I have always read that RCA-XLR cable was iffy. I tried to do a conversion using a passive pre-amp that had both types of input/output, i.e. RCA in/XLR out. It didn’t work. When you say custom made do you mean with your specific components in mind or custom made as regards shielding etc.?
In the case of adapters, I tried a set of male XLR to female RCA Switchcraft 324 adapters on my headphone amp input transformer. They wouldn’t seat. Tried a pair of Cardas adapters and they worked fine. Used the Switchcraft adapters in another place. Worked fine. Here’s the Blue Jeans Cable info:
You asked for it, so don’t blame me for inducing your headache here brother
(This is for when you don’t have balanced outputs obviously but amp does have balanced inputs).
From someone WAY smarter than me:
“First principles approach is there will always be some common mode ground noise currents flowing in your audio cables shield, due to low freq magnetic fields, RF from SMPS and cell phones, and from direct injection by the various products’ AC/DC power supplies.
And your audio cables all have non-zero ground impedance. So if this CM noise current flows in part of the cable that’s used for signal (e.g. RCA’s shield is also the signal negative wire) then it’ll corrupt the signal as hum, machine noise, hiss, etc according to V=IR where R is the RCA shield impedance.
But if this CM current is given a preferred pathway (e.g. separate shield wire in XLR cable) then it won’t corrupt the audio. Similarly if given a lower impedance path (e.g. very short RCA, or more-copper in the RCA shield) then it’ll corrupt less because the “R” term is smaller in V=IR.
An RCA>XLR cable, if built properly, will provide a mostly separate pathway for CM noise current to flow. It’s a half-way house.
Now the question is whether or not an amp can handle an unbalanced signal on its balanced input…. if the amp is designed right as a “differential” input stage, then no problem (as is the case with most product).
XLR pin 1 Zin is < 1 ohms to ground, and XLR pin 3 Zin is much higher around 1 meg ohms to ground. CM current can be modeled as a current source. According to Kirchoff’s Current Law and the ratio of these impedance pathways, a proper RCA to XLR-3 cable will force the vast majority of CM current to flow into pin 1. The pin 3 wire carries negligible CM current, so incurs no hum-inducing voltage drop.”
Built it myself. The recommended Belden1804 cable (star quad, silver plated) and shielding method came from John Swenson.
Yeah, i use BlueJeans XLR cable. A quote from the link you supplied, BOLD font mine.
"In general, where there is a choice between connecting your equipment balanced or unbalanced, it’s better to go with a balanced connection. This should be distinguished, however, from the situation where the device at one end of a connection supports either balanced or unbalanced connection, while the other device supports unbalanced (usually through “RCA” connectors) connection only. In such a case, it’s almost always better to hook your equipment up unbalanced to unbalanced, rather than try to rig a balanced-to-unbalanced connection.
It’s great advice to always first check with the amp designer. These days it’s crazily easy to directly reach the designers of the gear we enjoy.
In my case, I discussed directly with THX 789 design engineer Andrew Mason - he was also part of the design team of Benchmark’s HPA4 state of the art headphone amp… I assume he knows what’s best for his amp/s…
If someone wants to argue with the designer about what is best for his gear or if there is any technical merit at all to this hookup method with the gear he designed, then I give up
But yes - always check with the designer what they recommend (and why…).