I have a vintage Modsquad phonodrive with what I think is a series-pass 25v PS that scops out as being somewhat noisy. I don't have the schematic, but I believe it's controlled by a 25v zener. Would like to use a better quieter reference. I have read a bit about using LEDs, but the string of LEDs to reach 25v doesn't seem likely to be responsible or quieter (?). Changing the design is not really feasible. What's possible?
A good idea is to replace all PS regulator with a LM7824, or an adjustable LM317 is the 25V are need to exact.
Don't work on it without a schematic. An RC filter can be added after a Zener for filtering.
A capacitor across the Zener does nothing because of the low source impedance of the Zener.
A capacitor across the Zener does nothing because of the low source impedance of the Zener.
Using the mc output and preamp section through an internal jumper produces a noticeable hiss. The mc section flows through the mm section, then through the preamp through that jumper. I have scoped the drain terminal in the mm section and traced the fuzz to that supply. Thus would at least like to clean it up some. It appears to be 20mv ptp noise problem on my old scope.
If you want very, very low noise try Elvee's "De-Noizator" -- has very, very noise and output impedance (I tested). There's a formula toward the end of the thread outlining the resistor values you need for a specific voltage.
The De-Noizator will be more several orders of magnitude lower noise than the LM7824. D-Noizator: a magic active noise canceller to retrofit & upgrade any 317-based V.Reg.
Minor issue with the design -- it requires several tens of seconds to achieve the desired output voltage.
The De-Noizator will be more several orders of magnitude lower noise than the LM7824. D-Noizator: a magic active noise canceller to retrofit & upgrade any 317-based V.Reg.
Minor issue with the design -- it requires several tens of seconds to achieve the desired output voltage.