ARC PH3 SE which I really like for high output cartridges. It is a simple unit with just one switch in the signal path. It is a hybrid tube unit with a FET front end. It really does not have enough gain for cartridges with output < 0.5 mv. It is 30 some odd years old and still in perfect working order but it is time to move up. I do not like complex phono stages. It is too easy to damage the low level signal coming from a LOMC. The Lino C is a minimalist unit which is balanced throughout which I really like. Loading adjustments are not required and it is battery operated, another big advantage. I can use the ARC for high output cartridges which I prefer to MCs with certain music. The other candidate is the Sutherland Phono Loco. |
Not really... The problem is that by the time you get to 20KHz you're not going to be 20dB down. Please look at the actual RIAA de-emphasis curve: See that flattening out part in the middle? That's what I'm talking about. A strain gauge simply bypasses the flattened out part and continues rolling off the output following the 6dB slope as seen on the left side of the curve- so the highs simply won't be right. Imagine that red line just continuing down at 500Hz instead of flattening. You can see the error that is introduced. In reality it won't be as far off as the red line would be as I just suggested since were talking about a curve using analog filter theory. To properly EQ a strain gauge, a pole (seen at 500Hz) has to be introduced (a rising response) and then a zero (seen at 2100Hz) going back to flat, so the strain gauge can do its thing. So imagine an EQ curve that is flat from the bass up to 500Hz, then rises for 2 octaves at 6dB/octave and then goes back to flat. That's the math of it; the actual curve won't do that- it would be smoother of course in a way that is similar to the smoothness of the de-emphasis curve you see above. If this is not done the EQ won't be right.
I don't know if its the same one but I have heard Soundsmith SG cartridges before. Tracking isn't what I'm talking about- I'm sure it tracks fine as the one I heard did. I'm talking about equalization. I provided the information you need to set the EQ right above. Can you dial that sort of thing in? If you do it in the digital domain, be sure to include an algorithm that includes the phase shift and frequency bump of analog filters.
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