RF and Prefix

Worked superbly for me, but only an option if using a seperate supply from Prefix (i.e. SNAPS, *-cap)

Andy.

We tried to set up one of those puppies the other day.

The ARO was acting as an antenna..

NO prefix for me..At least not yet.

Regards,

Marc Newman

* prefix st into 52, no hicap --- very little rf
* prefix powered by hicap into NA526 boards via BNC's --- very little rf, but sound quality not as good.
* prefix powered by hicap into high level boards via 5/4 pin din --- much higher rf, but sounds better than above configs.

enjoy

ken

Also, did you solve the RF problem, and if so, how?

Ron The Mon

Yep new in the box....never used....stored in a closet for 10 years. Sounds fantastic...not the most neutral cartridge, but tunefull and huge boogie factor. I listened to the Helikon, but it was too brash and bright for my taste (to be fair it wa s the medium out-put version). I have a friend who has the clear audio insider which I hope to hear soon in my system, but so far the Troika is the only cartridge I have ever liked--never like the Regas when I owned a RB300.

It may have been the last of its kind, new.

Still have not found a solution to my RF. The inernal boards in my 52 are pretty good, but I want a prefix! I am hoping the 552 will have some new facncy super phono section with RF rejection circutry and will sound better than a Prefix/Supercap.

Chris Bell
More Fi, Less Hi
o

quote:

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Originally posted by Chris Bell:
Anyone had any luck shielding RF from their Prefix? I tried some fancy RF shielding fabric and it did nothing...my K boards in my 52 are fine, but a Prefix is worth the trouble to get the most from my new (yes new) Troika.

Chris Bell

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Chris,
I believe the shielding fabric is just some kind of silk material, works for some cables but not all the time.

Have you identified where the RFI is entering the Prefix ensemble? Arm to Prefix, Prefix boards themselves or SNAIC to preamp?

If it's the Prefix boards, I may have an idea.

Best Regards,
Mark Dunn

I would love to hear your ideas on shielding a Prifix. I am not sure where the RF is being picked up, but it is really bad. (I joke with my dealer that the Prefix is actually an FM tuner in disguise)

King,

I tried some RF shielding fabric a while back. I completely wraped a stageline (K) and it had no effect. The RF is comming from one station in town 94.9FM.

Thanks everyone for your interest/help!

Chris Bell
Seattle

Andy.

I talked to Dave Dever about your trick and he said it would not work with a 52.

Chris

Chris Bell wrote:

>Mark,

>I would love to hear your ideas on shielding a
>Prefix.

When I lived in Memphis, TN I had some quite bad RF issues. In the end I discovered it was due to RF getting INTO the 102 and RF getting OUT OF the SNAPSC and "infecting" other items.

I made Faraday cages for each unit and the problem was *completely* cured. The cages were made from aluminum screen door covering material that you can buy from Home Depot etc. for $10 a roll. I simply cut a (one piece) shape that fitted around each box with about a half inch gap all around.

Chris, *if* it's the boards that are the problem then you could try this with your Prefix but you'd have to have it simply laying beneath the LP12 and not physically connected. If you don't use a baseboard on the T/T this is a non-issue.

Hope this helps.

Best Regards,
Mark Dunnn

quote:

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I talked to Dave Dever about your trick and he said it would not work with a 52.

---

Dave would have more experience of the RF environement you live in and he may well be right.

There are no earth loop issues with trying this though, and my experience as an RF engineer tells me that shorter is, almost always, better. Returning the earth to the Supercap (or 52PS?) might help.

I'm happy to accept it may not, since it's not been 100% succesful for everyone whose tried it, but has Dave tried it?

I suspect though, with a local station nearby the breakthrough is via the arm / prefix wiring / prefix input. One of the daft things about the Prefix, bearing in mind it's wide bandwidth and huge gain, is the input cables are unscreened, and no connection is made to earth the arm.

The reason is most likely flexibility issues with screened cables, or the extra capacitance (which could possibly be taken into account in the input stage design).

It's a shame, it is a bloody good phono stage when working optimally.

Andy.

You may achieve better results by tuning the RF out, using the ground lead (disconnected) as an antenna of sorts.

Or move to a high-output MC or MM cartridge.

Dave Dever, NANA

P.S. The ground tag of a Prefix should connect to the 52 anyway if powered from INPUT 2, as it grounds the arm at the preamp; no need for a special connector.

quote:

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P.S. The ground tag of a Prefix should connect to the 52 anyway if powered from INPUT 2, as it grounds the arm at the preamp; no need for a special connector.

---

In my Ittok + Prefix there is no connection between arm earth and Prefix earth. The Prefix earth just connects to the Prefix enclosure to screen it. The Signal 0V connects to the cartridge, unless I'm are missing something (or my LP12 is?).

Naim made me a special DIN connector that fitted to an unused connector on the NAC112 to provide a connection for the Prefix earth, but RF breakthrough was still present although it was improved over no connection, or connecting to mains earth.

Returning the Prefix earth direct to the PSU powering the Prefix, rather than via the 112, it's internal PCB traces and a SNAIC cured my problems completely.

Full volume on my K type Prefix just results in hiss.

Even trying different DIN sockets can make a difference, as the effective RF impedance of the earth varies with length.

These problems are difficult though, without being there. You need to have an innate 'feel' for RF stuff, IMO. The problem is earth is very rarely earth if a cable length / PCB track etc. is involved. Ignoring cable velocity factors, at 95MHz a cable 1/4 of a wavelength long (approx 0.8m) can be grounded at one end but be effectively open circuit at the other, as it forms a parallel resonant circuit, which has high impedance.

Andy.

[This message was edited by Andrew L. Weekes on THURSDAY 03 January 2002 at 09:03.]

quote:

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In my Ittok + Prefix there is no connection between arm earth and Prefix earth. The Prefix earth just connects to the Prefix enclosure to screen it. The Signal 0V connects to the cartridge, unless I'm are missing something (or my LP12 is?).

---

The only difference between the SME-DIN Prefix and the Aro variant is the plug. The arm grounding is the same and does not make electrical contact with the Prefix chassis. On older units, the arm ground (black wire) passes into the unit and back out again.

The Prefix chassis is earthed via the SNAIC-5, to its power supply.

Dave Dever, NANA

I was thinking Farraday cage as the solution here too. Did you connect any wires to the mesh to ground it or just make a cage out of the mesh?

- GregB

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