16-01-2021, 06:16 PM
Well, floating would be a 3rd option. I doubt the expense of a good transformer is justified, but if you happened to have them, I think I'd put them at the receiving end to improve the CMRR of the speaker's input stage (still have the impedance balancing at sending end). Alternatively, a quasi-floating output stage could be done, but I've never really been comfortable with them, as they are a complicated mix of positive and negative feedback, and while they work well on the bench, there's not a huge amount of data about real-world performance as they are relatively uncommon. I often wonder how well they cope with the large amounts of RFI we see today. The designs I've seen in the wild didn't have much in the way of filtering, but those all dated from 30-40 years ago when the air was cleaner.
Yes, that's very high. If (reversible) mods are on the table, then I'd be tempted to try lowering R117 to 100R, and replace R118 with a short. I'd probably up R119 to 10k, as the TL07x op-amps aren't very good at driving low impedances (the increase in distortion is in the "measurable" rather than "audible" category, but when it's so easy to avoid it, you might as well). If the the muting transistors no longer attenuate enough, then a relay could always be used. The impedance balancing resistor for the cold signal could be installed externally in whatever plug/socket arrangement you come up with to get from the '34 to the T&E. It would be lovely to have XLR sockets on the '34 itself, but I've no idea how practical or reversible that is - it's been decades since I last had one in front of me.
That prompted me to look up some photos, and my attention was immediately drawn to the Rifa across the mains
No, I've never seen anything from Alesis, so no idea how they might sound or measure. Will keep an eye for them in future.
Yes, that's very high. If (reversible) mods are on the table, then I'd be tempted to try lowering R117 to 100R, and replace R118 with a short. I'd probably up R119 to 10k, as the TL07x op-amps aren't very good at driving low impedances (the increase in distortion is in the "measurable" rather than "audible" category, but when it's so easy to avoid it, you might as well). If the the muting transistors no longer attenuate enough, then a relay could always be used. The impedance balancing resistor for the cold signal could be installed externally in whatever plug/socket arrangement you come up with to get from the '34 to the T&E. It would be lovely to have XLR sockets on the '34 itself, but I've no idea how practical or reversible that is - it's been decades since I last had one in front of me.
That prompted me to look up some photos, and my attention was immediately drawn to the Rifa across the mains
No, I've never seen anything from Alesis, so no idea how they might sound or measure. Will keep an eye for them in future.
