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The RFI Problem
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Graham Slee View Drop Down
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Points: 16314 		Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 17 Jun 2008 at 1:56am
The only way to filter out RFI from unshielded cables is to introduce a resistor/capacitor filter which would screw-up the phase response of the phono stage. You did right going for the refund. In most other walks of life black-magic is laughed at with great scorn, but in hi-fi BS rules OK!
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damianwo View Drop Down
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Points: 8 		Post Options Post Options   Thanks (0) Thanks(0)   Quote damianwo Quote  Post ReplyReply Direct Link To This Post Posted: 22 Jun 2008 at 10:05pm
Hi again. 

My double shielded (both braid and foil) cable arrived from the USA (see the spec here: http://www.bluejeanscable.com/pages/technicaldocs/1694tech.htm).  Unfortunately the RFI problem persists.  This is whether the turntable is plugged into the Slee Bridge or not.  The RFI is only through the channel the Bridge is plugged in to and stops as soon as the Bridge is unplugged.

So I'm thinking the only things left that can be supplying the Bridge with RFI are the amp or the power supply.  The power supply is the one that came with the Bridge and the amp is a Marantz PM-57 circa 1999, so nothing experimental or fancy there.

I don't have another amp or power supply to test with.  I may be able to borrow an A&R cambridge A60 though, given a couple of weeks.

Any advice or thoughts?
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Graham Slee View Drop Down
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Points: 16314 		Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 22 Jun 2008 at 11:58pm
I simply just don't know.

We bought advanced spice modeling software recently to test our designs to ensure stability (phase margin) way beyond what the test gear is capable of. The results for the existing Gram Amp 1 Bridge show it is absolutely stable. If an amp can oscillate then it can detect RFI, but the gram amp doesn't oscillate. For an amp to oscillate it has to be very close to being 180 degrees out of phase where its gain is more than unity (one). It has a good phase margin.

It is a great shame manufacturers don't quote phase margins. The Gram Amp 1 EQ is built around the amp which guarantees it can't "take-off" (go unstable) and it would have to to be able to detect (rectify) a radio signal.

Thinking it through further however, there is a "kick" in the response at -120dB which is 100 times lower than the noise floor, but if you are running it into a very sensitive input, I guess this may be the problem - you're not going into the amps own phono input?

If your amp has got one, try the CD direct input as it has the lowest sensitivity.

By the way, this is a plot of the Bridge's response all the way from 0.0001Hz to 1,000,000,000Hz. The bit of interest is the part above the 0 on the Y axis - this is the RIAA curve, necessarily shaped like that to equalise your records so they come out with a flat response. The roll-off below 20Hz is the sub-sonic filter. The "kick" shown at 800MHz at less than -120dB has been ironed out a bit with a very subtle mod to the schematic, but if this is doing it then you've far too much gain.




Edited by Graham Slee - 23 Jun 2008 at 2:13am
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damianwo View Drop Down
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Points: 8 		Post Options Post Options   Thanks (0) Thanks(0)   Quote damianwo Quote  Post ReplyReply Direct Link To This Post Posted: 23 Jun 2008 at 2:28am
Thanks Graham, I'll try plugging it through the CD input, it's currently going in through the AUX.  I suspect that isn't what you mean though, my amp just has a standard CD-in rather than a direct CD.

I don't think there is too much gain as the volume produced by the Bridge through the AUX channel is similar to what I get from the amp's own phono stage which is a lot less than the CD provides.
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damianwo View Drop Down
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Points: 8 		Post Options Post Options   Thanks (0) Thanks(0)   Quote damianwo Quote  Post ReplyReply Direct Link To This Post Posted: 23 Jun 2008 at 10:06am
Tried the Gram Amp 1 plugged in through CD-in on amplifier.  No discernible difference.

But.  Went back to my old (RFI free) built in phono stage for a bit of a listen.  The Gram Amp 1 really does sound a heck of a lot better even with RFI.  So I'm sticking with it.  Maybe one day I'll upgrade my amp and the RFI will go away.

Thanks for the help and suggestions.
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