This is the first instalment of a series of blog posts around CCS for valve circuits. Hope you enjoy it as much as I did with the experiments conducted as a result of my interest in CCS-driven circuits.
The depletion cascoded CCS
It’s been long time since I’ve done some circuit analysis and algebra, hopefully I’ve got this right. Seems to get to the expected result, so hey: I’ve done it ok.
The analysis of this circuit starts by using the T-model of the MOSFET. I’ve omitted the parasitic capacitances to simplify the analysis. I leave you the challenge to add them in though. If we look at the typical self-biased depletion FET CCS we can find the output impedance by doing the following formulae crunching:
In summary, the output impedance looking from the source side is:
I couldn’t resist the temptation to try and build quickly the SLCF design proposed here. It was question of building a simple PCB for the tail CCS and the top MOSFET follower. Wiring it then point-to-point could be done in a matter of minutes and a “rat nest” was built fast enough to enjoy this learning experience.
The usual challenges we face when breadboarding circuits
One of the challenges we face when building a cathode follower with a high-gain / transconductance valve is that it can easily oscillate widely into VHF. So we then are a bit more precocious when building the test jig and “try” to have short connections (something which I didn’t do), add some ferrite beads to anode, grid and screen. Also some grid/screen stopper resistors (e.g. 300Ω) are always very useful. If you pay attention to this and check with an oscilloscope with sufficient bandwidth (e.g. 200MHz) you can spot out any nasty oscillation from the valve. I didn’t, thanks to the ferrite beads and stoppers.
The clear challenge of the SLCF is establishing the correct bias point on the top follower due to the high value of the resistor divider and the high-variance we typically get on the VGS(th) of the MOSFETs.
High-value resistors are available on 1% but the variance on the FET defeat the purpose of accurately building the resistor divider.
Thanks to Tim’s suggestion to implement Allen Wright’s Super Linear Cathode Follower (SLCF) which forces the valve to operate in constant voltage (in addition to the constant current) by bootstrapping the cathode to the anode using an extra follower, distortion is reduced further. In my case, it is halved!
The “extra follower” on top of the cathode follower could be another valve. But to avoid elevating heating, I just went for a straight sanded follower using a depletion FET, the famous DN2540. You can use the MOSFET of your choice here:
The stage gets a tad more complex, albeit not much. We can keep one single supply rail here but we need to elevate HT up to 180V minimum to provide the headroom required by the FET to operate well and minimise its output capacitances to ensure HF response is good. M1 is the top follower which provides a fixed DC voltage using the resistor divider formed by R7 and R8. The higher these are in value the smaller C4 is. C4 provides the bootstrapping between cathode and anode to force constant voltage operation at all times. This minimises the distortion of the valve. I found that 100nF was enough to halve the distortion down to 0.0035% at 2Vrms!
PSR is about 60 to 70dB across the audio band so great addition to have a top active device here!
I think this could be a fantastic follower to use in multiple designs. Worth breadboarding it, hopefully shortly.
A common challenge we all face when building HT supplies for our valve amplifiers is the inrush current at start up produced when the filtering capacitors charge up and blow the fuses. There are several workaround, albeit most of them are not effective. Increasing the value of the primary fuse seems like an easy solution, but is pretty dangerous. The fuse will not blow at start up, however, what is worse, it will not blow at all before any other damaged is already produced in the supply in case of a short circuit or any other issue. If we add some resistance to the secondary, this will drop volts, waste energy and increase the supply output resistance. If we add resistance in the primary, like an NTC, is a much better approach, however we want to bypass this NTC to increase efficiency and performance.
A nice solution is to bypass the NTC (or a resistor) after initial in-rush. A simple circuit is possible to implement using a timer and a relay. The same circuit is used also to apply a longer delay (e.g. 2 min) to turn on the HT supply automatically if you wish. Continue reading →
A typical challenge we may all face is how to drive effectively our amp via a stepped attenuator or an AVC. I have a 4P1L preamp which drives very well my AVC, however, I have now an LME amp which has a wimpy input impedance of less than 7K.
How do we deal with this? A simple line stage which is capable of driving the low impedance of the amp is what we need in this case. Several options are available, however I settled down for a simple cathode follower.
Why? Because I love valves, and I wanted to play around a cathode follower design here.
A heavy load for your preamp or music source may increase distortion and we don’t want that.
I set myself the challenge to design a simple linestage with a minimum number of power supplies. I could have gone for a MOSFET follower, but hey: I wanted some hollow state stuff in there! Ok, if we look into a cathode follower as the core design, this means that we need at least an HT supply and a filament supply. If we could leverage a bucket converter, we could provide the HT from a LT transformer, probably best to look into two windings to separate the filament supply from an HT one. There are cheap ready build step-up converters for peanuts, and this is what tempted me to explore this solution.
I tested recently some step-down bucket converters and was encouraged by the noise levels and the FR.