Excessive brightness - help & suggestions needed

It's a simple electronics fact that an inductor of 500mH into a 47k load rolls-off at about 16kHz.

Would you like the formula?

To hold-up the frequency response such that it isn't severely rolled-off by the time it reaches 16kHz, a capacitor is placed in parallel with it. This is a tuned-circuit in exactly the same way the tuner works in a radio.

The actual capacitor value required works out as being 200pF. That's 100pF from the arm wiring and arm cable, plus 100pF from the phono stage input.

So how come a moving magnet cartridge can do 30kHz? The simple fact is it cannot!

However, by making the shaft that holds the stylus and connects it to the magnet assembly (the "cantilever") resonate like a tuning fork, it raises it's output mechanically by the extra octave required to hit 30kHz.

If you read up about your cartridge you will fing they've used Gold plating to dampen the Boron shaft's tendency to oscillate - they call it "nano-resonance"...

Having done Gold plating industrially, I find it hard to believe anybody can guarantee that every piece will be plated to exactly the same depth, especially something so darned small.

Therefore experiment with tracking weight! By increasing down force the stylus is prevented from resonating as much.

Now, let's see, tone controls... they're OK if designed properly, or if they can be switched out of circuit. If the switching out-of-circuit only has the effect similar to parking the control's wipers at zero, the frequency dependent circuit and associated amp stage is still in operation...

Now, I have been busily running stability simulations for near-on 12 months on all sorts of circuits as part of our research and development. I think (if suitably interested) people would be quite shocked to find that many so called hi-fi circuits are unstable and produce a considerable amount of ringing from any high-frequency stimulus, whether from the phono cartridge via the phono preamp, or from parasitics generated elsewhere.

Yes, take some semiconductor manufacturers own application circuits (of which many designers take as being read) and simulate them and they don't do what the applications engineer may have thought.

Therefore, first thing, please try adding 10 - 25% more downforced, and let us know what happens.

Thanks for the suggestions, but I am already using my cartridge at near its upper range of tracking force. The AT150MLX has a tracking force range of 0.75 g to 1.75 g with a manufacturer's recommended setting of  1.25 g, and I have been using it at 1.60 g ever since I got it. I don't want to push it up to the limit and risk damaging the cartridge. I could in theory set it at 1.70 g, but I don't have a stylus force gauge so I have no way of accurately measuring the tracking force and making sure I don't accidentally set it too high. The tracking force adjustment of my Pro-Ject turntable is not very accurate.

With everything taken into consideration I think my cartridge is to blame for the excessive brightness of my LP playback. I already knew about the gold-plated boron cantilever before I purchased the cartridge. That was one of the features that convinced me to get the AT150MLX, although I was not aware it could contribute to a bright sound. When I recently asked me dealer, he said Audio-Technica cartridges tend to sound naturally bright, so maybe it's the cartridge.

On a different note: my Technics amplifier is a cheap piece of garbage, and the tone controls can not be disabled. They are permanently active. All I can do when I don't need them is to set them at the neutral 12'o'clock position.

I suppose the only cure to my problem is a new cartridge, for example the Dynavector 17D3 which has a 1.7 mm diamond cantilever. A diamond cantilever doesn't have high-frequency resonance problems, sharpness or metallic ringing that plague metal cantilevers. Plus, it has a ruler-flat frequency response in the audible range, so if any brightness is heard, one will know it comes from the LP and not the cartridge. Oh well, time to save up...









Edited by Analog Kid - 28 Jan 2009 at 4:42pm

Might be worth trying to get hold of a Canrong digital stylus guage - mine was around $75AUD so not too expensive and makes it much easier to get the VTF right.

Worth noting and in line with Grahams recommendation, I have seen reference to very experienced (and well heeled) users recommending 2.5 - 2.7 grams VTF for a DV-XV1S against the manufacturers maximum 2.2 grams.

Check http://www.high-endaudio.com/RC-Cartridges.html for the reference.