audiomixer said:
JohnRoberts said:
Looks like a lot of expensive parts for a mono sum bus.
quite obviously this is about very low noise summing and not about budget. I do know that in most topologies noise will be set by mic / preamp noise floor,
Thank you for saying that so I didn't have to. 8)
but there is a development in ultra low noise & distortion..... good thing.
New modern parts are always nice, but I tend to look at such things agnostic about the specific parts and looking at the pros and cons of different topology. So all things equal is this a better approach or just a better part. (while better parts are OK).
In my 1980 article about console design I identified the sum bus as one of the several problem areas for console design because of the noise gain associated with combining N stems (typically make-up/noise gain of N+1).
This "balanced sum bus" is a variant on virtual earth topology so subject to same N+1 noise gain. (noise gain in VE summer is responsible for N+1 noise, phase shift., and distortion.
I have the same OPA 1632 on my bench and my mixing bus is prepared for balanced buses for the main LR bus... looking forward to what this might bring.
others pay tons of money on transformers, so hey, let's embrace those low noise opamps and squeeze the last bit of performance out of them, yeeehaa!
- Michael
I am not a fan of transformers for audio paths so I expect this (1632) instead of transformers should be an improvement.
From my 30+ YO article I observed two different sum bus topologies better than virtual earth (in my judgement).
One was based on Buff's "Transamp" mic preamp topology, perhaps better known around here as "Cohen". While it too suffered from N+1 noise increase, the beauty of the transamp topology in a virtual earth sum bus topology is that the increased number of stems assigned to the bus increased the open loop gain of the transamp the same amount as the closed loop gain increased so, the phase shift and distortion from even a large number channels summed together was not much worse than an unity gain opamp. The transamp input noise was amplified N+1 though.
My favorite approach (then and now) was to synthesize current sources to take the place of the sum resistors feeding the bus. While the relatively small noise from these active synthetic current sources was accumulated (incoherently) in the bus the virtual earth sum amp did not suffer from any of the typical N+1 noise gain, so noise, and phase shift, and distortion were all quite low even with a large number of stems feeding the bus.
I have designed consoles using both of these approaches. They work.
I wish analog consoles were still relevant so I would have an excuse to apply modern parts to my 20-30 year old designs. My friends who are still trying to make and sell analog consoles have fallen on hard times, as modern digital consoles kick their ass with tons of features for silly low retail prices. Maybe they need to find some cost is no object customers. There is never enough of them when you really need them. ;D
JR
PS: Yes I know there are still some people using analog consoles for recording... problem is there are so many decent old ones for sale used, there isn't much market for new ones.
PPS: I am not trying to rain on EJ's design, get the 0V sorted and build it. let us know how it sounds.