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. 

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. 

The Mule was ready for some dissection at the operations theatre so I ripped off previous EML20A preamp, added a pair of source follower boards set at 20mA of bias CCS current. I have a pair of fantastic PTFE sockets for the Ba made by Luciano Bandozzi. I fit them on the Mule and replaced the filament resistors with an array of 6 SiC diodes. It was my first time I used them as part of the filament bias circuit, as previously I’ve used them as cathode bias. They were going to get hot, but not enough to add a heatsink device. So I soldered them making a string of them whilst allowing some space for proper air convection. 

The operating point was a bit of a result of testing it and looking at the harmonic profile in my workbench. From a hot operating point I went down to 0.8mA/150V on about 6V of fixed bias provided by the SiC diode array.

With this operating point, anode current is very low. With the high anode resistance (25k) and low anode current, we need definitely the addition of the source follower. The SF PCB is set to 20mA and is able to drive any cable and in my case obviously, the input of the Elrog ER801a driver stage.  

With a choke or transformer load, this is a tough valve to use, so the hybrid mu-follower is a better choice in my opinion.

We can see that the addition of the Source Follower helps significantly with the HF response of this stage, otherwise will be limited by slew rate

With a flat response up to 130kHz, the gain is about 14. Ideal for many 100% DHT systems out there. 

My workbench is a noisy one and obviously the Ba picks up the inducted noise from my bench power supply transformers. You can see above the FFT and the presence of H2 and H3 harmonics at 10Vpp. This valve provides  a rich harmonic content and needs to be quiet around it to provide the lower distortion and no IM distortion due to hum noise.

The addition of the SiC diodes isn’t providing higher distortion as some feared. At 500mA bias current, they are operating well into the linear region. The THD results correlate well with previously measurements made with fixed bias.

How does it sound

When I connected the stage to my system, I was impressed straight away. My fear was that the mains hum pick-up from the valve was going to be so high that I was going to wrap the valve in aluminium foil or put a Faraday cage around them. 

If you either set gain to maximum or put your hands around the valves, the hum is very very low. I was happy with this!

I played the stage for a couple of hours by now. I played the testing set of tracks we used with Andy Evans as well as test albums I use like “Harvest” from Neil Young and “Goin’ Home” from Archie Shepp. 

My initial impression is thumbs up. The Ba triode is very delicate and transparent. The sound is balanced and with a great bass. I like the voices and brass instruments. 

Will definitely play this for a while so I can make a more refined opinion.  It’s a challenging DHT to implement, but worth it.

Sunday 26th August, 2018. London