- Category: Turntables
- Written by Chris Heinonen
- Published on 08 August 2011
On The Bench
I'm sure many people are looking at this graph and see the THD+N number and wonder how I could have possibly written what I did. The Concept with its MM cart installed showed around a 45 dB drop between its primary harmonic and secondary harmonic values. You'll notice a spike there at around 60 Hz, which is noise from the power lines making it's way into the test results. Since turntables have far lower output compared to a digital source over their line level outputs, interference from the power lines can show up more easily. That spike changed in size as I readjusted the cables, and I never heard a hum coming while I played the Concept in my system, but that spike is present on the graph.
Next, you'll see a graph of the same turntable playing a rumble track. This is a track with no music at all, but it gives you an example of the noise floor of the turntable:
Here you see the noise floor is overall very low, except for that noise from 60 Hz. With a turntable, it's more important than with other devices to keep the output cables away from power cables, since the signal level is so low that any extra noise that could be introduced by a power cable should be avoided. After this was done, I installed the MC cartridge on the Concept.
The graph looks similar to the one for the MM cart, though the level of the harmonics drops off quicker, even with the higher output voltage. Unfortunately I don't have a rumble track for the MC cartridge due to a mistake I made during the bench testing. As an MC cart has an even lower output level than an MM cart, it's even more prone to picking up that AC noise on the bench tests, and also far more dependent on the quality of phono stage you use with it to amplify the signal and avoid introducing noise.
Using the recently released Platter Speed app from Dr. Feickert on my iPhone along with a 3150 Hz test track, the Concept measured remarkably close to it's rated speed. Just slightly fast at 33 RPM and slightly slow at 45 RPM, but very close and I could never tell a tone or pitch shift with my ears.