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Topic: Le Pacific J-FET phono preamplifier (Read 2398 times) previous topic - next topic
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Le Pacific J-FET phono preamplifier

Hello guys!! Long time ago I decided o try the schematics of the Le Pacific preamplifier.


If I try to power the entire bloc from a DC source the hum is higher, so I power it from 3 x 9V battery but they don't last so much. I also placed a 220uF/50V capacitor between + and - for reducing a little the hum.




The boozhound website recommands a unique power supply for eliminating the hum. http://boozhoundlabs.com/collections/all/products/power-supply-kit


I bought yesterday this metal case for the preamplifier. Should be good a shielded metal case.


Also, I thought at some things when constructing it:
1st: I recovered a transformer from a stereo system which included radio, 2xtapes + cd and also had a phono input!! The transformer doesn't make any noise and I think that also the fact the it is shielded itself with another perpendicular cover and with the copper tape on the coils would not perturb the good functioning of the preamplifier.


2nd: As it seems in the original power supply the author usess schottky diodes, and between those two caps of 4700uF there is a 1 ohm resistor. I am thinking that a good result could be in replacing that resistor with a small coil on the ferrite ring linke those from the PCs or notebooks power supply because the resistor has a low value and also could take the noise after. My question here is if I can replace that LD1085 with a LM317? Or LM150, 350? On the internet I saw different designs which used either LM317 or even zenner+transistors for the regulated power supply.

3rd: I'll redesing the entire board on a PCB. This is only "educational" but works fine. Something like this should appear finally:


For the rear pannel I was thinking to put 2 x RCA + 1 x 5 pin DIN for the older devices, and for the front pannel the same as rear.

 

Re: Le Pacific J-FET phono preamplifier

Reply #1 –
Quote
If I try to power the entire bloc from a DC source the hum is higher
Is that DC source the recommended LD1085 power supply, or something else?   Any good regulated power supply will have a minimum of hum & noise.    Sometimes in a preamp circuit you might see a filter (resistors & capacitors) following the regulator.   And, I've seen two regulators in series (such as a 15V regulator followed by a 12V regulator, etc.).

You can also get ground loops or other hard to track-down sources of hum.     

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2nd: As it seems in the original power supply the author usess schottky diodes
The 4 diodes making-up the bridge rectifier don't matter.   Regular silicon diodes are fine.    I'm not sure what the other two diodes are used for (maybe some kind of reverse-voltage protection), but I doubt they need to be Schottky diodes.    Those two other diodes are not shown on the manufacturer's recommended schematic on the LD1085 datasheet.

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and between those two caps of 4700uF there is a 1 ohm resistor
I'll bet you can replace that resistor with a short (with a wire) and there won't be any difference.

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I am thinking that a good result could be in replacing that resistor with a small coil on the ferrite ring
A ferrite bead (or small inductor) won't make any difference at audio frequencies (or at power line frequency).   It might block radio frequency interference from coming-in over the power line.

Quote
My question here is if I can replace that LD1085 with a LM317? Or LM150, 350?
You can use a different regulator, but follow the manufacturer's recommendation for whatever part you choose.   (Don't assume those parts are drop-in replacements for the LD1085.)

Quote
On the internet I saw different designs which used either LM317 or even zenner+transistors for the regulated power supply.
As long as there is a voltage regulator, the voltage regulator will reduce hum.   I wouldn't bother making a circuit with transistors and zeners, since it's so easy to use a specialized integrated circuit.    But...  I wouldn't have built a preamp from discrete components either...   I would have used a low-noise op-amp.  ;)    Or, I would just buy a preamp...  Maybe a preamp with an ADC & USB port...