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Hi,
I am hooking up my buffalo/legato combo for the first time using a balanced power amplifier but the only balanced interconnects I have at home now are unshielded ones. I am getting a very low level radio signal on the speakers with this hookup and have now been reading up on RFI noise and cable shielding and something they call the pin 1 problem. As I gather the recommendation is to always connect pin 1 on an xlr connection directly and only to chassis. This is so that any noise currents picked up by the shield (or ground connector for unshielded interconnects) should not get a chance to get into the circuitry of the device but instead be shunted to safety earth.
So my simple question is: can I simply connect pin 1 on the xlr going out of my legato to chassis instead of putting it on the circuit board GND? Or is this undesirable (or impossible) in any way. On the amplifier side pin 1 goes straight to chassis.
I know I could probably just use a shielded interconnect but I always like to chase down these little things and if nothing else learn something.
Thank you for a great product. Did I mention that the sound is simply amazing!
/seb
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Sure its OK to connect it to the chassis. You can even just leave pin1 floating on the DAC side.
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Thanks for the quick reply!
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Rank: Member
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Ok, so I tried this and what I get is a really loud hum. On both speakers. When I pull the plug on the amp it decays slowly.
Since the GND on the legato balanced out is now unconnected I guess that it is now floating with respect to the DAC chassis (previously having been tied to the amplifier case chassis) and this could be the potential difference that shows up on my speakers? Could this be right? Should I then just tie down the Legato balanced out GND to the DAC chassis?
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Sounds like you simply have a good old fashioned ground loop somewhere. You could try joining the power GND and the chassis ground, but something tells me that won't solve the problem.
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Ok, here is the situation. I have now reverted back to the original configuration but since I tried this mod the left channel is noisy. The right channel also has a very slight hum that was not present before but on the left I have a hiss(?) that is pretty loud. (I'm not certain about English vocabulary regarding these things but it sounds like a white noise that is high pass filtered. It's about three to four times louder than what I had before and what is still present on the right channel).
If I connect only the left channel to the left channel amp and speaker, leaving the right channel unconnected, I still get the noise. If I do the same to the right channel it is almost dead silent. Another symptom is that the left channel signal seems to be a little bit weaker than the right, tilting the center of the stereo image quite severely.
I worry that some component might be damaged since I had the big hum. It really did not feel just like a standard ground loop since it was so loud. It was more like 'wow, this is bad' and I immediately pulled the amp cord.
What do you think?
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Its very difficult to say without seeing some measurements. But I think if I were you I might try replacing the transistors on that half of the Legato.
I would also measure voltage across the R1-4 to see if you might have some wild offset (in fact I would do that first).
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Voltages across these resistors are the same for both channels, 7 volts (assuming these are placed in the same spots for my legato 2 and the 3.1 which is now on display on the webpage). So nothing seems to be going on there. Right now i just really hope its not the buffalo that is messing up. I think maybe I figured out what went wrong in the first place. My amps are DIY chipamps that are "pseudo balanced", a recipe by Thorsten Loesch. Its in Not just another gainclone thread. So the + and - signal from the Legato goes straight into the + and - legs of a LM3876 chip. Perhaps it is safe to fiddle around with pin 1 as long as you have a tranny or bal/se stage on the recieving end, but with this configuration it means disconnecting the signals from their original reference ground and amlifying it like that? Or am I talking nonsense? When I did tie down both pin 1 from the xlr:s and the legato outbut GND to the DAC chassis everything worked fine, except for the faulty channel that is.
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Ok, I think I've got it. Even though I had gone over it twice it seems like it was a poor connection to the shield on one of the cables. Sorry to have been wasting your time on this! Feel a bit embarrased  but I really thought I had excluded that as a possible error source, having reconnected everything and all. Thanks for everything anyway!
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As an addendum to this thread, I too had noise problems and disconnecting pin 1 from signal ground as well as disconnecting signal ground between the DAC and Legato not only fixed the problem, but had clear sonic benefits.
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