I’ve been prototyping a flexible CCS PCB. The intent is to provide a cascoded FET pair with some interesting features:
- The lower FET can be multiple devices depending on the choice of reverse capacitance and transconductance. These include jFETs and depletion MOSFETs like the 2SK170, J310, BF862 and of course DN2540. For this purpose several pads are provided for SMD devices as well as TO-92 ones, just like the gyrator PCB. A protection Zener diode between drain and source can be soldered when using low VDSS devices.
- There is either a string of trimpot plus a resistor to set the CCS current manually during test given the variance in the FET parameters. There is also an option to put a fixed resistor.
- There is a mu-output connection provided.
The board is very flexible and can be used for multiple purposes:
- shunt regulators (including VR valves)
- Anode load for phono preamps, drivers, LTPs, etc.
- LTP tail CCSs
I’ve been running some tests with excellent results.
If there is interest, I will run a batch of PCB to offer to the DIY community.
Here’s an excellent build of the 01a Preamp with the gyrator PCBs and the Source Follower PCBs by Paul. I will leave to him to write down his impressions of this preamp and the build experience.
If you like your VR valve glowing in the dark but you’re concerned about how quiet it will be, then you may be interested in this post.
I like the VR, I used them a lot. They are quieter than most people claim. However, they’re not the quietest if you’re looking for the lowest noise level.
Below is a very simple circuit which you can implement easily and use your lovely VR valves glowing in the dark! The VR (U1) is fed from the HT source via R1 .R1 should be sized to at least provide 10mA to the VR valve. C2 is the maximum allowed cap. R2 and C3 form the cap multiplier section. R2 isolates U1 from C3 to ensures it doesn’t oscillate. C3 to 10uF can provide about 50dB reduction at 100Hz, so great to smash out any remaining noise from the VR. Q1 provides current limitation in conjunction with R4. This will protect M1. The output is about 5V lower than the VR level. Well, that’s what you pay with a follower or cap multiplier.
The PSRR of the cap multiplier below is about 110dB @100Hz. Great performance:
The source follower PCB can be twicked easily to use for this purpose. It can also be used as an electronic choke (aka gyrator) and or a simple cap multiplier.
Q1 can be a BC547 thanks to the protection diodes D1 and D2 (15V) which will prevent from exceeding VCEO levels.
I’d use this circuit in many configurations, not just the screen supply.
Finally I managed to build a stable version of the 2P29L preamp. The wooden modular version inspired on DHTRob designs paid off. Here it is:
The result is fantastic. The 2P29L is a superb DHT, quiet and it sounds amazing. The circuit is similar to the original one but with a slightly different operating point due to the valves themselves:
I’m still running them at 20mA. This time the anode voltage has to be increased to 150V to achieve this. Consistent across the 2 valves used.
C2 was modified to fit a gift I received from my friend Vyacheslav. Instead of the usual 220nF, I fit a bigger 470nF FT-3 version which improves the LF response.
The frequency response is very good, as I published before. For the curious ones, here are some further pictures of the build:
I can’t say more than what I said before. This is a fantastic DHT, it provides a great detailed sound and is quiet. No microphonic noise and the dynamics are great. One of my favourite clearly.
Last time I wrote about the 4P1L in screen mode. It was great to see some DIYAudio member (Blitz) to post about his work on the 4P1L with screen as anode. I call it screen mode but probably is incorrect.
His post about G3 structure remind me to post this, I have tested it but never blogged about it. Yes having G3 as part of the anode structure will increase conductance and will form a nice “mesh” anode. Here it is how I implemented:
The pin 4 (G3) is now connected to 3 (G2) to form the anode. I reduced the anode voltage down to 110V to get 10mA. It could be increased, sure within the Pd limits.
The response is very good:
Here you have the distortion at 1kHz:
How does it sound?
Well, I wrote about it before. The 4P1L is one of my favourite valves. In this mode it sounds great, with a particular clear detail in the treble. I like this valve and will play it for some time to get further impressions.
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!
Top plate arrived as well, so we can put the UV-201a DHT preamp in its final frame. Now it will be time for soldering, yay!
It was time for the 10Y DHTs to do their beauty today. Sunday night with good music and single malt. Nothing more can be asked!
Well, after enjoying the 2P29L as part of the Mule preamp project, I decided it was worth building it so I can continue with my adventures on the DHT land. The 2P29L sounds so good that is worth having it as a standing preamp so here is my new version in progress. Thanks DHTRob for your inspiration using the IKEA chopping boards in a slightly different way I used them so far:
Simple and neat. The 2P29L were stripped out of their aluminiun cans. Naked as when we come to life. The gyrator boards are ready and tested. Just need to get the Rod Coleman regulators and complete the wiring.
Happy DIY audio!
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.
Continue reading “Ba DHT Preamp (Part II)”