Siberian DHT Preamp Gen3: filament supply

Time to upgrade my pre-amplifier (again) and is perfect timing to go back to 4P1L. The Siberian preamp had a fantastic bass response and detail.

Here is the new filament raw supply. It has split bobbin transformers, schottky rectifier bridge and input choke LC filter stage. It also has a CM choke and EPCOS electrolytic capacitors:

Power supply ready

Input chokes

Chokes and capacitor bank

Schottky Rectifier Bridge plus snubber

Building the power supply

Dead quiet at 16V output and 550mA which is the load used by 4P1L starved filaments in parallel with filament bias.

Soon to build the preamp!

A 26 DHT pre-amp from Albert

Albert sent me recently some great pictures and feedback of his 26 pre-amplifier using gyrator as the anode load similar to my design on the 26/01a.

Here are Albert’s notes on his 26 preamp:

“I have my 326 with separated power supply, 5U4G – 8uf PIO – 20H – 50 ASC – 30H – 50 ASC + 18uf pp feeding the Salas’ HV Shunt V2 Regulator, then to 2 ( left and right ) cascoded DN2540 gyrators. Rod’s filament supply coupled with filament bias @ 850mA with anode @135v. I don’t have anything to check the distortion, but this is the most good sound preamp that I have finished to date. It pleases my ears so much that I have the intention to turn it on once I’m in the house. It sounds gorgerous with very good image seperation. Sound stages are wide, deep and absolute quiet. The highs are sparkling smooth with lots of details. Mids are lush and warm with admirable vocals. I’ve found it even better at the lows if compare with 2x 156c. It goes deeper with clear notes.”

A new 4P1L Siberian from Martin

Martin kindly sent me a couple of pictures of his latest DHT preamp incarnation of the 4P1L Siberian:

Here are some notes from Martin about his pre-amp:
“A bit more about the circuit, I have built the power supply around the nice AZ1 mesh rectifier. First cap after the rectifier is an obligato 6,8 UF oil followed by a 10H choke and a 100 UF ASC followed by the SSHV2 that I purchased via DiyAudio group buy. On your suggestion I have used the CCS from your 301A preamp. If I refer to my previous preamp with the EF6 penthode in triode connection in parafeed configuration I can say that the 4P1L preamp sounds more detailed and with more air around the instruments. Finally I will do some tests with different output caps to find out if this makes a difference. At the moment I use the Vitamin Q.

Best regards,
Martin”

26 preamp (gen2) article

For those who are keen on the 26 preamp in filament bias and OT, here is my latest incarnation of this great pre-amp.

A preamp design with the LL2745

Lundahl LL2745 based OT DHT pre-amplifier

Before breadboarding a pre-amp with these nice OT provided by Thomas Mayer, I decided to simulate some options in LT Spice to see what results I got.

CX301a DHT pre-amp

First one is my preferred CX301a thoriated-tungsten DHT. I wired the LL2745 in 5.6:1 step-down configuration. This should provide a low output impedance which is what we want in this configuration where we need to drive the cables to the amplifier with sufficient capability:

Looks very promising. Of course gain will be much lower than a gyrator-based pre-amp, the 01a anode load is optimised providing very low distortion: 0.015% based on my SPICE model with curves taken from real CX301a. Gain is low at 3.1dB, but we don’t want loads of gain in this pre-amp. Sound is what we are after…

26 DHT pre-amp

Now it’s time for the revered 26. I used Dmitry’s model based on the RCA manual curves. I’d like to simulate this again using a model based on starved filament curves with a real 26.

Originally planned to bias the 26 in a different operating point based on feedback from Andy Evans, however after playing a bit with the OP I found that a more linear point was around Ia=5mA and Va=114V @ Vgk=-6.85V

26 looks more interesting in principle as the output impedance with this model is lower than the CX301a thanks to having a lower Ra (7KΩ against 11KΩ) so LF response will be slightly better in a side by side comparison.

Will be trying these two with filament bias and Rod Coleman’s filament regulators. The HT will be provided through a Salas HV shunt regulator.

Stay tuned…

Improving the 01a DHT preamplifier stage

After playing with DHT preamps (26, 71a, 4P1L, 46), I ended up staying with a CX301a version of it. I liked its warm sound and tone so I decided to stay with it. It’s been over 2 months so far that I haven’t returned to my precious 26 preamp (I will probably revisit my 26 preamp based on latest updates from Andy Evans).As I’m working on a version of the 4-65a SE amp by Michael Koster, I started investigating options and ways of improving the 01a as will be the first valve of my amplifier.

Using curves plot with my curve tracer and Dmitry’s composite model, I got a very well matched spice model for the 01a.

I modelled in LTSpice two versions of the gyrator loaded preamp with filament bias and Rod Coleman’s superb filament regulator:

1) Version 1: Using Anatoly’s PNP-FET Gyrator

2) Version 2: using classic cascoded-FET gyrator with mu-follower output.

Interesting results came out of this first simulation test. I think I haven’t managed to optimise (again) Anatoly’s gyrator despite have managed to keep good VDS (38V) to keep the CoSS low. I get slightly worse THD (0.03% versus 0.016%) compared to the MOSFET version.

Looking at output impedance, version 1 has 10K @1kHz whereas the version 2 can get as low as 734 ohms but impedance raises significantly at low frequencies (70k version 1 versus 80k version 2). This can be reflected in the frequency response where version 1 performs better than version 2.

Output impedance (Version 1)