What is the real purpose of 20Ohm trimpot at the input? Turning it, I reduced first stage offset to almost zero DC. Folded cascode current is set to 5ma. Reproduced sound is really outstanding, very low noise with good details, even preamp is still unboxed.
There is only slight hum with pot. at max., I hope it will be reduced with enclosure and careful grounding.
No, for loading is 10Ohm fixed resistor in series with 200Ohm trimpot to ground. I use 500Ohm trimpot for wider cartridge loading range. Twenty Ohm trimmer pot is between sources of input J-Fets. parallel with 6R19 resistors.
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You can remove both a 10 ohm resistor and a 200 ohm variable resistor. Put 1k resistor
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Yes, it is possible to put desired value at the input, but trimpot is more practical.
It is a still experimental version with 2+2 J/K J-Fets in parallel both with 8ma Idss. Soon I will put full 4+4 sets for lowest noise. But noise is very low at the moment.
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I have to repeat my question: Is 19.5 Volts safe for AD711 or OPA134 op-amps in the first stage servo? They are rated at18V max. I use op27 at the moment.
Vendetta first stage supply voltage according to unofficial schematics is 19.5V. Servo op-amp is AD711 rated at 18V max. Very confusing.
Second stage ( bass boost stage) op-amps has two 18V zeners. First stage servo op-amps are powered directly from local regulators adjusted to 19.5V.
From here, you can understand more about input noise.
If you can get jfets with more Idss ( 16 to 18mA ) I think you will get less noise too.
Hi Kamis,
I have done another simulation with some groundings so that the input jfets are as close as possible to the original fets (1 x K147 / J72 = 2.5 x K170 / J74) and included the 18 volts of power for the Opamp. Everything went very well.
I wish preamp to be faithful to original, without additional parts. And why six regulator transistors in parallel? Two transistors work perfectly without too much heat.
Yes, you're right !
The six fets regulators were simulation purpose only. I know less works.
The main modification is on the input. There, I wanna keep the same input capacitance and same noise level ( look at and compare the datasheets between both fets, original and actual ). The total transconductance has increased. I have decrease drain resistors values without problem.
Look, unfortunately the original fets is almost impossible to get. So, I did that changes trying to get the same results or almost.
The fets of simulation have 16.4mA of Idss.
Idon' t know if Linearsystems has LSK170/LSJ74 with this Idss. If yes, I think is possible someone to build that simulation version of MC pre-preamp and share results with us.
I like very much this circuit for a lot of reasons and today I have a real intention to build one but I need to prepare others things before.
Regards,
eD