I received the first batch of PCBs after several tweaks during the prototype phase. I’m very pleased with the result:
After doing all the soldering part (which I enjoy much), the preamp is now finished. It sounds as good as the original breadboard:
For the curious ones, here you have a picture of the inside:
The teflon sockets are bolted straight into the 4mm top aluminium plate. No microphonic noise this way. Rod Coleman V7 regulators set to 200mA. A pair of Russian Military NOS wire-wound resistors in parallel provides the filament bias. The gyrator PCB is set as per original circuit and each valve at 3mA. These are DC coupled to the MOSFET follower PCB set at 10mA each. The output is then taken out from a pair of FT-3 teflon caps.
Now to enjoy this beauty!
Travelling around Europe on business is paying its toll. I’m away from home every week and pretty exhausted now. I don’t have much time free and whatever is available I spend with my family. Hence, the lack of posts recently. I hope this will change in the future.
Anyway, what’s up in the DHT world? I listened the Aa/Ba valves long time ago but never played with them. Mainly due to their higher anode resistance. With the gyrator load and the source follower output, things take a different dimension.
I have a nice stash of Aa from Valvo (globe) and Ba from Siemens. Interesting to see that curves are not easy to find, so I submit them both to the mercy of the uTracer. Nice to see the linear curves with high mu about 14 on the Ba to 30 in the Aa.
Here is an example of the Ba loadline:
The gyrator PCB has been updated to fit now a wider variety of lower enhancement MOSFETs with low capacitance and high transconductance. The best examples are the BSH111BK and BSN20BK which are great options for currents above 25mA:
The board offers now all the flexibility needed in terms of different TO-92 and SOT-23 package pin-outs to use whatever FET you want.
The return of the Siberian
I have a pair of 4P1L/4П1Л dated 1968 which are properly burnt in. I’ve used them lately in my previous preamp incarnation with great results.
The circuit doesn’t need explanation, I think I’ve covered this repeatedly for a long time. I will only point out the differences:
The main change was fitting a pair of Russian wirewound 27Ω resistors in parallel to get closer to the 15Ω used in this position. I found these Russian wirewound resistors to sound extremely well as filament bias resistors. I tend to be skeptical about the “sound” of some components in circuits, however, they do make a big impact in the cathode of a filament bias arrangement.
The gyrator has my preferred combination: IXTP08N100D and BSH111BK. I have now an upgraded PCB Rev07 which fits the BSH111BK and similar FET and I will offer them shortly.
The latter benefits from the 30mA idle current. The result is lower output impedance whilst providing a great frequency response overall.
M3 needs a proper heatsink, it does get hot with about 2W of dissipation.
How does it perform?
Well, this valve has the reputation of amazing performance and low distortion. The gyrator setup provides the best out of this valve in my view. You can get a flat response as well as great bandwidht from 10Hz up to 3MHz loaded with 100kΩ:
The distortion is very low and is lower than 0.05% below 10Vrms. Dominant H2 with a lovely harmonic profile characteristic of this valve.
How does it sounds?
i’ve been listening and using this valve extensively since 2011. I have to say that it sounds amazing. I never get tired of its sounds. Before I listened to a 4P1L-4P1L system and found a slight edge on the sound (probably due to its H3 component) which I don’t hear on my system. The drive, clarity and tone is amazing. It can drive the 4P1L PSE perfectly well and you get a strong and clear bass. Very powerful. My +600 hours 4P1L are very quiet in this setup, no microphonic noise. I don’t have even dampers in the 4P1L sockets!
Anyway, if you need 19dB (x9) gain in your system or you need a driver for your SE amp, then this is the valve to go. I Still can be found cheaply and is a great contender to the thoriated tungsten filament DHTs like 01a and VT-25.
Build this one and enjoy!
Back in 2012 I did some experiments with this Russian valve. The 2Ж27Л / 2Z27L is a fantastic valve. I discovered that if I run it hot, it’s extremely linear and non-microphonic. Bingo! With its highish μ of about 16, it’s ideal as preamp stage or driver.
