[Elecraft] KDSP2 - SSB Gain

[Lyle Johnson]

Hello Stewart!

> When the DSP was switched ON with SSB Filter gain set to the default
> ( 0 dB) the audio level went up by about 3.5 dB. No wonder signals
> sounded stronger !
>
> I have set the SSB filter gain to -3.5 dB and now observe no change
> in audio level between DSP in or out.
>
> I am wondering if my K2/DSP has a problem as I would have expected
> unity gain with the default setting of 0 dB.

The short answer: it's normal.

The long answer...

The KDSP2 has an insertion loss of about 3 dB.  This is due to the voltage
divider at its input consisting of resistors R15 and R16 and the input
impedance of the A/D converter within the CODEC chip (TLV320AIC23) on the
DSPx module.

In bypass mode, the CODEC acts as a switch, and simply passes its input to
its output.  In this case, the insertion loss is present.

When you make the DSP active, approximately 3 dB of gain is added back in to
the system to overcome the insertion loss.  Thus, the 0.0 dB gain setting is
referenced to having no KDSP2 installed, rather than being referenced to the
KDSP2 in bypass mode.

73,

Lyle KK7P