I previously implemented a preamp with the UV-201a. These are very old globe valves, somehow fragile and hard to get in good shape. Despite all this, it’s a superb valve. I have managed to acquire a decent set of them to pair the best valves to use in my preamp.
Recently I developed a prototype PCB for the source follower circuit. The source follower is ideal to place at the output of this preamp due to its low driving current. My 4P1L PSE amplifier will be pleased with more current to pump the Miller capacitance effectively.
Here is the updated diagram for this preamp:
No major changes on the preamp itself. Just some component adjustment due to availability and preference. I’ve posted several times around this design using the gyrator PCB, you can read the blog if you want. The top MOSFET is IXTP08N100D and the lower jFET is BF862 which are my favourite combination for lower current (i.e. <20mA). The filament bias is formed by a pair of NOS Russian wirewound 51Ω resistors in parallel. Rod Coleman regulators V7 set to 200mA.
The addition of the source follower is very simple. The follower is DC-coupled to the SiC MOSFET (C3M0280090D). The back to back Zener diodes D4 and D5 protects the gate of the SiC. You can use your MOSFET of choice, ideally with low Crss and high gm. The SiC follower has a tail CCS formed by M5 and T1. The CCS current is set by R13. In this case is 10mA, however it’d be better to dial it up to 20mA. Penalty is the power dissipated across M4 and M5. Both need their heatsinks. With 10mA, you just need a clip-on heatsink for M5.
If you want, you can use any other FET of your choice. The PCB I designed includes both TO-247 or TO-220 pads for the top device and also you have back to back protection zener pads for the lower MOSFET in case it doesn’t have them built in on the same package.
The output coupling capacitor C2 is now connected instead at the output of the source follower.
Finally, C4 may be needed for HF decoupling and avoid oscillations which I experience with my variable HT bench supply.
The ground lift circuit was inherited from previous build and has a back to back Schottky power diodes which aren’t in the diagram. Not mandatory but useful to have if you have ground loops. In my case, I don’t need it as layout and grounding has been thought through carefully.
Prototype build
Here is another ugly breadboard for you:
Measurements
As usual, I conducted the measurements to check the preamp before the listening tests just to spot out any issues:
On my first test I found a poor FR due to a leakage cap in one Rod Coleman reg I damaged during careless testing. Gladly I replaced it and got the preamp working perfectly well. You can see the good response of this stage from 12Hz to about 400kHz loaded at 100KΩ. Now looking at the harmonic profile we can see:
The distortion is really low. More importantly, the even harmonic profile is nice with suppression of H3. At 10Vrms output THD is below 0.02%. Nice, but this doesn’t say much more until we listen to this stage.
Listening
Well, the most valuable part. After playing several weeks with the same configuration (4P1L into 4P1L), it was pleasing to get back the 01a. My amp has only one stage of 4P1L PSE and despite being manageable well with a 01a at 3mA in terms of slew rate, I can notice a nice improvement in the overall detail at the treble side. I already praised and enjoy Tony’s 01a preamp which has a simpler follower stage (not this one). It’s revealing to see that the addition of a well implemented source follower stage has no noticeable detriment in the sound. In fact, the 01a works in better conditions which allows it to perform at its best when loaded with a more demanding capacitive load (due to Miller effect).
I will continue to listen to this stage for a while to give firmer impressions.
I received several comments and questions since I publish this earlier today. Probably is worth clarifying a couple of points about this preamp version.
1) Difference between UV and UX types:
It’s worth reading this great webpage:
http://uv201.com/Tube_Pages/uv200_uv201.htm
There are minor differences between these valves in terms of sound in my view.
What is important is to find 01a globe ones in good shape, rather than whatever UV one you can find around. No rattle or loose parts in key. Finding UV ones in good shape is hard. Difference is minor in terms of sound, which can’t be degraded if the valve hasn’t been conserved well. Remember, these are very old devices!. However, the difference between an ST one and globe one in good state is big.
2) Should I build the preamp with the follower output buffer?
Time ago I remember we did an experiment with Andy Evans. We used the JFET follower designed for low level signal with a servo for DC bias between the 01a preamp and the 4P1L PSE Amplifier and the result was that the sound was degraded, muffled. This is not the case with the plain MOSFET follower designed for large voltage swing. The stage is as clear (or even sounds better) than without the follower in this case.
In my opinion, if your system has got a relatively high input impedance (e.g. 47K or above) and/or the input capacitance isn’t that high (e.g. step transformer coupling, long cables, etc.) then you don’t need a follower stage. The simpler the better, no need to add extra stages.
Hope this helps.
Ale
If building a version of the 01a circuit without the source follower, would it be worth it to make the component value changes shown in this post, or would I be better off using the values from your 01a gen 2 preamp?
Either are just fine, I can’t do side by side comparison at this point.