Thanks for all the help. Implementation worked out quite well.
I finished separating the AC portion of the power supply from the CNC phono preamp chassis. The hum that was being induced by transformer is now totally gone. Moving the toroidal to a separate enclosure, adding a rectifier and filtering was all that was needed. The PSU is connected to the rest of the circuit (regulation, filtering, decoupling, RIAA signal circuit, etc.) by a short 3 pin XLR I made.
The rectifier and C-R-C filtering (100uF-47R-220uF) in the PSU reduces ripple attenuation by 17.9 dB and has a fc of 15.4 Hz with 19mA on the + rail and ~10mA or so on the - rail. This is all before the DC hits the further filtering, regulation, filtering and decoupling in the main enclosure.
I do have a little bit of hiss in 40dB mode when the volume is maxed in my setup. This is reduced by replacing the OP2134 with a LM833 which I am currently using. I think this hiss is very much exceptable. I shorted the inputs to ground when determining this noise.
click pics to enlarge
I made a small plate to cover up the IEC cutout out of sheet metal. I then drilled the plate for the XLR jack and spray painted it to match the enclosure. The plate then mounted with the existing mounting holes for the IEC jack. Really happy with how this turned out. I thought this mod would be way messier and screw up the neatness I had before.
Previously I did have a IEC jack with a built in line filter. I left room on the PSU perf board to add the line filter back at a later date but made up of discrete components. Any ideas on part values?
I finished separating the AC portion of the power supply from the CNC phono preamp chassis. The hum that was being induced by transformer is now totally gone. Moving the toroidal to a separate enclosure, adding a rectifier and filtering was all that was needed. The PSU is connected to the rest of the circuit (regulation, filtering, decoupling, RIAA signal circuit, etc.) by a short 3 pin XLR I made.
The rectifier and C-R-C filtering (100uF-47R-220uF) in the PSU reduces ripple attenuation by 17.9 dB and has a fc of 15.4 Hz with 19mA on the + rail and ~10mA or so on the - rail. This is all before the DC hits the further filtering, regulation, filtering and decoupling in the main enclosure.
I do have a little bit of hiss in 40dB mode when the volume is maxed in my setup. This is reduced by replacing the OP2134 with a LM833 which I am currently using. I think this hiss is very much exceptable. I shorted the inputs to ground when determining this noise.
click pics to enlarge
I made a small plate to cover up the IEC cutout out of sheet metal. I then drilled the plate for the XLR jack and spray painted it to match the enclosure. The plate then mounted with the existing mounting holes for the IEC jack. Really happy with how this turned out. I thought this mod would be way messier and screw up the neatness I had before.
Previously I did have a IEC jack with a built in line filter. I left room on the PSU perf board to add the line filter back at a later date but made up of discrete components. Any ideas on part values?
After the fuse, PCAT switcher power supplies use a MOV surge supressor to kill 1100 v spikes coming from motor turn off (refrigerators, A/C) or down the line lightning. S07K150 is one part number of the wimpy 7 mm version. I tend to use the 15 mm ones salvaged out of motor drives which will accept more energy.
Following that, PCAT power supplies have a toroidal choke which is 10-25 turns. This limits RF entrance into the later circuits. I salvaged one of these to put in the DC coming into my disco mixer from the wall transformer: the mixer where I eliminated the 34 VCT transformer inside right next to the 50 gain RIAA circuit.
Some designs have a ,1 uf disc cap across and after the transformer to also kill surge. In the sixties these used to be 1000 v rated ceramic ones, but since the 80's manufacturers have been using X rated .1 or .47 uf film caps with a 250 VAC rating or above across transformers. These I also salvage from dead PCAT supplies. They can actually be bought new. Those 200 W PCAT supplies used to fail pretty frequently, usually not up in the front end.
Following that, PCAT power supplies have a toroidal choke which is 10-25 turns. This limits RF entrance into the later circuits. I salvaged one of these to put in the DC coming into my disco mixer from the wall transformer: the mixer where I eliminated the 34 VCT transformer inside right next to the 50 gain RIAA circuit.
Some designs have a ,1 uf disc cap across and after the transformer to also kill surge. In the sixties these used to be 1000 v rated ceramic ones, but since the 80's manufacturers have been using X rated .1 or .47 uf film caps with a 250 VAC rating or above across transformers. These I also salvage from dead PCAT supplies. They can actually be bought new. Those 200 W PCAT supplies used to fail pretty frequently, usually not up in the front end.
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Yes. Don't forget the fuse or circuit breaker before the MOS supressor. MOS supressors eventually fail shorted. Hasn't happened to me yet, but the application notes from the inventor - schimdt, schwimmer, schweber I forget and I misplaced the catalog - emphasize that. The PCAT supplies do it that way.
With all that, plus 33 pf disc caps from RCA center to analog ground in inputs, the yellow pickup truck that drives by emitting CB radio of dogs barking continuously no longer enters my amplified RIAA input or reaches the speakers.
With all that, plus 33 pf disc caps from RCA center to analog ground in inputs, the yellow pickup truck that drives by emitting CB radio of dogs barking continuously no longer enters my amplified RIAA input or reaches the speakers.
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