unbalanced to balanced conversion

[Lavcat]

Unbalanced to balanced conversion between components is an old problem, and I found several prior threads where it was discussed without concensus.

In my case I want to use a Threshold FET ten/hl to drive a Crown D-75A. The Threshold has RCA phono outputs and the Crown has XLR/phone inputs. I do not really want to use a transformer, or I should say I really do not want to use a transformer.

I intend to use a short RCA to XLR adapter cable. The question is how best to wire it. On the Crown XLR connector pin-1 is shield, pin-2 is signal plus, and pin-3 is signal minus. My inclination was to use two conductor shielded cable with the RCA pin connected to XLR pin-2, RCA frame connected to XLR pin-3, and cable shield connected to XLR pin-1 but unconnected at the RCA end.

However in the D-75A manual Crown shows to connect pins 1 and 3 together at the XLR connector. This seems wrong to me, or at least not as right as it could be. I wrote to Crown asking for clarification but had no response today.

In searching tonight I found an oft cited paper from Rane Sound System Interconnection that would seem to agree with me. See figure 4-17.

However Crown makes the amp, not Rane.

Given that I'm not about to change the amp or preamp at this point, what is the current thinking here on best practice? I appreciate any help.

[MarcelvdG]

Are there any other ground connections between the two apparatus? If not, I would connect the shield on both sides.

[AndrewT]

Convert your source to pseudo balanced output impedance.
From Jensen AN003

[godfrey]

Originally Posted by MarcelvdG

Are there any other ground connections between the two apparatus? If not, I would connect the shield on both sides.

Connected together at the RCA side so ground currents flow through the shield, not the signal - wire.

[MarcelvdG]

Originally Posted by godfrey

Connected together at the RCA side so ground currents flow through the shield, not the signal - wire.

Yes, that is what I meant.

[Lavcat]

Originally Posted by AndrewT

Convert your source to pseudo balanced output impedance.
From Jensen AN003

I saw that Jensen app note but I hesitate to modify a vintage piece of Nelson Pass equipment. Likely the only one I'll ever have.

Besides, I've never yet figured out how to non-destructively remove the cover.

[Lavcat]

Originally Posted by godfrey

Connected together at the RCA side so ground currents flow through the shield, not the signal - wire.

This is what Jensen suggests in their app note. Rane says to leave the shield open on the RCA side. Crown says to connect shield and ground at amplifier input. I am as confused as ever.

Would anyone suggest using a transformer? Good line transformers are rather expensive, which is one reason I would rather not use one. ("One," ha, make that two.)

But if a transformer would give better results I would consider it.

I never said what I am using now: I have some old phono to phone adapters, and I am connecting them with some old rather average cable with RCA on each end.

Right now I am getting a clean signal without any hum that I can hear, but depending on the phases of the moon and how I breathe on it I get either barely perceptible hum (in headphones) or somewhat more. I'd like to figure out the best solution whether I can afford it right away or not.

[MarcelvdG]

Hi Lavcat,

When you are mainly concerned about mains hum (as opposed to RF interference), you would normally want to have one and only one ground connection between the two apparatus. Without any ground connection at all you could get large common-mode hum levels on your input (because of mains filters or because of the equipment cases acting as electric aerials). Common-mode hum is suppressed to some extent but not perfectly. With two or more ground connections, you get ground loops and you have to ensure that the ground loop currents don't have any impedance in common with the signal path, which is basically impossible when there is unbalanced equipment anywhere.

So if there is already a ground connection, then connect the shield on only one side. If there is no ground connection, connect the shield as drawn by Godfrey. If you also have problems with radio-frequency interference, connect the shield as drawn by Godfrey.

Further refinements are possible by inserting an extra impedance in the negative line to improve common-mode rejection, as suggested by AndrewT, or by using a signal transformer after all, placed close to the unbalanced output and not in a strong magnetic field.

Best regards,
Marcel

[Pano]

Yep. Agree with the above. In general practice and for short runs (under 10 feet) then just do the pin 1 + pin 3 trick, it will work. I have a dozen such cables here, no problem.
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