I frequently mix using “Hardware Inserts” in Pro Tools through my audio interface / converters, so the chain is:
- AD/DA converter balanced line output > outboard gear (balanced) input > outboard gear (balanced) output > AD/DA converter balanced line input
However, I have some outboard equipment that doesn’t play nice or sound right when interfaced this way (say something like a Pultec -- tube stuff and/or other pieces with transformers...), so I’m thinking of a simple solution / "utility box" for interfacing outboard hardware between audio interface / converter AD/DA to avoid potential impedance issues.
My first thought was to make a line driver to go between the line outputs on the D/A and inputs on gear, particularly w./ transformers on the inputs. But am I overthinking this WAY too much?
- 1st option I thought of would be to lay out a little board based on the data sheet for the THAT1606 line driver. Little tiny QSOP-16 package, what a blast to hand-solder. My brain went to the 1606 (rather than1646) because it already offers a differential input (e.g. balanced line coming out of audio interface), but does that even matter? Would a THAT1646 be fine?
Found this old thread on some ideas for using the THAT1646 as a transformer driver: THAT1646 Current Booster and Transformer Driver - Pro Audio Design Forum
- 2nd option was to use an API application note from the 2520 datsheet for a “balanced transformer-less amplifier” to drive a transformer (not unlike a 325, I guess), and/or just use something w./ transistor buffers like the Aphex/B&B “hybrid DOA” scheme to keep things simple and still be able to drive low impedance loads.
- 3rd option, to piggyback off of the API idea, would be to lay out a differential driver into "a high-speed, high output-current buffer inside the op amp feedback loop", like this scheme using the LME49724 which "drives 600R loads with full output signal swing" but I'd use LM4562 (datasheet) because I already have those around, and a pair of LME49600 (datasheet) which are headphone buffers: Source / original link to the scheme: https://www.eetimes.com/product-how-to-differential-line-driver-with-excellent-load-drive/
Final option: just get some 600:600 transformers (or something) & wire to XLR pigtails to interface between the converters and the outboard gear & call it a day?
This thread was also helpful: Driving step-up transformer directly from opamp?
For background regarding the I/O:
I’m using an RME UFX+ and a Ferrofish Pulse MX (over MADI to the RME). Input impedance for the RME A/D line ins are: 8 kOhm unbalanced, 12 kOhm balanced. Output impedance for the RME D/A line outs are 75 Ohm unbalanced, 150 Ohm balanced. Ferrofish data sheet doesn’t show impedance specs but says it is using RC4580s on both the AD and DA sides. Output level is switchable between +4 dBu and -10 dbV (or -8 dBu, ish) on both units. Likewise, I'm over here assuming my issues are D/A, but could it actually be the A/D side of things?
We have Burl Mothership B80 as our main converters at our studio, which has transformers on both the input and output cards. I rarely run into any issues with "Hardware Insert" interfacing through the Burls.
