Calvin-TONDO

Quite some time ago my friendly helper with the tonearm had sent me a 2-layer PCB that another follower had made from the original single-layer RP1 layout. It had been laying around populated for quite a while now as I had no fitting power supply and casing at hand. Then I found that Fischer Elektronik had launched a new casing line, the TONDO at the end of 2015.

The TONDO are listed under ´Aluminiumkleingehäuse´, resp. ´minature aluminium casings´ (You find them at he very end of the list, no.56-59. Click on TON 86 36 to see the TON 111 30). The casings consist of two identical profiled halves, two identical covers for front and backside, four associating screws and two coloured plastic strips for the sides. The inside features fitting slots for 1.6mm thick PCBs. They allow for simple mounting of hardware. As the phono-PCB measures 150x100mm, I chose the TON 111 30 200 in 200mm length. The flat faceplates can be drilled or routed easily. The design may be tuned and ´individualized´ with different or additional faceplates.

This kind of casing is used quite often even for commercial phono preamps, but most profiles are formed rather rectangular in crossection. I immediately liked the rounded shape of the TONDO. The only TONDO for 100mm PCBs is the TON 111 30, which measures 11mm in width and 30mm in height. I chose the 200mm long version (204mm with faceplates) to give enough place for additional a PCB and the connectors. The inside dimensions of the TONDO are 100mm to 25.2mm. So the space in height is quite restricted, 6.7mm to the bottom and 16.9mm to the top. It´s probabely impossible to find a power transformer that low in height. So either the power supply should be external, or -and this is what I prefer anyway- one can use switching supplies. I will in fact do both ways. A external wallwart SMPS will supply for 12Vdc and a isolated dc-dc-switcher will generate +-15Vdc internally. The CC3-1212DF-E from TDK-Lambda looks like a decent candidate. It´s a 3W isolated switcher that generates +-12V/125mA or +-15V/100mA with good regulation and low ripple from a single 12Vdc. It also features a remote ON/OFF input and a 5-sided shielded metal case of only 22.9x16.6x8.5mm. As the Calvin-phono features a ripple-eater onboard no other means of voltage regulation are required apart from the pre- and post-filtering usually recommened with switchers.

The Calvin-RP1 has a jumper at its RIAA-output just before the optional balanced output driver. This is intended for an optional volume poti. As I won´t need balanced outputs I´m thinking of tweaking the stage in that a small OPAmp gain stage is used after the poti instead. This would allow for either variable Line-output or a headphone output with the heaphone jack and the poti mounted to the front faceplate.

Due to its small size the backside faceplate will be quite crowded with connectors. All in all eight connectors have to be mounted. Two RCAs for unbalanced In, Two Fischer S03 for balanced In, two RCAs for Out, one for PE/Gnd connection and one 5.5/2.1mm for the 12Vdc power supply input.