Amek AT8 Hybrid Line Driver Questions

[limo]

This is my BC3 circuit diagram on page5, but your schematic the R19 connection in your circuit diagram is different.
R19 and U1D+12 should be connected together, not to GND.

 

[GreyboxAudio]

This is my BC3 circuit diagram on page5, but your schematic the R19 connection in your circuit diagram is different.
R19 and U1D+12 should be connected together, not to GND.

Thanks for sharing! It adds to the confusion tho. Because if I connect the circuit this way I get +39dB of gain and a low pass filter at 12kHz

 

[GreyboxAudio]

For reference: I've stitched together several see-through pictures of the PCB and will try to re-trace everything from scratch.
Update: it looks like everything is connected identically to the schematic.

 

[mhelin]

I don´t know if I posted this already: https://mae.tcrcloud.de/index.php/s/rq8Kxt2A27Rc2c4?path=/Amek/Amek BC3

They oddly specified LL1539 to be used as an input transformer as it is a balanced driver output transformer really (must be an option as the schematic doesn't make sense otherwise)...

 

[jensenmann]

When I was reworking a BC3 years ago I was really astonished to find LL1539 at the inputs. Indeed it was an option for broadcasters. I don´t remember whom I asked but I´ve been told that Rupert Neve specified these to be used in this spot. Thanks to the very low DCR it´s a good choice regarding noise.

 

[Matt Syson]

AMEK used a LL1539 as an input transformer (option) in several broadcast orientated desks as it can handle LINE level signals and was still reasonable for Mic level. A 1:1 transformer doesn't really care where it is located in a circuit. it is worthwhile to note that you should be careful not to accidentally magnetise the core of any transformer as it will become more distorted and possibly a touch microphonic as I vaguely remembrer a discussion between AMEZK and Lundahl about it (33 years ago). Lundahl Transformers are good candidates for negative resistance LF distortion improving circuits as used by other desk manufacturers. AMEK just used them as straight isolation when the customer needed galvanic isolation.

 

[sahib]

They oddly specified LL1539 to be used as an input transformer as it is a balanced driver output transformer really (must be an option as the schematic doesn't make sense otherwise)...

I have the test report and the internal memo for that. I just have to look through which folder it is in.

 

[GreyboxAudio]

For reference: I've stitched together several see-through pictures of the PCB and will try to re-trace everything from scratch.
Update: it looks like everything is connected identically to the schematic.

I've modified one of my MZ-15-RN channels so that I can easily swap the Insert send between the original AT8 and my own PCB. Even with all resistances matched the original AT8 still behaves differently and I'm starting to wonder if the compensation caps of the two NE5534 did serve a bigger purpose