Most all the Neve's I played with had BC184C transistors, so a later iteration. 
It is possible to bump up current on those for sure but I honestly don't know about the BC109C's

I think only mentioned BC109s after you did but I may be wrong.
For some reason the BC184C does not appear in the Blue Mullard book from September 1974 (when I was at Neve) even though the later BC549 does. Wikipedia argues these were all part of a sequence BC109-->BC184-->BC549. From the 1974 Mullard data book the BC549 has identical noise curves to the BC109 so it is reasonable to assume the BC184 is the same.

If you look at the NF curves for BC109 and 549 you will see the lowest NF of 1.75dB can be obtained with a collector current between 0.1 and 1mA and a source impedance of around 800 ohm which I think I said is the typical impedance a 200 ohm mic presents to the circuit. It is difficult to see how simply altering the collector current could make much difference in NF. (pdf of NF curves attached)

These days we can cheat and put a low rbb high Hfe BJT in there, although it requires re-jigging the circuit as everything in those things is intertwined and there're only 3 transistors anyway.

Which low rbb high Hfe BJT do you have in mind?

You might change one resistor, and then the 2nd transistor gets starved and farts on ya.

Yeah but that is just tinkering not proper design.

Cheers

Ian

AARRRRRGGGGGHH! So, that weird overtone series is also there if I run the signal through my FET/500 in the same lunchbox, and also there if I run the signal through a standard racked 1073 clone So, looks like its more of a Logic Pro or Apogee Symphony overtone problem (which sounds much more expensive).

Thank you to all of the kind people would made suggestions on how to fix things. Looks like the filters were just fine (despite me un-soldering and resoldering most of the components) and I can get on with making faceplates.

All the best,

Marty

Hello!
would you guys help me reasoning what is happening with this Allen heath gl4000 mixer?
I made a video to better show the problem. It’s a noise buzzing in all the channels and master section constantly, but with some rises at moments (if you turn up the volume you can hear it on the video).

https://youtu.be/fkKeS8yi6yM

I got this mixer for cheap, and it’s really old, but the strange thing is that I gave it an extensive try at the buyer’s place measuring every channel and function with white noise and a spectrum analyzer and it was damn quiet. The buyer said he had the ground lifted on his mains power though. When I got the mixer in my studio I found this noise showing up magically, I tried to lift the ground too and it lowered a bit but the noise is still there.
Electrical power in my room is not stabilized or filtered, but I would not think it’s impacting so much.

I am no electrical engineer but in my ignorance i was trying to figure it out. I opened up the power supply unit to see if the condensers were consumed, but they were not swollen or bad looking at all. I don’t know if that means I still have to replace them. But I’m also thinking there might be some ground loop somewhere else.

Overall the mixer looks functional, many faders are hard to move like they need some deep cleaning or replacement, but they work. Only one channel and a couple of groups are not passing signal, but everything else looks good.

Do you have any suggestions about what should I look for? It would be great to have some kind of idea or strategy before I open it up.

Thank you!

Spend some time on getting the gains right using the pots on the front. The best I get is mild dips and bumbs with a max of about 0.2 dB off unity gain. Not bad, I'd think.

For me this was an academic investigation into how well 'filter' frequencies can be optimised using only resistors (not trims). I knew from the start that this was a futile endeavour, because I use pots without centre detents. So it was clear that by the time I screwed the knobs onto the pot shafts, the centre positions would most likely be slightly off again -- yet they are all at about 12 o'clockish now.

Centre-detented knobs are still an option, although knobs without centre-detents and without indicators might even be more enticing to me. With only a few knobs per channel, I now simply dial the knobs without looking until it sounds good (in testing I mostly ended up with mild settings anyway), and if I need zero, I simply flip into full hardbypass.

To anyone who builds this unit in future:
- there is no need for high-precision caps or resistors. Get caps, install and measure them. Then the correct resistor values need to be calculated anyway. Could use resistor combos for that, but trims are probably better.
- meticulously matching of caps makes sense only in the air bands (L and R)
- stepped pots are an option but I'd believe they would require that the gain of the individual stages is spot on before any signal hits those stepped volume switches (probably a trimmer before the stepped volume switches ?).
- full boost (up to 20dB on paper) is most likely not needed
- clip indicators can easily be implemented (using the two unused opamp halfs of U4 for input and U6 for output), but I skipped this as I don't use that much gain in the first place
- a volume knob before the output might be useful (or not)
- highly recommended is being able to hard-bypass (at least my unit sounds a tiny tad 'bright' and/or a tiny tad less 'beefy' in the lower range even in soft bypass -- not necessarily a bad thing though.

Sorry for being persistent;) but i assume that 2x6 choke should do? Or anything above 10 Henry?