The Korf Blog

The inside story: our research, development and opinions

21 August 2019
The GrooveMaster II Tonearm
The backlog's cleared, and we are back in our shiny new lab, ready for new adventures in analogue reproduction!

We can't wait to continue with headshell experiment. Remember, we also wanted to do a subjective comparison of the worst and the best headshells? I think the time is right for that.

Wouldn't it be nice to accompany the listening test with some more measurements? We probably need something steadier than our usual Jelco SA-750DB to truly separate headshell and armtube performance. What could we possibly use?

Oh, I forgot to tell you! We already have a thoroughly modern tonearm that takes replaceable headshells. Please allow me to introduce the Audio-Creative GrooveMaster II, straight from the Netherlands:

We've bought the GrooveMaster II this April. Paid our own money and got no special deal on it (as is the rule with all the equipment we use to test things). It's available at https://audio-creativeshop.nl/groovemaster-2019/ or at your friendly hi-fi dealer, sometimes under the Timestep brand (confusing, I know). The arm comes in 3 versions: with 9", 10" or 12" armtube. Ours is 10".

I think it's a pretty cool-looking tonearm. Its design evokes both an iconic EMT 997 "banana" and a somewhat less-known SONY PUA-9 (look at the counterweight). Texture-wise it's all anodized sandblasted CNC, all the time. The titanium armtube and the heavier parts of the counterweight have a slightly darker colour, and it livens up the otherwise monotonous look a bit.

Technically, the arm is a mixture of the sublime and the infuriating. The former includes the bearings. I have no way of measuring them, but you instantly feel the quality when moving the arm by hand. The JIS/SME bayonet is custom made and has a longer sleeve to make changing headshells simpler. There's magnetic antiskating, which I usually prefer to all other types. The micrometric screw for setting it is a nice (if mostly cosmetic) touch.

Most tonearms available today suffer from being built too light for no meaningful reason. GrooveMaster II is built just right, with reassuring heft to its armtube.


So what's infuriating? The tonearm designer's love of set screws. Look at the counterweight: it slides freely over counterweight stub, and is fixed by single set screw (circled in red). It's nearly impossible to reliably dial the required downforce⁠—and to keep it while tightening the screw.

Also, I simply cannot understand the logic behind the counterweight design. It consists of 3 parts: the light aluminium sleeve (a), replaceable steel counterweight (b), and the steel end cup (c). They're held together by one invisible and almost unreachable bolt.
Even the tiniest counterweight movement results in a sizable change in downforce. This is probably the least precise and repeatable downforce arrangement I've seen in a production tonearm.

These difficulties might be understood if the user-friendliness was sacrificed for rigidity—but no, the counterweight stub design is essentially the same as with our earlier tonearm prototype. It's aluminium, CNC'd out of one piece with the arm's central part. Not rigid at all.
The tonearm's vertical column is set in the base with another set screw (circled in red again). No matter how tight you make it, the column still rocks back and forth a little bit. And if you properly tighten it, there's this fingernails on chalkboard sound of anodized layer being ruined. My relatively inexpensive mid-1970s Telefunken S-600 has two screws set at 90 degrees to fix the tonearm column—guess that's progress.

And the slider in the SME-type base... yes, fixed with another set screw biting into anodized aluminium. Painful.

Now let's have a look at the GrooveMaster II's geometry:
There's a little dimple on top over the arm's center of rotation. Great touch that makes setting the pivot to spindle distance a lot simpler.

The specified geometry is pretty standard, with offset angle α of 20.26° for a 10" version. Our example's actual α is about 19°—not exactly as specified, but close enough.

The line between the two horizontal bearings is not tangential to the tonearm axis A-A. Now most tonearms's horizontal bearing axis is rotated clockwise to minimize azimuth error on warps, but here... it's moved in the opposite direction! β is -8° instead of 20.26°. This isn't a good engineering decision, and it's hard to see any rationale for it.
Measurements
My hopes were pretty high because of the titanium armtube. We couldn't source one for our experiments, but given that titanium's Young's modulus is 1/3rd higher than aluminium, it should perform quite well.

We used a Jelco HS-25 headshell for our initial measurements, because that's what we measured the Jelco SA-750DB arm with. To keep the measurements consistent across all our experiments, the same Audio Technica AT7V cartridge was used.

To separate the influence of the headshell and the rest of the arm, we've employed two identical Picomin 22 accelerometers. One was fixed to the headshell as we usually do, and the other to the armtube just after the collar (see the illustration on the right). Their ouput was conditioned and recorded on two channels simultaneously.

First, let's see how the GrooveMaster II compares to the Jelco SA-750DB with the same headshell:

(please ignore the 150Hz peak, it's the 3rd harmonic of 50Hz on a noisy DAQ channel)

Ouch. This isn't what I expected at 4 times the price of a Jelco. The 350 Hz main armtube resonance is higher in frequency (good), is higher Q (good), but also significantly higher in amplitude (not so good).

How much of it is the headshell, and how much the arm?
We can only blame the arm for (almost) all that low frequency resonances.

What would happen if we try to isolate only the headshell resonances?
The dotted line is the total energy from the IET measurement with the cartridge attached. Does our measurement here fit the IET results we got from the Jelco HS-25 with the cartridge attached? Yes and no. The low frequency peak at 400Hz matches ideally. The next one at 750Hz is rather more prominent on the IET.

As our measurements of the Body Cap for Denon DL-103 showed, peaks at 5.5kHz and beyond are very much influenced by the cartridge itself. The IET "ping" doesn't contain enough energy over time to reliably excite these resonances. The recorded sweep that we used for excitation today does. We'll pay a particularly close attention to this area of measurements in the future tests.
Subjective Evaluation
The subjective performance of the GrooveMaster II is in stark contrast with its measurements
GrooveMaster II is being sold as a tonearm strictly for low and very low compliance cartridges like Denon DL-103 and Ortofon SPU—but the whole idea of effective mass/compliance matching is mostly nonsense.

It's in daily use in my system with a Goldring 1042 moving magnet cartridge. We have also evaluated it with the same AT7V that we used for the measurements. Despite the supposed gross compliance mismatch, there is never any mistracking even on the most warped LPs. Neither did we notice any undesirable bass artifacts. On the MC side, I've tried it with the old Dynavector Karat 23R.

The subjective audio performance of the GrooveMaster II is a lot better than the measurements suggest. It is, of course, in a different league from the Jelco, despite a similar-looking vibration profile. I think that GrooveMaster's bearing quality and sturdier build play a significant role in the way it sounds.

While not having the resolution, tonal correctness and the outright "steadiness" of our own Prototypes 5 and 11, the GrooveMaster II is somewhat similar in character. It's a bit darker-sounding, and there's some desire for better spatial resolution, but fundamentally it's enjoyable and mostly done right.

I was quite surprised by the disappointing measured performance of the GrooveMaster II. The extent of main armtube resonance suggests either low rigidity of the tube itself, or the way it is attached to the central part of the arm. It isn't helped by the counterweight arrangement. I have a feeling that the multitude of set screws may have also impacted the measured rigidity.

For lack of a more solid arm, we will be using the GrooveMaster II to evaluate the best and the worst headshells in our series. That would be the Orsonic AV-101b and the noname stamped aluminium one.

Stay tuned!
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