Q Can I balance my mixing desk inserts using a patchbay?
I recently bought an old analogue mixing desk, where all the channel and bus insert points are unbalanced, which is quite common, I think. But all my outboard gear has balanced inputs and outputs! I want to bring all the inserts out on to a patchbay for obvious reasons, but am not sure how best to avoid noise or signal-loss issues. There are 40 inserts in total, so putting transformers on every channel would get expensive!
Preston Baker, via email.
SOS Technical Editor Hugh Robjohns replies: There's no simple off-the-shelf solution here, as you may have to employ different strategies for different devices, depending on the nature of their balanced I/O interfaces. You'll certainly have to break out the soldering iron, too, and make up some custom interface cables. The easy bit is that the patchbay should be a standard balanced type, and all the outboard connected to it in the conventional way.
When connecting unbalanced devices there's always a risk of creating ground loops and suffering unwanted noise and hum as a result. The best way to avoid that in the situation you describe is to wire the unbalanced insert sends to the patch bay in a 'pseudo-balanced' form, using the same basic idea as the SOS pseudo-balanced cables we offer for connecting unbalanced synths and the like to balanced inputs. Basically, the unbalanced sends are wired to the patchbay socket tip terminals, and the unbalanced send sleeves to the patch-bay ring terminals. The patch-bay sleeves are left isolated. In this way, the balanced outboard will receive the unbalanced send signal across its differential inputs — so it gets the full signal — but there will be no direct ground path to create ground loops.
Dealing with the insert returns is slightly more complicated, because it depends on how the outboard equipments' balanced outputs are designed. You'll need to check with the outboard manufacturers' manuals for the preferred wiring format when interfacing with unbalanced destinations — it will vary depending on the type of output circuitry involved. In most cases you will need to wire the unbalanced insert return to the patchbay tip and link the insert sleeve to the patchbay ring and sleeve.
It's possible that some outboard devices will cause problems with ground loops, and in those cases you may need to consider using isolation transformers to provide balanced/unbalanced conversion, as well as galvanic isolation. However, rather than wire isolation transformers permanently into these specific signal paths, the more flexible approach would be to wire them into spare patch-bay sockets so that you can then patch through them only when necessary. In this way the transformers also become available for other purposes — such as introducing some deliberate 'iron saturation' when mixing or mastering, perhaps!
You could use one of the commercial line-isolation transformer boxes like those from ART, for example (the T8 or multiple CleanBox 2s, perhaps) or install individual line transformers from Sowter, Lundahl or whoever directly into the patchbay itself if your DIY skills allow it (just make sure they are protected from stray external magnetic fields!). The transformers would be wired balanced in/out in the usual way, as the insert return sockets' wiring (as described above) will enforce the required format conversion.