Send 0dBU to unit, check output, adjust trimmer for 0dBU output (this is fine tuning, if you just leave them, youll still be within range of the classic GssL)

R59/R60, blend summing gain.

I like to dial these in, so correlating signals at 0dBU from wet and dry come out to 0dBU, some will prefer to calibrate for 6dBU, some will prefer something in between. In real use, youll never have correlated, full gain signals here, so calibrating it +6dBU make sense to every workflow, and we have a make-up gain in-circuit to control total gain anyway.

R153 - meter calibration

Send xdBU to the unit, measure output, add compression and adjust threshold until output measures x-5dBU, adjust R153 until meter reads 5. (in essense, inject signal to the unit, add a known amount of compression measuring difference between input and output, adjust the meter to that amount)

Changes from rev 4 to rev 5

Ratio switch pinout was reversed, and has been fixed

Revision was done on the ratio board, so previous builds can be fixed with a rev 5 ratio board, or you can cut and jump 6-1 to reverse, 5-2, 4-1, as well as 12-7, 11-8 and 10-9.

Changes from rev 5 to rev 6

Corrected a few values (see below for resistors to change if needed on your BOM)
220K to 160K in the threshold
2 x 10K to 2 x 20K in the blend
2 x 14K7 to 2 x 6K8.
2x 500R trim to 2 x 10K trimpot

Changed the trimmer from 500R to 10K in the gain adjust.

Changed series resistor from 14K7 to 6K8.

Set resistor feedback resistor at static 15K (varied with the use of 1206 vs IC input).

The IC input holds unity gain, while the output adds 6dB, which means we have +6dB if we dont adjust it over the VCA stage.

When using the 1206, input gain is -6dB, and output +6dB, and it evens out, so

These changes were made to enable us to trim for both scenarios when calibrating.

Changed 2 x 10K to 2 x 20K in the blend circuits (marked with asterisk on rev 6 boards)

The dry signal is tapped before the VCA stage, and the wet is tapped after, before they are summed in the blend mix. When we gain compansate over the VCA (using the Standard IC input), there is a discrepancy of 6dB between the two. It is doubtful that you will hear the difference, since we dont have a "50/50 clean/compressed" absolute reference, but this dials in it so they are functionally identical. If you are using the THAT1206, keep the 10K value

Changed 220K to 160K over the threshold pot.

This also has to do with the 6dB discrepancy between the standard IC and THAT1206 input. The 220K is correct when we have a 6dB drop of signal over the 1206, and it will give a range from +20dBU to -20dBU over the threshold. Replacing the 220K with a 160K resistor shifts it back to proper reference when using the standard IC, which would otherwise shift it by 6dB.You will notice the changed values are kept in line with the IC input set-up, and while using the THAT1206 "adds" 6dB headroom, you can easily replace it with the THAT1200 chip which will mimic the standard IC input.The two 20K resistors marked with an asterisk need to be 10K, if you are using a 1206

The 160K resistor marked with an asterisk needs to 220K if you are using a 1206.

You can keep them in line with the standard values, if you replace the 1206 with a THAT1200 chip.

These explanations are pretty long, but the discrepancies are small.

Also moved the left channel 3 pin connectors for colour trim and blend on the board, and separated the large PSU capacitors for a little extra space.

Isak & Gustav

GssL 20th anniversary

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