Tag: Source Follower

Ba DHT Preamp Build

Just in the last days before the arrival of our second child, I had a spot of luck and managed to get some free time to work on my DHT preamp experiments. 

A pair of NOS Siemens “Ba” valves under initial testing 

Originally I was put off by the hum pickup susceptibility of the Ba German DHT. However, I decided it was time to listen to the design I worked and experimented in my workshop time ago. I remember listening to this valve and was very pleased with its sound. 

Continue reading “Ba DHT Preamp Build”

Another DHT Preamp

Another DHT Preamp on the way with Rod Coleman regs. I will do a A-B comparison with some Tentlabs modules, it’s an interesting experiment to undertake. The preamp has the classic gyrator Rev07 and Source Follower boards…

UV-201a final version – finished!

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!

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.

Continue reading “Ba DHT Preamp (Part II)”

Source Follower PCB

I’ve been posting about the use of source followers in the circuits with some interesting results from testing. Some interesting posts to read, If you haven’t read them so far:

  1. Slew Rate, Slew Rate (Part 2), Slew Rate (Part 3) and Slew Rate (Part 4).
  2. 6SF5 driver for 300B/GM70/813 SE Amps
  3. DHT Phono Stage Test. 

After several tests over a variety of circuits, I finalised the prototype for a Source Follower PCB. The circuit is incredibly useful. Some examples of uses cases are:

  • Amplifier output stage grid drive
  • Screen drive amplifiers
  • Screen voltage stabiliser for pentode stages
  • HT voltage stabiliser for preamps
  • Buffer stage for high-mu/high-anode resistance stages – either triodes or pentodes (e.g. Phono)

Some key aspects of the board are:

  • The PCB has been designed to accommodate all sorts of power MOSFETs (both TO-220 and TO-247), in particular the high transconductance and low Crss ones which perform the best in this role.
  • The tail CCS is simple and leverage the option of using same MOSFETs.   
  • The board takes into account the use of any bipolar supply up to 450V diferential. You only need to change a resistor depending on the supply voltage levels and make sure there is a sufficiently big heatsink on the MOSFETs.
  • There is a current limiter circuit built in to protect screen or grid from excessive current. This is also very useful when the board is used as a voltage stabiliser for a preamp. You can limit the peak current and avoid destroying the MOSFETs when capacitors are charged or if accidentally the output is shorted. This circuit can be bypassed easily with a jumper.

Here is one of the boards submitted to the usual abuse during testing:

This is a very useful PCB in my view which can be used extensively in preamps, line stages and amplifiers.

If there is sufficient interest, I will run a batch of PCBs for the DIY audio community: