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PostPosted: 12 Feb 2016, 11:03 
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Joined: 03 May 2011, 15:16
Posts: 178
Location: Romania,Iasi City
Hello guys! I remembered recently about a Romanian magazine since 1994 where was presented an interesting phono preamplifier based on FET transistors. As previously most listeners preffered to buy or construct either "Le Pacific" or "Boozhound" preamplifier, I decided to come in help for them with this diagram.

Short description:
"The preamplifier for the magnetic cartridge presented can satisfy a series of lately wishes such as:
-very good signal/noise raport thanks to using the FETs;
-very low THD for a high gain and a high output voltage;
-high capacitance for preloading the signal peaks.

Technical characteristics:
-Gain at 1KHz = 40dB;
-V out = 250mV;
-S/N = 82dB;
-overload for the normal output voltage (250mV) = 40dB;
-THD = 0,01%;
-Z in = 220K;
-minimum Z out = 1K.

The preamplifier is built after the pattern of an OA equipped with FETs at the input in a differential mode. The stage mixes or combines the advantages of the BJT (regard frequency and high amplification) with the low noise of the P-channel FETs.

Regarding to the type of cardridge, C1 can vary between 10-90pF.

Diagram
Image

Power supply
Image

Part list
Image

PCB
Image


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PostPosted: 12 Feb 2016, 13:34 
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Joined: 08 Aug 2009, 03:11
Posts: 2160
Location: Chilliwack, BC
Hi,

Looks like one of the better Sansui or Yamaha vintage preamps. Awesome sonics!

I wouldn't bother with the input cap inless you're picking up local transmitters. Most phono cables do a good job at loading the MM cartridge. Otherise make it quickly adjustible for your cartridge, with socket pins instead of soldering direct.

Also, try R2 at 47K (metal film) .... unlike BJT's, jFET's don't have enough input bias current to give them a naturally low input impedance.

Cheers!

_________________
-= Gregg =-
* Ratings are for transistors - tubes have guidelines*
Home: GeeK ZonE
Work: Classic Valve Design


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