Hello everyone and compliments to LH forum.

I am using two Xilica XM-2040 DSP cards to drive a pair of 4-way loudspeakers with JBL components.
The problem is that these cards have a hiss tone at the outputs that is much above -115dB S/N declared by the manufacturer. First I tried Cosel LDC30F-1 switch mode power supplies that were supplied with cards but the problem was emphasized and even worse when one PS was used for two cards. Then I used Traco Power TML20212 20w switch mode PS, the result was slightly better but the problem persists. Finally, I used two 12V batteries connected in series in order to get +/-12V. The result was better but this hiss tone (between 3-4KHz) was still present, at lower and acceptable level although audible in close vicinity of JBL 2450SL-8 with PT-H95HF waveguide.

Otherwise, those little cards are doing the job quite well.
Does anyone know how to suppress this annoying hiss tone?
Grateful for your help.