Perhaps, but the telephone equipment I work on at Collins Radio was 0.1DB up to 3.3Khz, and 3db ± 1db at 3.4Khz. This was the stuff that stuffed the channels into a baseband signal, at some point it has to be audio. Interestingly enough, 1458's were commonly used for this.

Only way this applies to this thread is these channels were converted to SSB suppressed carrier AM modulation, then stacked on top of each other in 4Khz increments.

 

The RC time constant is τ= RC. The resistance R is R4 + R6 in the Thevenin equivalent. Or 60Ω. The capacitor is 3μf. So thus τ = 0.18ms. Times this by 5 = 0.9ms.

Looking at the output terminals of the schematic you provided and the information along with it. It appears to me that you are not applying Thevenin's theorem properly.