One positive thing I can get out of this COVID-19 lockdown is that people are building more audio equipment. Here is a great story from Sridhar Ganti who has built a few DHT preamps over the years.
VT-25 with SMPS filament supply
Sridhar strived to get the VT-25 preamp working with SMPS supply for the heaters. As everyone knows, SMPS are challenging to implement due to their wide-band ripple noise. They are ok to implement in an output stage where the step-down effect of the output transformer and the SNR is significant to make this noise very low at the output . However, at a preamp level is another story. The signal levels are tenfold or more lower so the noise injected at the cathode is crucial. Think about the DHT as a differential amplifier. Is not just what gets at the grid, it’s also about the cathode. What the triode amplifies is the signal difference between the grid and the cathode.
I tried and failed with the VT-25. I couldn’t get rid of the midband noise. I added a series choke and it wasn’t good enough. In the end I gave up as I needed to add such a filtering stage which wasn’t worth the trouble.
On the other hand, Sridhar went through this. Despite my discourage to avoid the SMPS, he went through the experimental journey and succeeded.
Here is Sridhar’s write-up on his preamp:
The 6 diodes give a drop of approx 7.8V. The 801’s are shock mounted on a sub-chassis using 6.32 sandwich mounts from McMaster Carr, link given here : https://www.mcmaster.com/catalog/126/1548. This really helped controlling the micro-phonics. I also mounted the power transformer on small rubber grommets to further isolate power transformer vibrations.I used Meanwell SMPS supplies with the filtering circuit provided my friend and mentor John Levrault. Getting the preamp right was a little bit of a journey, I had messed up the filament connections, on one tube I connected the +ve to Pin1 and -ve to Pin4, and on the other tube it was reversed. The results was, when I powered up, there was power supply white noise. Since I had not used meanwells in the linestage before, I suspected that to be the culprit. Needless to to say after 1 week of tinkering around, John pointed out my mistake, and when I connected the pins with the right polarity as given in the 801A datasheet, the preamp powered up correctly, and it was dead quite. The initial Meanwell I had went up to max 14V, and with the 7.8V drop across the Sic, it was cutting it fine to get the 7.5V across the filaments. While trying to fix the initial hum problem, I ordered a pair of 24V meanwells, I used those with about 6.2 Ohms of dropping resistors to get the voltage to 15.5V The dropping resistors provide further filtration anyways.Now coming to the sound, but before that, a little word on my old preamp — it was a 10Y, using LCRC-Coleman filament supply and cathode biased and had a 4:1 step down transformer from Dave Slagle — Intact Audio. So it was a very good benchmark. However, the gyrator load with duelund caps and meanwells filament supply took the sound to another level – more blacker background, more details, the three dimensionality with the music improved in all directions. This is undoubtedly the best linestage I have ever built.Attached are some pictures of the line stage, its internals with the external filament supply, and picture of my system. The amps are Class A2 805 monoblocks using 211 as the driver with a source follower driving the grid of 805.Ale – thank you for the experiments you share with the community, and also being very patient in working with novices like myself :-). Owe you a nice dinner next time I am in your part of the world.CheersSridhar
A VT-25 from South Korea
