Input transformer: floating input = distortion

[ErikdeBest]

Many thanks to all for the replies!

Hi Dsavitsk

"E" is currently grounded to the earth bus from the amplifier. I also tried the IT's without the center tap grounded, as per your recommendation, but that did not improve matters for the moment: distortion is still terribly high.

Hi Zigzagflux

That is sure worth a thing! I know a lot about the trial and error thing, I spent quite some time trying it different combinations, loads, earthing... I had this gut-feeling that the non-grounded CDP could have an influence on the performance, therefore I cited it in my opening post. Plans are to build an external DAC with balanced output in the near future, so I won't bother with modifying the CD player. The DAC will be grounded, as will the amplifier, so that will probably work!

I just went to the room to test your proposed solution with the resistors, and guess what, turned on the amp and it was playing perfectly with no ground reference at all. I even plugged in the cliplead again to see if it changed the situation, but it kept the same. Also no hum with inputs floating... maybe because its Sunday and she wanted me to listen to her! So I have not tried your proposed mods yet, but I will keep it in mind just in case.

Hi Peter

I measured for DC with my cheap DVM but could not detect any. Still I will keep that in mind as well for future debugging.

Erik

[Gary P]

My experience with balanced inputs and 2 wire connections (RCA cables) has shown that the chassis of each component must be connected to earth safety ground or you will get distortion and buzz/hum. 2 wire corded sources are very bad this way as they rely on the ground of the interconnect cable to reference the source to earth ground. If you measure the AC voltage (or AC current) between your CD player and earth ground (no interconnects attached, just the line cord plugged in and unit powered up) you may be surprised at the results.

To this end I have added IEC sockets and use 3 wire power cords on all sources and amplifiers in my system.

At this time all of the components except a vintage tuner that is not mine have XLR connectors and use standard XLR cabling. With balanced XLR cabling pin 1 provides chassis bonding while the signal is carried between pin 2 (+) and pin 3 (-). If the components are connected to safety ground the connection on pin 1 can be opened and terminated with ~47 ohms and .01uf for RF grounding.

Jensen recommends that the ground connection on pin 1 be connected to the saftey ground tie point and not to the signal ground. This is to keep ground loop currents between components out of the signal handling stages. This information is available from Jensen Transformers and other pro sound vendors.

See the Tabor schematics on my web page for details on hard ground vs RF ground on pin 1 of the XLR. The pin 1 ground point shown in the schematic should show the saftey ground symbol instead of the signal ground symbol as it does now. More things to update...

Tabor schematic

[pmillett]

Just to expand a little on what Gary said...

Most 2-wire CD players and similar equipment has an EMI filter that contains two caps (the "Y" caps) that connect each side of the line to "ground". In this case "ground" is the case and also probbaly the "ground" side of the RCA jacks.

Since the Y caps form a capacitive voltage divider, then the "ground" of the RCA is at 1/2 the line voltage when referenced to the real ground. It has a fairly high source impedance - a typical Y cap may be ~1nF, which is ~3megs, so it isn't a shock hazzard. But apply this to the primary of a transformer with no reference to real ground, you are asking the transformer to reject ~100V of common mode signal at 50/60Hz! That is a tall order. Interwinding capacitance of just 10's of PF would couple enough noise to be annoying.

You are better off keeping the transformer and connecting the "ground" to a real ground. I haven't found that it matters much if you connect it to safety ground or signal ground... each has some advantages.

Of course the best possible thing to do is to connect the Y caps back to the wall socket ground with a 3-wire cord.

Pete

[ErikdeBest]

Thanks for the extensive replies! I think I got the most of it: it is time to wire in a 3-wire power cord!