Repurposing channel overload light on TAC scorpion

Hello, Everyone,

After I finished up cleaning and modding my TAC Scorpion, I got to think about the metering, which wasn't great even when these things were new. With the stock LED drivers in the metering circuit being proprietary and the circuit function itself leaving much to desire, I came to the conclusion that a new board and faceplate would need to be designed, which is low priority right now. But I would like some indication of signal present on the channels and I thought about tweaking the overload indicator circuit and the lowering the threshold of where voltage is sent to the LED to something that corresponded to what would be around -40dBFS on the DAW. In my mind, this would be a simple resistor change, but I would need some help verifying that and if so which resistor(s) would need to be changed. I've attached the schematic of just the overload indicator section of the channel.

I also thought about doing something fancier like a tri-colored LED, but this is probably much more involved than I want to be on this project.

Am I correct in thinking it's just the resistors that need to be changed or will I need different transistors as well?

Thanks!

Paul

Potato Cakes said:

Hello, Everyone,

After I finished up cleaning and modding my TAC Scorpion, I got to think about the metering, which wasn't great even when these things were new. With the stock LED drivers in the metering circuit being proprietary and the circuit function itself leaving much to desire, I came to the conclusion that a new board and faceplate would need to be designed, which is low priority right now. But I would like some indication of signal present on the channels and I thought about tweaking the overload indicator circuit and the lowering the threshold of where voltage is sent to the LED to something that corresponded to what would be around -40dBFS on the DAW. In my mind, this would be a simple resistor change, but I would need some help verifying that and if so which resistor(s) would need to be changed. I've attached the schematic of just the overload indicator section of the channel.

Click to expand...

That's a typical British circuit that cannot be easily modified for significantly lower threshold. You could decrease R70 to a point; that would put you close to nominal operating level (about -2 dBu) but the circuit would generate significant distortion.

  will I need different transistors as well?

Click to expand...

Changing transistors won't change a thing (unless you use Germanium types). The threshold is intrinsic to Silicon.
The only viable option is to insert a one opamp gain stage. It may not be too difficult.

From looking at the schematic I thought the overload circuit did not have any interaction with the actual audio, hence my inquiry. A simple op amp stage makes sense, but I am curious as why how this circuit would affect the actual audio path. I've attached the full channel schematic for reference.

Thanks!

Paul

Potato Cakes said:

From looking at the schematic I thought the overload circuit did not have any interaction with the actual audio, hence my inquiry. A simple op amp stage makes sense, but I am curious as why how this circuit would affect the actual audio path. I've attached the full channel schematic for reference.

Thanks!

Paul

Click to expand...

The detector stage's input is two b-e junctions, highly non-linear. R70 isolates the circuit so it does not load unduly the Insert return amp. But if you decrease R70, in order to lower the threshold, the opamp has to drive this non-linear impedance and starts to sweat.