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In Reply to: RE: If you had really done it balanced there would be no hum. posted by Ralph on May 30, 2017 at 10:25:34
Thank you for your insightful posts!
In hindsight it seems so obvious to me and I feel so foolish now...
I made the mistake of tying each of the preamp signal ground to pin 1 instead of leaving it floating.
I don't know what I was thinking about.
As your previous post suggests this is not how balanced is done!!!
I need to rerun the experiment.
I'm especially feeling the pain since right after I posted yesterday I thought the experiment was over so I decided to switch my SL1200 back from shielded twisted pair cables to coax cables. Anybody who knows how much work it is to pull apart the SL1200 chassis to change wires knows how much fun I'll be having doing it all again to rerun the experiment. Grr!!! probably somewhere around a half an hour just to swap out tonearm cables probably. Grr. I like to do things the hard way sometimes.
Maybe I'll get back to it next weekend.
You don't need to rewire anything on the Technics SL1200. All you have to do is construct a different cable to go from the arm to the preamp.
The specs for doing this is in the owner's manual for our preamps which is a free download- but the important thing is that you don't allow the barrel connection of the RCA to come in contact with the shield of the cable, which ties to the ground post on the 'table, and to pin 1 of the XLR input on the preamp side (which is also ground).
I probably mis-spoke. What you recommended is in fact all I've been doing, ie swapping out the interface cable that solders to the little circuit board inside the TT. I had previously had some shielded twisted pair installed for the experiment then put some coax on there.
Just a little whining. No show stoppers.
I swap between twisted pair and coax depending which preamp I'm playing with at the moment. I didn't mention earlier I also have a full balanced preamp I'm playing with. The problem with this particular balanced phono stage is the noise. It's self noise, at equivalent gain, is around 5-10dB higher than where the hum of the 651p's are sitting which is also 5-10dB above their self noise. In other words the balanced performance of the balanced preamp is offset by it's crummy electronics design. Though it has no apparent hum visible in the FFT scans I've captured from it, it's wideband noise is pretty massive.
Yes, the noise is present with cart connected. I also terminate unused inputs with shorts.
I like to do my noise testing and troubleshooting with the equipment hooked up as I plan to use them.
The coils in my moving magnet cartridge seem to pick up a bit more noise than when I just use shorting plugs but not too bad really.
and ultimately built myself a balanced phono section. As far as I know, it was the first in the world. The preamps we make were the first balanced preamps for home audio. Seems normal now, but back then they were kinda weird.
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