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Help please - Naim phono RF mod?

YNWOAN

100% Analogue
I hope someone could help me with an issue I am having with my modified external 323 cards?

I believe Naim have an RF mod that they applied to the Prefix; this consisted of moving a couple of resistors so they connected more directly to earth and the addition of a couple of capacitors. I've moved the resistors but am not sure about the capacitors. Could someone tell me what value capacitors are required and where exactly they go (preferably with regard to 323 boards)?

Many thanks :)

NaimPrefixRRFMod.jpg


This pic is of someone else's Prefix - I can see the capacitors but can't make out what value they are or where exactly the connect to!
 
It's one cap that ties the cartridge phono cable shield to the incoming 0v star, avoiding that large loop in the middle of the board.

The capacitor value isn't too critical, try 4.7nF or 10nF or so - in something with the shortest leads possible (= low inductance), which is the bit which matters. Ceramic caps fine for this - as the Naim version you picture shows. NB Maplin sell the excellent Wima FKP film/foil caps for pennies, which are also ideal.
 
So that's incoming tonearm earth straight to the output earth on Les's pcb via cap, I have wima if you need. It won't make any difference though it's your unshielded tonearm wire that is picking up ;-). You already have a similar shunt fitted on the main pcb.
 
Hmm..wondered if I did. Will try your Wima though - thanks :) (can't do any harm - could always take it off again).

He has a resistor parallelled to that cap if I remember correct - perhaps it would be better removed?
 
RFMod.jpg


The circled cap is what you are referring to Si (you can see the resistor I mean - I don't know what the purpose of that is?). There is a lot of circuit track though, The conceptual earth point on the board is B, but I suspect the cap would be better fitted at A - thoughts?

I'm not sure it is the arm cable, I've tried disconnecting it and the issue remains - I think it's all the unnecessary circuit track on the motherboard acting as an aerial.
 
I notice your BNC connectors are not insulated - so you connect the signal shields direct to case earth.

I think you want an insulated BNC connector, with each channels shield (the BNC shell) going to the respective point where you have a bit of bare wire coming in.
 
I know it is sometimes radiostations on various wavelengths.

I have a prefix or two with rf mods but still sometimes gets specific and very familiar noises through the speakers. I think it is mobile phone noise as the radio at home and in the car picks it up if the phone is nearby.

The best way I can describe it is a short burst of low frequency morse code. Da di da dit dit dit. Any ideas?

Not a good contribution to the thread, sorry. I had hoped to post a photo of Prefix internals, but I cant find it! Love the Avondale pcb though.
 
The best way I can describe it is a short burst of low frequency morse code. Da di da dit dit dit. Any ideas?

Yeah, that's your mobile phone searching for a provider - I don't think there is anything that can be done about that as the signal it sends out is really big; just put your phone further away.
 
I notice your BNC connectors are not insulated - so you connect the signal shields direct to case earth.

I think you want an insulated BNC connector, with each channels shield (the BNC shell) going to the respective point where you have a bit of bare wire coming in.

Thanks for that. Actually, the situation isn't quite as that pic shows, the return of each BNC is individually wired back to the ground pin of each phono card.

I've just wired a 0.1uF XRF ceramic capacitor from the shell (return) of the left channel XLR (input) to the earth point A. is that OK? Do both L and R Inputs returns need to be commoned first and then connected to earth via the ceramic cap?

I've also been given a 0.47uF Wima cap (thanks Si for both caps :)) which I could use if that would be better?

The returns of the L and R BNC's are wired up like this - going directly to the Ground of each phono card:

BNCwiring.jpg


At present I have wired the 0.1uF ceramic cap as per diagram A, but I could do as per diagram B (and I could use the 0.47uF Wima instead)?

RFModoptions.jpg
 
I notice your BNC connectors are not insulated -I think you want an insulated BNC connector..

Actually, I think they are insulated - the pics aren't a very high quality (sorry).

They are insulated on the back from the panel and internally the + and return emerge from the insulation. The fitting nut makes direct contact with the case, but it doesn't make contact with the electrical return (just the insulation).
 
I would use two small caps (say 2n to 10n, ceramic fine) between the BNC shell, and the nearby case earth - DO NOT TIE THE BNC EARTHS TOGETHER
 
Yeah, you can see the ground coupling cap and resistor move have been incorporated into the circuit board.
 
I would use two small caps (say 2n to 10n, ceramic fine) between the BNC shell, and the nearby case earth - DO NOT TIE THE BNC EARTHS TOGETHER

OK, thanks PG, I've done as you suggest; the only difference is I used 10nF Wima's (as suggested by Martin earlier) as that is what I had (Naim told me the value of the cap they use is 10nF).
 
Oh, well these are tiny :). I've fitted them just as you suggested and so far I can hear no RF breakthrough at all; it is very variable though and seems to like times of high pressure (cloudy now).
 
B. looks spot on Mark

Well, following PD's advice above, I've actually wired it up so that each of the + and - returns are individually wired to the case earth via a 10nF capacitor; rather than commoning them and then connecting to earth via a cap as diagram B shows.

your bnc's are insulated.

Yeah, I had a check, they are.

However, it's not resolved the issue :( - reduced it perhaps. Do you think the caps have to be a different value?
___________________

DO NOT TIE THE BNC EARTHS TOGETHER

Can I ask why not? I have done as you suggested, but I think they are tied together at this point in the Prefix. Presumably it is because it will effectively be one long cable connected to earth by a cap as opposed to two discreet cables that are each connected to earth via a cap?
 


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