Preamp wanderings.

[buzzforb]

AS if i didnt have enought going on, I am starting this thread to play with preamp ideas. Here is the first. It use LTP input in order to try to avoid increasingly scarce reistors. THe y are 369's, but are easily bought in large quanitities. It is based on a mix of the Salas folded RIAA and the Pumpkin. Havent decided to keep final stage common drain or switch to buffer. WE will see.

[PKI]

Is this mine? ;-)

[DUG]

Could someone please post a schematic.

Figuring out what it is from a board layout is a bit of a pain.

IMHO

[buzzforb]

I will post the schematic later. I have a tendency to skip that and go straight for the pcb.
@PKI,
You can choose between this and complimentary version. Gonna be one late birthday present

[DUG]

"...skip that..."

Sounds like an idea.

Thanks.

[Itsmee]

Hi Buzz, I know you like playing with circuits, just as a suggestion, why not replace the cascode voltage referance with a TL431, that way you can adjust the voltage with a pot to find the 'sweet spot', you wouldn't need to match the LED's and you could save those JFETs for another project.

You could also replace the LEDs in the output stage with a transistor (or TL431) for an improved current sink, obviously you would then need to match the FETs but I suspect you would do that anyway for minimal differential offset; the LTP current sink would also need the same treatment.

[buzzforb]

All very good ideas. I have the luxury of knowing that these are good operating points as offered in other circuits, but you are right, it would give more freedom.

[buzzforb]

I need some help with folded cascodes. In a typical cascode circuit, the voltage seen by the gate of the cascode device sets the drain voltage of the gain device. In a folded cascode, the cascode device does not set the drain voltage of the gain device, which is now determines by either CCS or Rl. What purpose does the Vref on the cascode serve? What determines the output of the collector of the folded cascode. If not loaded by a resistor or transistor, isnt it functioning as a transimpedance amplifier.

[buzzforb]

Found what I thought was a good reference. It confirmed my suspicions.
The Shunt Cascode Driver | Bartola Valves
Will have an updated schemtic soon.

[Zen Mod]

that one originated from our very DiyA , toob forum

it's already implemented in one veeeery secret (still in evaluation phase ) ZM's shebang

however , one need to not confuse it with modulated cascode hints from Pa