The Korf Blog

While we are busy with our tonearm prototype and with the next batch of headshells (watch your inbox if you want one!), we have received many questions about the role and effectiveness of cartridge spacers — small flat parts that go between the headshell and the cartridge. Currently, one of the snake oil merchants is running an advertising campaign for their miracle spacer and, naturally, people's interest is piqued.

I thought this is an excellent opportunity to finally break the shrink wrap on my Audio Technica AT633 ceramic cartridge base. It has been with me for a few decades, and, if not for your questions, I would have completely forgotten that I have it!

We'll do what we usually do: the measurements, some listening, and then we'll try to explain the difference if any is found. There's a question at the end of this post, and I would very much appreciate your feedback.

AT633 came about on the crest of the "ceramic" hi-tech wave back in the early 1980s. Kyocera came to the market with zirconium dioxide ceramic knives in 1984. To really drive the "ceramics are the future" message home, they've commisioned Micro Seiki to build a fantastic Kyocera PL-910 turntable with alumina ceramic platter.

It's very easy to install. In almost all headshells, the ceramic piece slides easily between the headshell proper and the cartridge. Tighten the screws, adjust the downforce, and you're all set.

I've decided to test the AT633 on the Rega RB300 arm. Previous measurements showed its headshell as the weakest part. Many correspondents wrote how similar carbon (or even brass) spacers improved the RB300's sonics. I believe Rega's old arm should highlight both the measured and the subjective difference of using the Audio Technica ceramic base.

We're still using the same methodology we have described in the post about the typical tonearm resonance measurements. The accelerometer is attached to the top front of the headshell, and the vertically modulated 0 to 20 kHz sweep recording is played. The output from the accelerometer is conditioned, amplified and converted to g. We're keeping our tradition of using the Audio Technica AT7V cartridge for the vibration measurements.

Fortunately, we have a much better measurement system now compared to 3 years ago. Less noise, more resolution, lower own resonances. Do our findings about the original RB300 still stand?

Yep, we did pretty well back then. The data above 10 kHz is a little better now: the accelerometer's own resonance is still at 11 kHz, but its extent is somewhat lower. It's clear that there is still a lot of energy in this region to excite the resonance.

Now let's bring the spacer in. What's going to change?

The picture is conceptually similar to the one we had with a "cap" for the Denon DL-103 cartridge. The main high frequency peak is lower in amplitude and in frequency. The frequency change is not as pronounced now, but the amplitude of the 5-6 kHz resonance becomes a lot lower. The spacer shaved 0.05g off it, which is impressive. It does add quite a lot of rigidity to Rega's headshell.

But it's not all positive. There's a new peak where there was none. I've repeated the measurement several times to make sure it isn't a fluke. No, it's there, the wide 850 Hz hill. Wonder what that is, and whether it would be audible?

Normally, an Audio Technica AT7V and a Rega RB300 tonearm are not a very happy sonic match. It's not really bad, but the music seems to be lacking the proper foundation, and on the slightly hissy recordings the sibilance is highlighted. The stereo image is flat and seems to "cling" to the speakers. And the magic just isn't there. AT7V doesn't sound materially better than cheaper AT cartridges in the stock RB300.

With the AT633 spacer, the differences are immediately audible. The improvement in bass is expected and very welcome. You can put the numbers into our compliance calculator and see that adding mass to the Rega RB300/AT7V combination brings the acceleration line into the green zone (add 1.75g in the "headshell" field. RB300's effective mass is 11.5g, AT7V weighs 6.5g, my best guess at its real world compliance is 12-15).

Sibilance is not just diminished, it's completely removed. With the spacer in place, one can finally appreciate just now neutral the AT7V is. The imaging is also a lot better, and some depth can be felt. Unfortunately, the tendency to cluster the images close to the speakers remain.

That 850 Hz peak? I didn't hear it. RB300 has some coloration in the mids, and I think it masked whatever changes this peak might have brought.

To sum it up, very impressive improvement for something so simple. An effective cure for RB300's main aliment. And the spacer should be equally effective with any headshell that can benefit from more rigidity.

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