While a perfect accommodation of my application is welcome, I want to be clear that the supporting circuitry has to work for other applications, as a standalone gain stage. This needs to be usable with not only the LO2567/LO1166 and modern knockoffs, but also the Carnhill and Sowter LO1173 equivalents, and even the API 2503 and 2623 and their equivalents. Basically any step-up between 4dB and 10dB gain whose secondary is set up for 600Ω.
All those transformers are known types with good response so no reason it wouldn't work. We already know the amp can drive 50 ohms. The other original configurations of the Neve gain block with transformer use lesser known types with a tertiary feedback winding etc. That's why I thought this the best way.
I'm not sure I see this as a group project for widespread use though. Maybe I'm wrong? It's a lot of output amps on a single board, perfect for you but not many folks have a Neve desk with spare transformers inside 😃
As I mentioned above, the values of components that have a user option associated should be indicated by their function and not their value (these being Rfext., Rg or RVg, the series output resistor, and Rp1/Rp2/RVp should they be a good choice)
The parts can be indicated with whatever system you want or think best, doesn't matter to me.
The 150µF on the inverting input is a size advantage over the 470µF, so I'm glad that's possible. No BA489 version uses a 1000µF smoothing cap, and because of its size (12.5x30 as opposed to 10x25), I'd like to understand if it is completely necessary in the 1081 version. Also I am curious how running series/series will change the output choices as opposed to the 1081 parallel/series.
Force of habit on upsizing the 470uF to 1000. My bad.
Not sure what you mean about the output choices, transistors or transformers? If transistors, then using 2N3053 over BC441 etc has no bearing on how the transformer is strapped.
Does it change the operation of the amp to bring back NFB into the inverting input at the node between the fixed gain resistor and the blocking cap (1081), versus bringin it back in to the node between the blocking cap and the inverting input? Regardless of values, this won't be a user-selectable option.
If the cap is at the inverting input junction (1081), the cap is within the loop. Non linearities of it are then equalised.
I don't think we need a separate set of gain boost pads in parallel with Rg, because its value will be set by the user prior to assembly. It may however be smart to give Rg additional pads for a trimmer (RVg), which potentially could negate the need for a pad. A trimmer in this position would need a range fix in series.
Having a place for another gain boost resistor in parallel was just replicating the function that was brought out to the rear connector on Neve amplifiers. A trimmer is fine too if that's best for you.
I see the advantages of a slugged rotary fader. Are you thinking of putting this onboard as a trimmer, and providing hookup points for folks to offboard it if they wish (i.e. a pin at the non-inverting input as opposed to just a solder pad)?
Yes to both.
Any difference in function within the 10Ω-47Ω range of the series output resistor, or is that just a way of saying "anything within this range will do"?
It's not known if you will need that isolation resistor in your situation given the distance to the transfirmers Ideally you don't as anything there will raise the output impedance of the amplifier. If you do need it then start at 10 r and move up if necessary. Stop as early as possible.
Hope that helps.