How many times I’ve been asked “how do I lower the gain of the DHT preamp?” many builders love the DHT sound and yet don’t really need that gain. Of course when you use a transformer load, step down is the obvious choice. And it works really well when you invest in a very good quality OPT.
However, when you use DHT with high rp, generally most of the low current ones, you face a tough challenge with regard to frequency response.
With the hybrid mu-follower topology (aka gyrator) we have the problem that the gain is fixed to nearly mu.
I’m in love with the DHT sound and in a preamp a 01a and the 801a/VT-25 DHT Preamp Update preamp cannot be beaten in my view. They all deliver a gain of 8 with a hybrid mu-follower topology.
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…
Barry French has recently build his version of the 01a Preamp Gen2. Here are a couple of pictures:
And Barry’s impressions:
“The 01a Amplifier is a stunner, personally I feel it leaves the 26 out in the cold, better top & bottom by a Country Mile, this was built using the Russian FT-3 Caps on the Output, Russian PIO Caps on the Boards with Jupiter 0.1 μf Wax/Oil Caps from B+ to Ground, the Power Supplies for both Filaments & B+ are from my original 26.”
It’s been a while since I played with preamps. Here is my third iteration of the 26 preamp. I love the sound of this valve as many of you out there. I found my previous build a bit inflexible to modify bias points and play around so decided to go back to the workshop and update my preamp, again. The design is very simple as you can see in the figure below. I’m reusing an existing supply but you can get away with a 200V HT and probably a 15V raw DC for the filament circuit. Key characteristics are:
Filament bias as I’m a fan of it, no output or input caps sir!
Added a Salas SSHV2 regulator to provide HT voltage flexibility and vary the operating point of the 26. The SSHV2 is extremely quiet so will give a clean HT supply to the 26. Yes, the lovely colour of the glow valves are lost in this version 🙁
The Kelvin capacitor C1 (ala Morgan Jones) will help keeping Salas noise input low, specially HF.
R1 is a simple resistor to measure the anode current. It is located in a handy place to allow easy measurements
T1 is the Lundahl LL2745 in Alt R mode for 5.6:1 ratio. This provides the lowest gain and the lowest output impedance. In my new system, this preamp is more a line stage as don’t need further gain.
SW1 is helpful in case ground loops are to be broken
The Rod Coleman regulators are set to 800mA to starve the DHT and reduce microphonics and distortion. I found 800mA to be better sounding than my previous 760mA.
P1 is my stepped attenuator and R2 will help providing grid bias when breaking before making action of the attenuator.
RF is 2 pairs of 10Ω/20W parallel/series wirewound.
The preamp is extremely quiet. The below measurement is with a noisy Sylvania ST valve which picks up significant 50Hz hum. Notice that 100Hz harmonic is very very low (can’t be seen at -100dB). Distortion at 5Vpp input (8Vpp output) is lower than 0.03% which is what you would expect from a 26. With a good selection of valves you can get this down to 0.02% for sure:
The most important part of the build work is grounding and avoiding ground loops. A combination of star grounding is recommended. I do the following star ground combination:
Input stage
Filament bias return
Output stage
You definitely want to avoid the filament current ground introducing hum in the output stage so you want to keep the three star ground points separate.
26 DHT Gen3 ready to be tested
Listening to the 26 is a fantastic experience. If you haven’t built a DHT preamplifier then do it. The detail and colour of this stage is unique. I found ST valves being more detailed on the top end, a richer treble compared to the Globe ones. Globe ones are sweeter and mellow. Definitely prefer globes for classical and ST for rock and Jazz.
26 Preamp connected to the 4-65a SE Amplifier
The preamp is extremely quiet. Best build so far, I can’t hear hum on my high-efficiency FE167E full-range speakers and that is a real challenge.
I will try this new combination of 26 preamp and 4-65a SE for a while and report further impressions at a later stage.
Just need to fix my Lenco turntable and will be a happy man again 🙂
Hope you enjoy this post and encourage you to build one of this.
