A heavy-weight driver

Rod Coleman came up with a brilliant design recently which baptised as “shunt cascode” driver. For those who cannot stand a pinch of sand in their circuits, I suggest you skip this post now. This hybrid circuit is actually a folded cascode if we consider the book terminology. What makes attractive of this design is its outstanding performance against the classic multistage designs aimed at achieving a large drive signal for output stages such as 300B, 6C4C/2A3, etc. I personally haven’t build it yet but according to Rod the sound is superb.

Before building a stage which will replace my current 45 SE driver, I thought it made sense to analyse the circuit and understand why is claimed to be such a great alternative for today’s designs.

First of all, let’s present the “shunt cascode” circuit:

The circuit is formed by a triode in common cathode mode loaded by a CCS and the output is feeding a common-base bipolar stage (yes, a bipolar transistor!). The purpose of the BJT transistor (Q1) is to fix the anode voltage and force the triode to operate in a (nearly) vertical load line. The triode is then operating as a transconductance amplifier (or current pump). As the input voltage varies (grid-cathode) this is translated into a change in the anode current. If we look at the anode which has a fixed voltage, the current variation will be reflected at the Q1 emitter as the CCS current is fixed (and it has a very large impedance). Q1 presents a very low input impedance as it is connected in a common-base mode. The current variation at the emitter is “buffered” and reflected at RL. Here is where the “amplification” is manifested as the current is forced through RL which creates an output voltage change (Vo). The BJT does not convert current to voltage, you can easily demonstrate this by setting RL to zero. Then, the voltage out is zero, but the current swing is almost entirely unchanged.

So you will be probably asking yourself: so what? What is the benefit of this configuration? Well, it happens to be that the gain of this design is dependent on the triode transconductance (gm1) and RL only. In fact, the gain is:

What makes it really interesting is that the gain can be much higher than the valve’s mu. You can easily achieve 200-300 gain with only one stage. The gain is dependant on the valve’s transconductance and RL only. So any high transconductance valve is a good match here. In fact, most of the TV high-frequency pentodes triode-strapped (e.g. EF80, E180F, EF86, etc.) are great candidates.

Some key points from the DIYAudio thread to highlight from this topology are:

1. The amplification is entirely determined by the triode
2. The shunt cascode operates with a static anode voltage, therefore Mr Miller is not crushing you with his habitual burden. The input capacitance is just the Cgf + Cga of the triode, without upscaling from the gain. The is the killer for CCS-loaded high-gain triode stages – and demands very low impedance preamp drive.
3. The quality of the cascode voltage-source is critically important.
4. The base drive to the cascode has a strong influence on the sound.
5. Looking at the hfe as a function of collector current and that the base current adds to (subtracts from) the collector current, any variation in base current will be reflected at the output to the valve.With a carefully chosen transistor, and operating conditions, the base current robs a minor linear fraction from the output current. A FET would fix that, naturally, but the FET’s gm is too low, and capacitance too high (in most circuits).
6. Compare previously stated flaw in point 5 with other means of getting the same gain. With 2 triode stages, you multiply the nonlinearities of the first stage in the second stage. These nonlinearities include the variation of gm across the large-signal swing (this alone is worse than the base current error). The power-supply rejection (lack of it!) is also amplified by stage 2.
7. Since the music current only circulates between the triode and the load, the supply current is constant instantaneously (not just on average, like a normal SE class A stage). This means that the power supply capacitor carries no Music signal – a huge advantage in itself. And the gain is so high, that the triode’s cathode resistor can be unbypassed – eliminating another perennial nasty. It can also run into the following stage with dc-coupling (lose the coupling capacitor, too), so you can have a capacitor-free driver stage!
8. The BJT is not acting in an amplifying capacity at all – its one and only function is to fix the anode voltage to the value set by the (fixed) cascode voltage. If you prefer circuit analogies, think of it as an emitter-follower – with only a dc input. The emitter has very low output impedance, driving a very high output impedance (the triode’s anode). Therefore the emitter totally defines the voltage. An emitter values holds its properties well, regardless of collector current – when correctly designed.
9. The BJT’s contribution should be very linear, so whatever the triode expresses with be transmitted to the output grid. Keep in mind that the amp’s output will be the combined contributions of the input and output tube. To the extent that it’s 2nd harmonic, they will cancel or partly cancel each other, as they are in antiphase. So a very horizontal load line for the input tube (less 2nd) could actually mean more 2nd at the output.
10. Comparing its performance to a CCS driver stage, the CCS loaded amplifier would only have a gain of about mu – meaning you would need to swing more volts as grid voltage to get the same voltage output. In almost every case, this would take the voltage into the high-voltage region of the triode, where the curves begin to compress, and the low-voltage region, where they expand. This distortion would be many times worse than the effects we are witnessing from Vbe changes. Add to that the serious problems of input-loading caused by the high Miller capacitance, and it becomes clear that the horizontal load-line stage has major distortion effects to contend with, that are almost completely avoided in shunt-cascode.
11. A darlington BJT pair instead of Q1 is better because there is effectively no base current error, and the capacitances are lower with BJTs (compared FETs), and the gm higher.
12. The high R-load works better, because the grid has to swing less for the same output. This reduces triode distortion.
13. A resistor is better in cathode because:
    – it degenerates the gm, but linearises it (with zero phase error,unlike loop feedback)
    – it holds the anode current steady (important for dc stability)

Let’s look now at the gain of this stage in more detail:

To simplify the analysis, I will use a fixed-bias stage and ignore the CCS output impedance. So if we replace the triode and the BJT with their models like in the diagram on the left, we can see then that the following expressions can be derived from this circuit:

The gain of the shunt cascode is then:

So considering that:

Then we arrive to the simplified gain formula:

The driver in practice

So, how will this circuit perform to drive a 45 SE stage where at least 100Vpp is required?

I could use any of these Russian pentode drivers, but I thought about testing the lovely 6e5P first. Rod has kindly helped me in refining a 6e5P driver with this topology. Here is the initial take on it:

A simple DN2540 CCS set to about 42mA and a darlington BJT formed by a pair of PBHV9040Z set to 250V at its base by a shunt regulator to ensure stability of such a high gain driver. The 6e5P (triode-strapped) is biased at about 37mA and 251V. RL is a 27K which gives about 135.

Looking at the THD performance we can see that its lower than 0.22% for about 160Vpp. The predominant H2 component is then reduced and odd harmonics increase significantly:

A DC coupled version can be achieved by placing a gyrator in parallel with RL to set the output voltage to the bias requirements of the output valve and provide a high impedance to let RL set the gain of the shunt cascode driver. This will allow removing the coupling cap (C4 in the diagram above) and provide an end-to-end cap-free system 🙂

Well, enough for today, I will probably look at refining this driver for a 6C4C or 4P1L PSE output…

Now it’s time to build this driver and judge its sound!

Ale

Author: Ale Moglia

"A mistake is always forgivable, rarely excusable and always unacceptable. " (Robert Fripp) View all posts by Ale Moglia