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Old 27th January 2008, 11:11 PM   #1
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Default RFI and shielding help needed

Hi all,

I'm posting in SS hope this is the right venue.

I have made a phonostage - a solid state 2 stage one, some of you may know it, its the phonoclone. Link to more info:

The Phonoclone and VSPS PCB Help Desk


Its a superb performer - but a few of us have noted that its particularly sensitive to RFI problems - and at least in my case, it is the interconnect from the phonoclone to the amp/preamp that is acting as an antenna. Careful routing of the interconnect is needed to minimise the RFI. If you go to the last few posts of that thread you will see one member having big problems with it.

So I'm wondering, what tricks can be used to lessen this type of interference (interconnect is shielded)?

Can you do something like put a cap across the outputs or something?

Balanced is not an option here!


Fran
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Old 27th January 2008, 11:27 PM   #2
sam9 is offline sam9  United States
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Going back to the decades before CD's, RFI and other stuff getting into the phono input was perhaps the single most common audiophile problem next to surface noise. At least it was mine. Here are few things I recall:

Some interconnects were better than others. This had no correspondence to price. I think (but I'm not sure) that foil shoelding worked better than woven strands since when bent, strands can open up a gap.

Re-routing the interconnects can help sometimes. May just a an inch, sometimes several feet. Unless you have expensive instruments, this is a trail-and-error task.

Depending on construction of the component at each end, the shield may or may not actually be connected to earth rather than just internal 0-volt reference. I'm not sure there is a fixed rule about which is the right way plus what kills the RFI may give you ground loop buzz in exchange.

Sometimes no reasonable solution existed, especially when the neighbor was a CB radio fan.
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Old 28th January 2008, 03:01 AM   #3
Eva is offline Eva  Spain
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If the problem is really related to the output wiring, then there are a lot of chances that your op-amps are oscillating.
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Old 28th January 2008, 10:11 AM   #4
Mooly is offline Mooly  United Kingdom
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Hi, If R.F. is getting into the input it might be worth trying a low value resistor of say 33ohms in series with the inverting input of first op-amp together with a small ferrite bead around the leg of the resistor. Try something like this for starters. If it works try replacing the resistor with a wire link+bead. The bead should be as near the op-amp input as possible. This has the effect of increasing the impedance to high frequencys.
EDIT , I have just looked at the photo in your link, if constructed like this just thread the signal lead, the one from the centre pin of the phono socket inputs through a bead, one for each channel.
Regards Karl
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Old 28th January 2008, 11:27 AM   #5
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Mooly:

thanks, I will try the ferrite bead.


Eva:

I have an oscilloscope - but have never used it to do such diagnosis before. Exactly what would I look for and what should I expect to see?


Fran
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Old 28th January 2008, 12:53 PM   #6
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Very high gain amplifiers -- here are some hints -- but you might want to take a look at the excellent tutorials on grounding on both the Analog Devices and Texas Instruments websites.

1) bypass the power supply pins of the opamps with 100nF ceramic caps.
2) a ground strap between the chasis of the phono amp to preamplifier, as well as the usual ground strap between phono preamplifier and turntable.
3) are the IC's socketed? if you are using a tin plated sockets replace them with ones using gold plating -- they don't cost a lot more but are worth it in this application. You can also get snappable machined sockets -- these would have been used in the high quality instrumentation from Hewlett Packard, Boonton and others back in the 1980's.
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Old 28th January 2008, 01:32 PM   #7
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Which cable do you use to connect RIAA to the preamp and with what kind of shielding?
Balanced (cable, not connection) might be an option here, with directional soldering. You can use + and - leads for the signal and solder the shield at only one end. Normaly, that end should be on the source side, but you can actually reverse it and decide when you get less interference. The third option is to solder the shield on both ends.
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Old 28th January 2008, 03:10 PM   #8
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Thanks again.

jackinnj:

I will check but I think the PS into the opamps is already bypassed.

I don't have grounding connection to the preamp - I will try this, but the case/circuit is grounded to earth

Gold sockets are used.

I think the opamps are either OP27 or 37 - would need to double check but I think either will work.

Willi Studer:

Copper braid on the interconnect - one end connected to ground. I do have some cable that is 3 core with foil shield and I could try that - although it sounds poor. I have also tried a number of cables, both homemade adn commercial and they make very little difference.

Thanks for the help so far,

Fran
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Old 28th January 2008, 05:02 PM   #9
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You need to keep RF out of the box. I'll assume you have a metal box. If you seal the doors of a house in a flood, you stand a chance, but once the water comes in, all the bailing in the world won't help. What you need is to address RF right at the connectors. Small caps from the signal lines right to the chassis. The input signal is mostly current at very low voltage, perfect for a ferrite bead. I'd say put the ferrite bead right near the connector, just after the cap to the chassis. You want to create a filter right at the connector, not let the RF get any distance into the box on the wiring. Do something similar on the output. Remember that the usual phono input will have an isolated ground, brought back to an internal ground point. If this is the case, you have another RF leakage problem. Put the same cap to chassis followed by ferrite bead on the ground. Pick up a copy of Ott's noise book if you can find it, hopefully used, as new books are getting out of sight.
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Old 28th January 2008, 09:29 PM   #10
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Hello i am the other person with noise problems
i hear voices
and also hum that is more or less when i move or touch the cable or sometimes come close to the cable ore case.
It is the same amp as Fran build and i have only problems with the input side
and i tried every ground and earth connection i think.
it is written in the last posts of this phonoclone toppic.

on this pages are some pics of the case i build :
The Phonoclone and VSPS PCB Help Desk

Thanks a lot so far
Robert
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