For more than 10 years I’ve been experimenting, designing and enjoying DHT pre-amps. Without over-dwelling on this subject, to me DHT preamps bring the unique sound transparency and detail we all seek for. They can make night or day difference in any system. And is not about gain, hence I’m focusing on sharing this blog entry.
Tag: Source Follower PCB
The return of the Mule (RE084)
It’s been a while since I experimented with preamps again. I settled as a permanent setup with my 01a/ER801a which I love. However, I wanted to continue with my experiments so I built a new Mule, let’s call it “Mule 2” for now. It’s based out of multiple PCBs which simplifies the construction process and reduces build time:
Pentode Differential Pair (Hybrid Mu-follower)
Pentode drivers are very interesting. You can get excellent results out of them. Here is my version of a differential pair using pentodes. I want to try them out in the 45 PP design I wrote previously Continue reading “Pentode Differential Pair (Hybrid Mu-follower)”
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 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.
Continue reading “Ba DHT Preamp Build”DHT Audio Shootout @ London
Here’s the summary of the shootout day we had yesterday at my place with Tony Rees and Andy Evans
Test Music
For testing the pre-amps Andy suggested the following 2 tracks:
- Ravel Feria (Cleveland/Boulez)
- Janacek The Cunning Little Vixen Act 3 Scene 7 “A vixen’s running through the woods” Prague Theatre/Gregor
Andy Evans
The Ravel is orchestrated for a large orchestra and percussion, and listening was particularly for the timbre of strings, brass and woodwind and the clarity of the percussion. The Janacek is for several different voices and chorus, and listening was for the tone and character of the voices and how well they were differentiated from each other. In both cases the overall clarity, life and crispness was considered.
Although I wasn’t familiarised enough with these 2 tracks myself, I found them to be an excellent choice and a reflection of Andy’s musical experience and knowledge as musician.
Continue reading “DHT Audio Shootout @ London”GM-70 SE Amplifier
I keep getting requests about SE amp designs using the 6SF5 driver. Here is what I have come up with for the 6SF5 driving the GM-70. With only one stage you can drive the GM-70 to full power and get 18W out of this lovely transmitting valve: Continue reading “GM-70 SE Amplifier”
Valve test rig with fixed bias
My test rig for DHT/IHT stages (and even Pentodes) has evolved over the years. Lately I settled with some nice modifications to allow testing the majority of valves I have. I use a modular socket system, nothing fancy and can add/remove a source follower stage at the output. There is also a screen regulator in case pentodes are submitted under the mercy of the jig.
Here is the simplified diagram. I added a nice fixed bias supply formed by a SMPS PCB board which delivers up to 400V, however the output is dial to about 100V. Then I use a Swenson Regulator to knock down the noise by about 100dB. A simple pot provides the voltage needed between 0 and -100V. It can be tweaked for whatever range you need. The pot is 20T wirewound so allows a fine adjustment on the bias:
For the curious builders, 
here is the rig mounted on a piece of floor board:
01a Preamp build by Paul
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.
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:
