Add Neve 183 output to Neotek Series II

I want to experiment with adding a balanced Neve 183 output to my Neotek unbalanced 2-mix output section. In an ideal world, I'd like to be able to switch between the two so I can choose which "color" I might like for mixdowns. I have some Never 183 boards on the way, a couple of the Altran Neve transformers (DC gapped), and will be ready to go shortly.

The question I have is where to tap into the existing 2-mix output and how best to do that. While I'm pretty handy with the DIY stuff and have built numerous projects from this website, I'm painfully inadequate when it comes to understanding what I need to consider by adding this new output section, especially if there are impedance issues I need to consider when tapping into the existing output circuitry. I was hoping to learn a bit by doing this mod.

I've attached a schematic of the Neotek 2-mix output. The output ICs (U4, U5) are Burr Brown OPA 134. I'm thinking I need to take the signal after the 10K stereo fader, but I'm not sure if I need to tap in after one of the following gain stages or just off the fader itself.



Any help will be GREATLY appreciated.

Cheers,
--
Don

it depends how close you put the BA183 card to the fader. the 10k fader will have highish output-Z at roughly where you will be usually running it, -6dB. too much screened cable to the amp means capacitance and therefore high end rolloff. IIIRC the input Z of BA183 is about 50k, not quite as high as the opamp followers U4 U5 but high enough.

if you cant put the new circuit physically close to the fader then I say build it in box external to the console and just patch into it. you could eliminate the current booster and take the signal from pin 7 U4/5.

now I gotta go jump in the pool!

[quote author="mikep"]it depends how close you put the BA183 card to the fader. the 10k fader will have highish output-Z at roughly where you will be usually running it, -6dB. too much screened cable to the amp means capacitance and therefore high end rolloff. IIIRC the input Z of BA183 is about 50k, not quite as high as the opamp followers U4 U5 but high enough. [/quote]

There's plenty of room to install it on the 2-mix PCB close to U4/5, so I might get by without having to use shielded wire. If I do use shielded wire, though, it would be very short, most likely less than six inches.

if you cant put the new circuit physically close to the fader then I say build it in box external to the console and just patch into it. you could eliminate the current booster and take the signal from pin 7 U4/5.

Click to expand...

This is a good idea and one worth exploring.

So if I do it either way, I should tap off pin 7 of U4/5?

Thanks for your help!

Cheers,
--
Don

Hi Don,
take signal from fader slider (patch point 37/40) and skip U4/5 completely. IIRC the BA183 has a DC-blocking cap on its input. Remember to flip the output transformer secondary as you will be missing the inverting stage (pin 1,2,3 of U4/5) after the summing inverter U2/3 for correct phase.
Don't know why you'll try to make your Neotek a Neveotek :? . Anything wrong with it's sound?

[quote author="Harpo"]Hi Don,
take signal from fader slider (patch point 37/40) and skip U4/5 completely. IIRC the BA183 has a DC-blocking cap on its input. Remember to flip the output transformer secondary as you will be missing the inverting stage (pin 1,2,3 of U4/5) after the summing inverter U2/3 for correct phase.
Don't know why you'll try to make your Neotek a Neveotek :? . Anything wrong with it's sound?[/quote]

No, just wanted to tinker to see what happens, and I want to learn more about the theory behind all of this stuff! I'm very pleased with my Neotek as is, but I'm incurably inquisitive and just like to know how things work. I'm a LONG way from truly understanding much of the theory behind electronics, so I take the small steps I can when I can.

I'll still be able to use the stock Neotek output as I plan to either wire the Neve output as an outboard box that I can patch in or have it available via relay or switch.

Cheers and thanks,
--
Don

Well, after nursing a herniated disc in my back for the last three months, I've finished up racking the two 183 cards and am ready to patch it into the Neotek. The 2-mix outs are available on my patch bay, and I think they're taken from 15 and 16 on the schematic above. Other than dropping the master fader to accommodate the extra 15dB of gain the 183s provide, is there anything wrong with patching in at this point? I wasn't able to find the input impedance for the 183 card, but I'm guessing that it's high enough to work with the output impedance at patch points 15 and 16. Is this correct or am I way off base?

Cheers, and thanks for any advice.
--
Don