Serge ' 73 VCF , suitable for eurorack format , PCB is tested and working with single cv input , so simply don't install the bottom IC and you got a working '73VCF as it was meant with single input , low freq option is added (edit: this is not all that great layout ,i can suggest you to use the original serge '73 vcf layout ,the only difference is it uses CA3080 )