MeToo2
Well-known member
Thanks for posting and sharing.neodyms said:this the pdf
sorry
Agree on the previous suggestion increase the fb pot to 470K and Kingston's other notes on transformers.
I'm not 110% convinced about your tl783 phantom regulator.
You are only supplying 2 channels of phantom at say typically around 4-6mA each = total of around 10-12mA.
You currently have a worst case no load of 1K+2k7+82R which is around 12mA. If the 1K trimmer is at minimum you'd have 2k7+82r for 48V = 17mA.
With 2 mics connected you have a worst case high load of less than 50mA (17mA+2*48/6k6||6k8).
1. 4*220uF looks way too much for a load of only 50mA for 2 channels. Maybe this was a design to supply more channels? So is there too much capacitance on input?
You'd probably be better off using CR filters, rather than lots of parallel capacitors. 4 high uF caps will cost quite a bit relatively speaking.
2. The TL783 needs 15mA to maintain regulation (from the data sheet) so the output voltage may not be regulated to 48V under very low loads.
Add on one single low power mic like an AKG 414 which only needs 2mA and you only have 14mA-19mA total in the worst case. I know they suggest 82R for the adjustment resistor, but I'm not sure that's enough. It might just be enough to make the TL783 work. Then again it might not.
Suggested changes:
Change the 220uF +220uF + 220uF+ 220uF to a simple single 220uF + 120R + 220uF CRC filter, The maximum permissible value for Rfilter (120R) will depend on your transformer no load voltage (TL783 needs 10-20V across it for good ripple rejection).
Simply "throw away" a tiny bit more current (3mA more) by connecting a 15K 0.25W resistor to ground from the output of the regulator. Or perhaps reduce R44 to 68R and R46 to 2K2. Then you'll always have at least 15mA-20mA flowing even without a mic connected, so the TL783 will always regulate.