Tag: DHT Preamp

Ba DHT Preamp (Part II)

I’ve been posting not very frequently lately. This is mainly due to lack of time and the level of business travel which reduced to nearly none the time available for DIY audio. 

Nevertheless, the scarce time always pays off. It’s incredible how selective I have to be in order to prioritise which project I should work on. The list is long though. 

Last time I did a quick exercise on the Ba DHT based on the curves I traced and the LTSpice simulation. Well, you always need to build and test in order to check against simulations. The result is, that you may need to adjust and learn from your practical experiences. 

The Ba (like the Aa) are tricky to use. They pick up any electrostatic induced noise. You don’t need even to place your hand close, the mains noise is induced already in its plate. This force you to shield these valves if you want to use them. Am afraid, that is what it is. My friend Rob (DHTRob) warned me, thank you.

The circuit I posted here, had to be readjusted.  Distortion was way too high. The operating point wasn’t good enough. You’d normally get inclined to run the valve as hot as you can, but I was wrong here with this one.

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Ba German DHT Preamp, here we go…

Flying around

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.

German precision

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:

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UV-201a DHT Preamp Revisited


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. 

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Gyrator PCB Update – Rev07

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.

 

2Ж27Л / 2Z27L DHT Preamp

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. 

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2P29L DHT Preamp

The Mule saga continues and it was time to modify the 3B7 preamp and to test the 2P29L valve. This was quite easy as they both have loctal sockets. I had to modify the Rod Coleman filament regulator to set the filament current down to 120mA. Then a bit of wiring work, and in less than an hour a new preamp was ready. Job done, this is why I built the Mule:

The circuit is quite close to my original design time ago. I modified the filament resistor to use an existing Russian NOS wirewound part I had in stock and suit this preamp quite well. Added grid and screen stoppers as well:

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Gyrator PCB Hack: final Enhancement Mosfet design

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)
 
How well it performs? Here you can see – no guard ring here, just adapted standard PCB for testing purposes:
Not bad at all with 3MHz bandwidth. However, considering the circuit complexity, I much rather stick to the depletion version which performs much better in my view:
 
Nearly 5.7MHz under same conditions!
 
Cheers, Ale
 

Gyrator hack: Enhancement MOSFET option

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!

 

VT-25 DHT Preamp Update

Well, it was just a question of time to make an update to the VT-25 DHT preamp. I wanted to try the BSH111BK as lower FET as shown below:

The result is visible on the frequency response. It does have an improved FR and the bandwidth gets up to 800kHz on same testing conditions. If you increase the HT to 280-300V you can increase the anode current to 30mA which should be good as well. 

I love the sound of this stage driving the 4P1L PSE output stage. It has a deep bass as well as clear treble. Dynamics on the system are fantastic. 

The bass on my system is fantastic now. Stronger than with previous setup. That is what you get when you marry low ra (4P1L pair) with a high-quality OPT.

I have a pair of pre-soldered BSH111BK boards (which are painful to solder) so I will give them away on a first come first serve basis

Weather is really nice today in London, so I’m going out right now…

cx-112a DHT preamp

Introduction

This was one of my first DHT preamps. I found a quartet of NOS CX-112a Cunningham (globe) back in Buenos Aires many years ago and built one of my first DHT preamps. I loved it. I played with it before I moved into the 26 and then started the long exploratory journey with DHTs. 

The CX-112a can be easily fit in an existing 01a preamp. Take a look at what Thomas Mayer recently blogged about this valve, worth reading it. 

Well, you can get more current drive than 01a (nearly double) but no thoriated tungsten filaments. Anyhow, the gain is slightly higher but is very easy to adapt to my gyrator-based circuit, that I couldn’t resist to take the quartet out of my valve stash and make them sing again after so many years.

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