Vendetta Research SCP-2A

[kamis]

Why 120000pf capacitor at the input?

 

[john curl]

you worry too much about Idss. you are ok as you are. the 20 ohm pot is for 2'nd harmonic reduction. the input 200 ohm pot is for optimum cartridge loading. you adjust it by ear.

 

[EDDELARUE]

(( Why 120000pf capacitor at the input? ))

Don´t worry about this. They don´t exist in the original schematic.

I was testing some moving coil cartridges behavorial.

 

[EDDELARUE]

you worry too much about Idss. you are ok as you are. the 20 ohm pot is for 2'nd harmonic reduction. the input 200 ohm pot is for optimum cartridge loading. you adjust it by ear.

Because the fet datasheet shows equivalent noise input is lowest for Id > 11~12mA.

With actual version, the 2'nd harmonic can be changed almost 30dB.

 

[kamis]

you worry too much about Idss. you are ok as you are. the 20 ohm pot is for 2'nd harmonic reduction. the input 200 ohm pot is for optimum cartridge loading. you adjust it by ear.

I am using 500Ohm loading pot. Turning 20Ohm pot also changes DC offset. My preamp works well without second harmonic reduction.
What about 19.5 Volts with original AD711 servo op-amp?

 

[EDDELARUE]

I am using 500Ohm loading pot. Turning 20Ohm pot also changes DC offset. My preamp works well without second harmonic reduction.
What about 19.5 Volts with original AD711 servo op-amp?

I think there is something wrong with your circuit because the 20ohm pot wouldn't to change DC offset due to the servo opamp.

SERVO ---> OPAMP + 1Meg + 1uF

Maybe you must to change the 2k49 for other value. Before, verify the servo circuit.

 

[kamis]

I think there is something wrong with your circuit because the 20ohm pot wouldn't to change DC offset due to the servo opamp.

SERVO ---> OPAMP + 1Meg + 1uF

Maybe you must to change the 2k49 for other value. Before, verify the servo circuit.

I adjusted offset to zero first without op-amp. Op-amp regulate input J-Fets thermal drift.

 

[EDDELARUE]

The pot is there for 2'nd adjust ONLY.

Zero offset is reached by servo.

 

[EDDELARUE]

Because the fet datasheet shows equivalent noise input is lowest for Id > 11~12mA.

With actual version, the 2'nd harmonic can be changed almost 30dB.

...almost 30dB for 1Khz , 5Vp from SCM-2B output.

 

[EDDELARUE]

Note that the 3rd harmonic is rebellious.....


Hi John.
You used Resista resistors in the Vendetta in the past.

Which would you suggest we could use in an alternative version of your Vendetta?

 

[kamis]

You can remove both a 10 ohm resistor and a 200 ohm variable resistor. Put 1k resistor

This is a simplified version of the first stage with halved numbers of J-Fets. Have you trimmed 20Ohm trimpot for minimum distortion? What is DC offset?

 

[ticknpop]

The original jfets were 2SJ72/2SK147 ( impossible to find)
parralled 2SJ74/2SK170 can be used ( and to get a high enough input stage idss

 

[kamis]

The original jfets were 2SJ72/2SK147 ( impossible to find)
parralled 2SJ74/2SK170 can be used ( and to get a high enough input stage idss

To get the same noise as in original, 4+4 K170/J74 must be used. But even with 2+2, noise is very low.

 

[VACUUMLAB]

Hi all.
Someone could let me know what is the function of the multiturn trimmer in picture on the SCP-1 and what is the proper value and procedure of setting?

An externally hosted image should be here but it was not working when we last tested it.

 

[kamis]

Why 20Ohm trimmer must be multiturn wirewound type? Where to buy suitable one? What is the method of distortion reduction with it? If untrimmed, is distortion too high?

 

[kamis]

My Vendetta power supply. Very useful for other projects.