Originally Posted by Sforzando
I recently bought my first turntable, an AR EB101 with dynavector 10x MC cartridge (yea, hardly a beginners setup but it was at a bargain price), with the intention of digitizing some old vinyl to use in my productions. Unfortunately everything I record includes an insufferable amount of noise (mainly an overbearing 60hz hum) when it passes through through my TC 760LC preamp. Attaching the turntable ground to the preamp terminal only makes the noise louder
I have read quite a few times that the purpose of a phono preamp is to 1) apply the RIAA EQ curve to the signal and to 2) bring it up to line level. So after a few failed attempts at resolving the noise issue I decided to instead run the RCA outputs straight into my audio interface and apply the EQ curve and boost the gain in my DAW. Can anyone explain in straightforward terms what is wrong with this approach? Apparently it boosts low-level noise but how exactly? Personally I think the audio quality sounds great, but I'm paranoid about how it might be perceived by others. I'd also appreciate suggestions about how I might be able to eliminate the noise using the preamp.
You have a good turntable and an excellent cartridge. However the Dynavector is a very low output MC cartridge and requires either a high-gain, low noise "MC" preamplifier or more typically, a set of low-Z to high-Z step up transformers
to properly load the cartridge (output load should be a very low 10 ohms or so) and to match the [47k ohm] input impedance of a moving-magnet phono preamplifier. With proper cables and proper grounding of the turntable you should have essentially no 60 Hz hum with the right preamplifier
. I have no idea of the quality of your preamplifier, but doubt that it is designed for use with a MC cartridge.
The output from a MC cartridge is very low (10's to 100's of microvolts), so it needs plenty of local amplification and proper grounding of the turntable chassis and the tonearm is critical. A correct MC preamp will have a grounding connection near its input jacks. Connecting the turntable and tone arm grounds farther up the signal chain will almost always lead to hum/noise issues.
A proper MC preamplifier is expensive as are correct MC to MM step up transformers. If you're planning on transferring a limited number of records, a simpler solution may be to just replace your cartridge with a good moving magnet cartridge like a V-15
or a M-97
which will have 20 to 30 dB more output and will be much less prone to hum using your existing phono preamplifier.
Using a mic input and applying the 40 dB of equalization needed to match the RIAA playback curve is an option if you have two very low noise mic pres and are able to properly terminate the [low-Z] MC cartridge. That approach requires the most gain at low frequencies, and decreasing gain as the frequency increases as the RIAA curve dictates. Hum and LF noise will be amplified the most. Ideally
, for the Dynavector, you will need approximately 75 to 80 dB of clean gain and a set of well-shielded phono, leads. That kind of gain is not often seen in mic pres except those designed for ribbon mics like the AEA TRP
or the True P-Solo
. And, of course, you would need a pair of them. Also, the mic pre input is balanced, and again is the wrong impedance for proper operation with a MC cartridge. It might
work if you use special cables: 4-wire "star-quad" twisted-pair mic cables brought all the way to the output leads from the tonearm and then connected as single-ended there, but may again be prone to induced hum pick up.
Using post-pre EQ gain (boosting the low end by the 20 dB required by the LF part of the RIAA curve) will, of course, also boost the hum and noise. It's essential to be rid of that before applying the two RIAA EQ curve components.
If your current mic pres sound acceptable using a post-pre EQ, then that's fine. Using a "user-applied" EQ curve does have the advantage of being "tunable" to the program content, and even the track location on the record since the amount of LF roll-off does vary with radius on some records (sometimes more LF boost is needed for the inner tracks).