Aa DHT Preamp (Part I)

Obviously it was time to test the little brother of the “Ba” DHT. In particular, as I have such a low DAC in place now, I need the gain. With nearly a gain of 30, it’s an attractive fellow to work with.

First good sign is that it doesn’t pick up as much hum as the Ba. That’s good, nevertheless I placed a back copper plate which acts as enough earth shielding to keep the Ba quiet.

I worked with my breadboard to find a good “sweetspot”( at least electrically) for this valve. Given the low signal source level, I aimed for a low bias. I played with my fixed bias setup before replacing it with the SiC bias board. I found that 2V/160V was very good in terms of keeping the distortion profile to minimum. Here is what I ended up with:

Only 2 SiC diodes are enough. The source follower PCB is mandatory given the low anode current. I run it at 20mA (hot) to get best results of the stage. Rest of the circuit is very simple, achieving a gain of about 30. Here is the distortion profile:

There is no shielding and you can see some minor IM distortion with mains hum. Harmonic decay is nice with H2 being strongest. THD is very low at 4Vrms which is good sign. Let’s see the frequency response now:


Nearly 140kHz of bandwidth which is plenty for the stage. Great response. I just need to plug this one in and listen to this promising German DHT.

 

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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