Nagra 3 - some questions

[thomasdf]

Hi all

I own a Nagra III which I love a lot and use regularly. My only complaint is that in Automatic mode it engages a HPF, by design. I'd like to remove it or tweak it to a lower frequency. I've looked at the schematics but can't figure where this happens in the circuit, since it has positive ground. I suspect it has to do with C1, C2 or C3 in the "automatic level control" section but I can't be sure ?

My other route could be to add an insert connector between the output of the mic preamp and the input of the line amp, but that's far more intrusive. (But would allow me to compress and EQ to tape)

Any help, pointer or ideas welcome

Cheers

Thomas

 

[zamproject]

Salut Tom

Not used to Nagra schematic, nor sure about the logic and signal direction for the automatic level control section.
Annoying the ref numbers start to 1 for each section ! there is dozen C1 in the schemo

But it seem the auto control is handled with feedback from the output trafo up to base of Ts3 (auto section).
I'll say you better look at the RC network between collector and emitter of Ts3 that probably modulate signal from preamp (O-G wire) and output it at R-B wire (switch full CW), especially C10, 0.22u (between Pre and Auto section), also check possible HPF when this network is engaged and loaded to the line amp section input (R1 R2 ?)

In half blind understanding like this I'll probably try a 100n or 10n in parallel of C10, temporary clamp or wire soldering at the back of the pcb, if the HPF cut off get notably higher, then it's the spot. You can then remove C10 and put higher C value for C10 like 1u ?

I'm sure someone will do the math of this network...

Cheers
Zam

 

[zamproject]

Looking again...because I have a doubt.

The filtered audio is the one that output and sidechain the limiter, the HPF is probably there to reduce low freq pumping ? especially for field recording (Nagra purpose), wind etc...
Probably if you lower the audio cutoff freq, you'll need to add a HPF in the sidechain/feedback ?

Any idea of the actual cutoff ?

To me (autolevel section) C1 is a HF filter (20kHz LPF with R1), C2-C3 are time constant for the autolevel.