Vendetta Research SCP-2A

[johnhenryharris]

The DC offset at the output is handled by the DC servo, the opamp, but you need to start with matched JFETs at the input, and matched FET at the output are a nice touch, to minimized DC offset. Remove the servo when measuring for raw DC offset.
Good Luck.

 

[kuly]

what purpose? What should I get? same offset mV

 

[johnhenryharris]

You measure DC offset at the output, without the servo. Not sure about the differences where you are showing whether they need to match, probably should be minimize distortion products.

 

[kamis]

Where did you buy K170/J74? Looking at the top of the TO-92 package in your preamp I am sure they are fake. There are not original Toshibas with short line at the side of the top. Measure Vgs using Erno Borbely measuring jig. It should be 0.3 to 0.6V.

 

[kuly]

bought in china - nowhere else
he. writes the original - probably a hoax

see attach. 2sk170

 

[kuly]

signal input - 1 kHz signal - 1 mV(computer)
12 mV output !!!!?????(vendeta)
Is this normal for MC head ? or is it a small gain?

I do not have a real head to check MC

 

[kuly]

I took made a layout of this circuit (attach) - I burned the amplifier! High direct current output - why? Then slowly falls

 

[kuly]

I figured out the legs of the transistors had a different label - now turned on the circuit. This is normal !? I have 0.3 volts of constant current at the output! He slowly falls! -This is normal?

 

[EDDELARUE]

Looking at Federico Paoletti' schematic , first stage, you can change the value that resistor parallel capacitor in the rails to get 20V. Be sure that combined Idss from parallel jfets voltage supply is much more than the current used by each rail on first stage.

 

[kuly]

that's exactly what I did . but it remains a constant current, it's probably bad Chinese radio components and a microcircuit

 

[EDDELARUE]

More recent simulations with MODIFICATIONS

For purpose to learn more about how to get this circuit working with available jefts, I did this simulation with very good results.


Input Jfets: Idss = 16.3mA

Power Supply: 6 parallel Jfets per rail ( plus 1 Jfet for current source )

C1 and R15 values were modified for better FFT presentation.

C134 and C135 used for my own tests. They don´t belong to original topology.

 

[kuly]

Thank you try this option.!
The second part does not change? In your?

 

[EDDELARUE]

Thank you try this option.!
The second part does not change? In your?

Yes, changes.

** The servo circuit has changed some values for simulation only. Other parts of circuit has values changed for correct results after modifications such as JFETs, Mosfet, etc.

 

[kuly]

thanks is great! ordered the PCB at the factory

 

[kuly]

grounding the wire on one side only! -It's right?
on the second side you do not need to make contact with the ground connection! - right? ------- so right?

 

[kamis]

Are original Vendetta PCB holes plated with copper? There are ground pads on both sides but soldered only on one side.Is there connection to other?

 

[kuly]

I think so too ! - solder grounding on one side!

 

[kamis]

Are components on the left side grounded on the top layer? There is no solder, but leads may touch copper foil. Plated holes?

 

[kuly]

two contacts with the bottom layer

 

[kamis]

There is a solder on the leg of this resistor and top layer ground pad, which means electrical connection with the bottom (fully soldered on bottom layer).