High gain stage with DHT
Some time ago a colleague (Shawn Fox) contacted me to find out whether I could test some rare high-mu DHTs. I didn’t have them in my stash, so he offered to send them across for testing. He was quite keen to find out the performance with a gyrator load due to the particular characteristics of the DHT in question. The valve in question is the CX-340. There isn’t much information about this valve am afraid and coincidentally, Thomas Mayer (Vinyl Savor) wrote not long ago a review of this valve.
Tracing the curves, the first step
The high anode resistance as well as the low anode current in which this valve operates makes it a real challenge to implement successfully. Hence, this is why the gyrator load plus an output follower stage comes into play as the best companion for this valve. Before we look into the circuit itself, I submitted the 40 valve to the mercy of my tracer:
I evolved my previous design here, thanks to the help of Rod Coleman and fruitful discussions with him.
There is an option to improve the design by bootstrapping the top MOSFET to avoid using a bias Zener and allow the bottom device to have a constant VDS. This can be achieved by double bootstrapping the FETs. Here it goes:
Similar design as before. Only difference is that R7 is used to create the bias of T3, and thanks to the bootstrap of C2, the bottom FET (T4) now operates freely regardless the swing. D1 is needed to protect T4. R7need to be adjusted considering the output voltage expected as well as the maximum VDS before D1 starts to conduct.
There is an stability challenge and it can be addressed as Rod Coleman clearly points it out, a “guard ring” :
The other pro trick is the guard ring: this will dramatically reduce problems of dc-drift, if the PCB surface gets contaminated, e.g. when soldered with some old or poor-quality solder. Or damp air, fumes etc. It’s a conductor (pcb trace) around the high-impedance network formed by the 10M resistors. A staggered-pinout version of the TO220 is needed to implement it, as the TO220 is the hotspot for leakage (B+ of drain to the 10MΩ-driven gate!).If there is a leakage path, it leads only to the guard ring, which is only a few volts away from the intended bias – rather than if the leakage can reach ground or B+, which would drive the circuit crazy. Connect the guard to a low-Z source – the Output in this case.Anyway – I hope it is useful in some way!(Rod Coleman)
Happy Easter to all! (whatever you celebrate, doesn’t matter, it’s always good to have some days off)
I have my preferred gyrator setup which includes a top (depletion) MOSFET IXTP08N100D, which has a unique high VGS(th) which helps improving the performance of the bottom FET, in my case the BSH111BK. The combination of both is superb and they do measure (and sound) superb. The frequency response is flat until 3.4Mhz (-3dB). Yes, a high bandwidth amplifier, so you need to be mindful of this when using high gm/gain valves. I read somewhere people complaining that gyrator “oscillate”. Well they don’t, however they create a high bandwidth amplifier which is therefore prone to oscillate if you don’t take the right measures. If you don’t know what you’re doing, it will oscillate for sure, you have been warned.
Ok, if you can’t get hold of (any) depletion MOSFET as the top device, there is an option, a la Gary Pimms.
The circuit can be tweaked slightly, as can be hacked the PCB (I can show you how if you’re intending to use this circuit)
Here is the design:
The main difference is that D4 provides a stable reference voltage (18V) which ones you subtract the VGS(th) of the top MOSFET (typically 2-5V) then will give you enough headroom to allow the bottom FET to operate under low output capacitance due to higher VDS. This is the common limitation of the cascoded pair of depletion devices. You can’t get more than 2-3V. As the top device forms a “cascode” with the bottom, it also limits the maximum voltage possible to the drain of the bottom device. The protection zener of the bottom device can be removed to ensure maximum swing. This stage can do 20Vpp easily. C5 provides some filtering to the zener noise, which is very low. I can’t see an issue at the driving levels in place.
The protection zener (D2) for the top device is needed unless the MOSFET comes with a pair of back to back as some do.
There are multiple options for the top MOSFET. I like the (nearly EOL) STP3NK60ZFP which is a FP TO-220 device, very handy for heatsinks and high voltage and comes with the bonus of the protection zeners. The best option is the AOT1N60 and also the easier to get hold off FQPF2N60C.
So, the performance is great. You can get flat response up to 2.1 Mhz. Here is a snapshot with my buffer which limits to 1.5Mhz:
However, my prefered stage can do 3.4Mhz under same conditions!