Distortion at Peak Dynamic Levels

I generally second ZD542's post. The most likely culprit is either (1) phono stage overload/slewing distortion, or (2) mistracking by the stylus.

If the problem is the latter, you may be permanently damaging your records. So you should investigate that first.

After making sure your records and stylus are really clean, try playing an offending passage whilst increasing VTF in tiny increments (like .01 or .02g if you can). Make several trials, increasing VTF each time. Does the distortion lessen as VTF goes up? If so, mistracking is the culprit. Eliminate it immediately by improving your setup, else you risk destroying your vinyl.

If raising VTF several times makes no improvement, the odds are it's your phono stage. Frankly, that's my bet. Many phono stages are incapable of reproducing powerful vocals or hard blown horns (for example) without slewing distortion. This can sound exactly like mistracking to all but expert ears. This can happen even if even if the phono stage is perfectly gain-matched to the cartridge. You should check that, as ZD542 said, but you may just need a higher quality phono stage.

Get a phono set-up test record, and check your cartidge's tracking. You may simply have the tracking force too light. Are you using a GOOD/accurate digital sylus force gauge? The Shure test records run you though a series of test levels, in various frequency ranges, which is more comprehensive than most. I'm still using my ERA IV disc, from the 70's. (http://www.ebay.com/bhp/shure-test-record) (http://www.needledoctor.com/Hi-Fi-News-Test-Record) All the Shures and the Hi-Fi disc have tracking tests. If you get a Shure disc: try for a NOS one. If a disc has been mistracked severely, it's probably permanently damaged and there's no way to tell visually.

A good VTF scale is indeed essential, but why spend money on a test record? The vocal tracks he's having trouble with provide exactly the test tracks he needs.

If he gets those playing cleanly, all will be well. Artificial test tones won't make it any easier. Nor will they provide any more useable information.

Like you I have multiple test records, including Shure, HFN&RR and others. None of them has been out of its sleeve in years. I've set up dozens of rigs from entry level to very high end without needing any of them.

Hey everyone thanks for the advice. It looks like the down force was the issue. It was at 2.2 grams and I brought it down to 1.8 grams and that seems to have fixed it.

Then tracking force may not have been the issue. Decreasing VTF moves the tangent point of the stylus to the grooves. That's because the cartridge suspension has a spring rate- less load equals less deflection. So the stylus position relative to the pivot point of the tonearm just changed. Different stylus shapes can tolerate more or less the error in the tangency to the record groove. Of course, with a pivoting tonearm, the tangency error can be 0% at only two points and if the set up is off a bit- to the point that error exists at all positions on the record, then the sound can break up even worse at certain areas on the record. Additionally, the VTA of the stylus changed too. And that could have changed the sound as well. All of the settings are inter-related. When I set up a new cartridge I start with VTF, then set VTA then set overhang (tangency point). I play some records and if I need to go back and make small adjustments, to VTF or VTA, then I recheck overhang.