Valve cascodes for Phono input.

[velvetsunrise]

Hi,

What are your thoughts on valve cascodes for phono input duty?.

The hybrid Fet/valve phonos have remained popular, both commercialy and from a DIY standpoint, but the all valve cascode seem to have been abandoned since the 80s.

Good sound? or not?

[SY]

They were very popular in the '80s, but the reality is, they don't bring a lot of advantages to the table other than Miller reduction (at the expense of less than optimum noise performance) for the additional complication. And the FET-on-bottom versions work quite well, so...

I suspect a better one could be done than in the past, but there are easier and good-performing ways to accomplish the same goals.

[Rod Coleman]

IME, a valve in the top of the cascode neither measures nor sounds as good as a BJT. Even a clumsy, high capacitance BU508 TV Line output NPN nails the cascode voltage much better, and sounds distinctly better than even purpose built cascode triodes, like ECC88.

On the bottom, the question is noise. You may be able to make something quiet enough for MM, or MC + step-up trafo, but direct-to-MC is not really practical, if even possible at all.

For MC, the 2SK369 NJFET [or its relatives] shunt-cascoded with a PNP, and a triode second stage, [also shunt-cascoded in my design] give the best performance of anything I've tried.
Perfectly quiet with 0.7mV pk Lyras, amazingly resistant to EMI, very high gain, and splendid sounding.

[DF96]

You need to think of a cascode as essentially a pentode without partition noise. This is good for RF, but has very little benefit for audio where noise is dominated by 1/f or flicker noise. The reduction in Miller capacitance is helpful, but some MM cartridges (e.g. Ortofon) need more capacitance anyway.

I was at one time thinking about a valve cascode preamp, but when I realised that it has poor PSRR and may need a higher supply voltage I went off the idea.

[Rod Coleman]

Originally Posted by DF96

............. but when I realised that it has poor PSRR and may need a higher supply voltage I went off the idea.

This is also true, but only in the case of a series cascode.

For a shunt cascode, the PSRR is better than most available alternatives, and the gain and output impedance can be traded off at will, since the dc operating point is not influenced detrimentally. And the fact that both input and output are directly ground-referred, gives high immunity to conducted and EM-coupled noise.

[DF96]

How does shunt cascode improve PSRR? The second/'upper' valve still has a high anode impedance.

[Rod Coleman]

Originally Posted by DF96

How does shunt cascode improve PSRR? The second/'upper' valve still has a high anode impedance.

Shunt Cascode can't use a valve on the first floor, unless someone developed a P-channel thermionic FET while I've been away.

The schematic shows the basic connexion:

Use your favourite current source [depletion FET, BJT twin, etc] to supply current into the stage. Naturally, the CCS should be designed to heighten the HF insertion loss between supply and amplifying parts.

Then, the cascoding element [PNP BJT] pins the anode of the amplifying device to the working voltage. The combination of these elements presents high dynamic impedance to the +ve supply, while rigidly holding the operating voltage to a ground-referred reference.

The PSRR is excellent, and unbeatable if you take good care in the generation of he cascode voltage (shunt regulator, to ride first class).

Stages built like this can have huge gain (my 300B driver has x280, with 27K output resistor) or can use low value resistor, and drive cables.

In the RIAA stage, you can put all the time constants in the output, and calculate the resistors based on your available C values - you don't have to compromise with the requirements of the load resistor, in the way of the usual valve designs.

[velvetsunrise]

Hi

Some interesting views on the cascode, i did play around with the shunt or inverted cascode, though quite some years ago, with a very simlar circuit to the one shown, though i used a CCS load for the inverted cascode transistor, fed from a negative supply rail, with riaa eq network tied to ground.


Of course if you take this hybrid aproach one stage further, you are on the slippery slope to a fully solid state design.

[DF96]

Originally Posted by Rod Coleman

The schematic shows the basic connexion:

Thanks, that makes sense now.