Make/buy rumble/warp filter for pre phono inputs?

This is possible, and I had hoped another member would offer a suggestion by now. I hesitated because I think this is a terrible idea.

Any low pass or rumble filter means cutting off the bass. Unfortunately this occurs over a much larger frequency than you might imagine. I have heard filters that affected the lower midrange, when they were aimed as solving excessive woofer excursion.

I had a phono stage that a manufacturer was experimenting with. He had gotten complaints, primarily from Japan about woofer movement.

Bottom line is that he put the "fix" on my phono stage and I went crazy looking for problems (tubes, set up, etc.) for days. I finally called him and told him my problem and he confessed he had tried the new circuit. After removing it my sound was perfect again.

If you have a good system, I would forget about the filter. If you have a turntable that excites the system excessively, spend the money to repair or replace the source of the problem, rather than filtering it.

Thanks for the concern Albert, but maybe I need to make my situation a little more clear. I have a more-or-less audiophile system, but I don't buy audiophile software. My record collection contains a lot of less-than-pristine rare vintage vinyl (33 and 45), and some of them are unavoidably warped, so this is not an issue about acoustic feedback problems. I am less worried about unadulterated sound with these disks than am about just being able to play them at all without the much worse effects (obvious distortion, compression, and severely limited volume capabilities) of excessive infrasonic modulation on the amplification chain and woofers. I can play the records in question by tracking heavier than normal, but I wouldn't want to do this routinely. What I want to do is make dubs of these onto CD-R with the infrasonics already filtered out before the signal even hits the phono pre-preamp stage. The points you raise are the reasons why I want to make the filter external and easily removable.

Zaikesman what were your results trying to transfer to CD-R without a filter? Are the warps so great that they use up all of the A/D's headroom? I've read that records can be de-warped by clamping the disc between two non-heat conductive surfaces and placing them in an oven at a low heat setting. Maybe someone will comment on this further.

I am currently transferring a large part of my vinyl collection to a digital format and I've come to the conclusion that for bad quality discs that computer-based DSP processing is essential. A record cleaning is the first step. Then once in the digital domain low-cut filters can remove rumble, gain can be altered as needed, pop/clicks and broadband noise can also be removed. All of this can be done relatively transparently. I use t.c. works Spark XL (Mac).

Onhwy61 - I will be using Sound Forge on some of these (which has the filter feature you mention) for scratches, but I suspect the results on the warped disks will be superior if the phono pre-preamp never has to deal with the heavy sub-bass modulation in the first place. As for the ADC, the meters on my HHB burner don't seem to register these frequencies' energy very well, but the overall result is poor enough, even when played back at low volumes, that some digital overloading is certainly a possibility. Of course, reducing the recording levels to compensate would reduce resolution and S/N ratio as well. While I haven't yet tried the digital warp filter, I will still want an analog pre-filter, not only for the reasons above, but also for making cassette copies, or just auditioning records in real time without having to digitize the signal and perform "post-production" on the computer.

Zaikesman, now I understand. I admire you for caring enough about music to save these less than perfect favorites, and now your filter idea makes perfect sense.

The problem is that putting a network of resistor and capacitor at the input of your RIAA stage can mess up the phono cartridge load and the input impedance of the preamp.

I suggest calling Conrad Johnson and tell them your situation. They have engineers that have probably already addressed this problem for others. My guess is that the filter should be in a tape loop. Other possibility is an equalizer, allowing you to cut and add for each situation as needed.