Cascode RIAA

[revintage]

As I said earlier, there will be a peak if simulating with the totally unrealistic Lipshitz reverse RIAA!

 

[jackinnj]

As I thought. Just a &^%$#@! simulation. Try building and measuring and if all else fails, listening, then there are no peaks!

Regards, Allen (Vacuum State)

I've found that the simulation matches up pretty neatly with the results on an AP Analyzer. Less so for tubes, more so for semi's.

 

[revintage]

If you go back to my first post I changed the original cascode for a pentode. It is also below. Remembered I had a bunch of 6F12P lying around. Simmed they look much better than than cascode so I will eventually give them a try. Have no idea of how prone they are to microphony and noise, though. The inverse RIAA is as exact as it can be and also includes the fourth...

 

[audiodesign]

Has been opened a group buy about the pcb of a my phono preampl.

http://www.diyaudio.com/forums/group-buys/161394-hi-end-phono-preamplifier-andrea-ciuffoli.html

 

[revintage]

Why the thread-jacking? There is a Vendor´s Baazar for commercial stuff!

 

[dsavitsk]

I can't get the schematics in the original post to show, so I may be reinventing the wheel here (and I am certainly stealing from Vacuum State), but I have been contemplating a simple phono stage with a hybrid cascode on the input. Any thought about the wisdom of the design? As SY pointed out, the cascode keeps the Zout of the first stage constant. The resistor under the JFET should allow for some gain tailoring. The RIAA values were just from a calculator and will need to be tinkered with a bit. What else am I missing?

 

[DF96]

Bear in mind that a cascode has poor PSRR so you need good smoothing/decoupling on supply rails.

 

[SY]

What Dave said. Additionally, you might want to put an RC filter between the voltage reference and the tube's grid.

 

[dsavitsk]

Both good suggestions. I added a cap to bypass the grid reference. I also used a simple shunt reg for the HT, and a regulator for the heaters. The heater reg uses schottky rectifiers and a 6.3V reg with 0.2V of dropout. Also, since power transformers with dual 6.3V windings are cheap and common, for the ~10V supply for the grids, I used a voltage doubler + regulator + CCSes feeding the shunt reg.

Anyhow, here's a first go at a PCB. Still plenty to do, but it is a start (there are ground planes that are not shown).

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