Nice phono stage with subsonic filter?

[Infinitely Baffled]

Hello, All. Is anyone tempted to part with a nice quality phono stage? Can be MM/MC or MM only, but must have an effective 20Hz subsonic filter. Cheers. Gary.

[bob orbell]

Hi Gary, can I ask why you want the filter?, I could take a guess and say that you have a turntable with idler drive. BOB

[Infinitely Baffled]

Hello, Bob. No, it's not an idler drive, but a Notts Analogue model with an ultra low torque motor, belt drive and high mass platter. It's just that, having recently switched to using some new loudspeakers that have no cloth grilles, I can see for the first time what my mid/bass speaker diaphragms are getting up to, and I don't like it at all!

What I can now see is that with some of my records I am getting slow, exaggerated wobbling motions of the cone. There is no accompanying sound, just lots of unwanted activity. I have done a bit of reading around on the subject and it seems that this unwanted subsonic information can be from a variety of sources: badly pressed records, tonearm/cartridge resonance, acoustic feedback from the speakers or heavy footfalls. You name it!

I have done some experimentation to try and eliminate some of these possibilities and haven't been able to identify the precise cause. But I keep coming back to the fact that I get it on some records and then not with the next - even though there has been no change in the set-up (same arm/cartridge combo, same turntable and loudspeaker positioning etc). I'm coming to the conclusion that it's simply down to the records themselves - some of them just carry more subsonic information than others and that's that. Only thing to do, it would seem, is to fit a subsonic filter into the setup somewhere. I prefer, if possible "single box" solutions, so would have liked to find a combined pre-amp with phono stage and subsonic filter built in. But that is a rare beast indeed so I am considering the possibility of a phono-stage with an integral filter. But they are not exactly plentiful either. Seems like it is a problem that not too many folks are worried about, and I wonder if I am getting it out of proportion. But sapping the power of the amp and overworking the loudspeaker cones and surrounds seems to me like something to be avoided if possible. Cheers. Gary.

[Richard]

Hi. what cart, phono, pre, and power amp are you using at the moment? It would be very easy to fit a filter which you could adjust to suit yourself (ie remove the cone wobble without losing the bass). If you tell us the system we could suggest where to fit it and what value components.

[Infinitely Baffled]

Well, there's an offer I can't refuse! That's very helpful of you, Richard.

The cartridge is a Decca Gold and it is feeding into a Croft Epoch pre-amp which has its own integral MM phono stage. And therein lies one of the difficulties, I imagine. I have read that the ideal location for the filter is just downstream of the RIAA stage and just upstream of the line stage. To achieve that I would need to install the device inside the Croft's box, and understand where and how to break into the circuit between phono stage and line stage. (I'm not at all knowledgeable about electronics, but I can solder and do simple stuff.) Then of course I would have to equip it with a power supply, which would have to be external (since it would undoubtedly be beyond me to tap into an appropriate power supply already present within the Epoch's case).

All in all it would be simpler for me to implement a filter as a stand-alone external module located in the line from the pre amplifier to the power amplifier (which is a WAD Kat 6550, only using 4xKT88s instead). I would probably want to equip any such module with a signal pass-through switch for when I was playing good records or using a different programme source.

So, not a straightforward undertaking for me. You can see why I am enquiring about the availability of a decent phono stage with a filter already built-in!
Regards. Gary.

[bob orbell]

Hi Gary,/Richard, Morgan Jones book, VALVE AMPLIFIERS, has a section on the RIAA disc stage, a disc stage with IEC equalization includes a 20Hz (7950us) time constant, I do not know what manufacturer to point you to, most just use RIAA, but a Google search may help, just to add to this, WD Phono 3S has 3 swichable low filters, and is a cracker , Jones also points out in his book that the problems you are describing are mechanical, and should be addressed mechanicaly. Back to Richards suggestion, I think you implement the low filter before the RIAA stage, and is simply a capacitor and resistor, but of what value I do not know, so its over to you Richard.