XLR Attenuation

[Ascari]

Alrighty then,

I have had my BeePre up and running for at least 5 years now.....    It has worked flawlessly and besides some tube rolling I have not made any changes or adjustments.    They simply were not needed.   Then within the last month or so, I upgraded my power amp and got the urge to see what XLR cables were all about.   Everything I have read says they are superior to RCA's especially with longer runs of cables.    Since I moved the new amp further from the preamp, I went for it and purchased some cables.    I plugged them and and they work....  Except for one little issue.  The volume is extremely attenuated.    I have to turn the BeePre up to about 80% of max to get a descent volume.   With RCA's I never really go beyond or even to 45% of max.    My assumption, even at this time is that has to be something wrong with the build, my usage, or even expectations.    For reference, I am using them with the following equipment.   VPI Classic 2 with a Soundsmith Boheme cartridge  2.5 mV, Project RPM 1.3 with the Hana SL .5 mV, homemade SUT using cinemags, the Eros phono, BeePre, and either an Emotiva XPA-2 or the Vincent SP-332.   Both of the power amps have XLR inputs and the result is the same with each one..    I am hoping it is really something simple, but with my luck.....       

[Paul Birkeland]

What we would hope is that the input circuitry in either of those amplifiers would be differential.  That would allow for the balanced output of the BeePre to work properly, as the XLR output jack has equal impedances to ground for +Ve and -Ve.  I went through the manual for the XPA-2, and it's "differential this" and "differential that", but then there's this:

" Each Differential Reference Amplifier channel consistsof two complete signal paths, one for each phase of the audio signal,"

That's not differential, that's push-pull!  Another hint is that the balanced/unbalanced switch completely switches out the RCA input, which is an indicator that this likely feeds both the + side and an inverting op-amp going to the negative side.  With a differential amplifier, you just ground out the unused pin on the XLR to use the RCA jacks, and you'll often see a little metal bar that is made for just this purpose to stick in the XLR jack.  In the case of this amplifier, you would want to use input transformers placed close to the power amp, something like a 10K:10K with either a CT on the secondary or a pair of 5K resistors from each end to ground.   

I downloaded the manual for the other amp and it was in German.  I'm not feeling that motivated today!

[grufti]

I'm German, so it didn't take as much motivation for me to go through the manual for the Vincent amp. Even so I still have no clue why you are having the problem that you are having. It is very counterintuitive. I use mostly balanced studio equipment and this sort of thing has never happened to me.

Both of those companies are reputable. They sell lots of gear. I doubt that either one of them would mess up something as simple as a balanced input.

Measuring is always a good idea. Measure the AC output of your BeePre set to about 50% directly at the unbalanced and at the balanced output. That takes the cables and everything else out of the equation. Play a 1kHz tone from a test CD or a PC app or xxx. Measure between pins 2 and 3 on the XLR, between center pin and shield on the RCA of your BeePre. Measure with your DMM set to AC. You might have to play a tone closer to 60Hz, if you are using a very basic DMM.

[Doc B.]

Try measuring resistance from pin 1 to pin 3 at the input XLR of the amp with the switch set to XLR.

[Ascari]

Sorry everyone.   I have been really busy being locked down and haven't been able to give an update.      OK,   I can't use the locked down excuse.    How about, I've just been busy?     Anyway,  I did check into the resistance from pin 1 to 3 on both amps and they were very similar.   Can't remember the actual numbers but since there were reasonably close that eliminated them as a culprit.     I then checked the cables themselves.   Oh my...    Without going into all of the trials and tribulations, the answer was the Cables........   I was seeing 30 to 35 Ohms.    I have a set of Magneplaner speakers, and I can tell you from other experiences that are really sensitive to this type of issue.    To be honest, I really didn't think that cables could really impinge on my system.    (Learning mode one.....   Wrong.)     The first set of cables were really meant for guitar amps and how shall I say??  Cheap.    I opted to go with a better quality, not really that more expensive set and what do you know....   All better.     Just another learning experience for me.    Thanks all for your input.....