Gyrator PCB boards arrived!

Great pleasure this morning to receive the first batch of the gyrator PCB. After extensive testing we refined the layout and options for the PCB and now we have the final product ready for shipping! Unfortunately I will not be able to do some testing and shipping before end of February and I received a large number of requests already. I’m sure this batch will fly very quickly so please confirm your requests.

I will post soon the specifications and some circuit examples for this flexible gyrator board which can be used for DHT preamps (e.g. 4P1L, 01a, 26), amplifier drivers, A2 drivers, LTP drivers, parafeed output stages and more!

So check the For Sale section soon for more information.

Some days ago we discussed in DIYAudio using this board for a 2J27L preamp like this one:

The output FET follower is needed to for valves which have low current and high anode resistance which will struggle to drive large capacitances. The FET follower of your choice can be used instead.

A minimal circuit which sounds fantastic is the basic configuration of this PCB. With few changes this circuit can be used on many DHTs like 26, 4P1L, 10Y, 30, 30sp, 12, 71a, 45, 46, etc:

If you want to send me your requests please use ONLY the form below:

4P1L Siberian Gen1 upgrade

Some of the DIYAudio fans have built this version of the 4P1L with great success. There are several upgrades that can be easily implemented to improve this. I haven’t tried this myself, but my recent experience with the Gen3 and the 01 preamp gen2, I think are worth trying:

Replace the gyrator FET for a cascoded pair (M2 and M4 below) to improve PSR
Replace voltage reference by a cascoded LND150 for better HF and PSR response
Optimise the LF pole of the gyrator load by increasing R4 to 4.7 MΩ and reduce C1 to 220nF
Bias 4P1L to about 30mA. This will reduce distortion

Hope this is useful

Ale

01a Preamp Gen2: Build (Part 1)

The first step in the building process of this preamp has been to get the active load boards ready. A reasonably quick assembly process this afternoon:

Continue reading “01a Preamp Gen2: Build (Part 1)”

4P1L Siberian Gen3: Loctal socket board

Here is the 4P1L Siberian DHT preamp (Gen3) socket board. I’m using a pair of custom made teflon sockets mounted on a PCB sandwich with a 4mm silicon rubber sheet. The lower board is mounted over 4 silent blocks:

drilling the socket boards

Loctal sockets fit and silentblocks

finished socket board

4P1Ls

4P1Ls ready!

This should be a great improvement to reduce any further microphonic noise in the preamp!

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 collection of latest images

After upgrading my 26 preamp, turntable and adjusted the system to a great sounding state. Looking forward to play many records now…

HT power supplies

Latest System

26 preamp and 4-65a SE Amplifier

DHT filament supplies (and many of them)

26 and bigger brother 46

Reddish 4-65a

26 preamp Gen3

Lenco turntable

Setting up to play in the evening

26 Pre-amplifier Gen3

An updated DHT preamp

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.

26 Preamp Gen3

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:

26 preamp gen3 THD test1

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.

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.

Ale

01a preamp (revisited 2013)

Looking at improving the CX-301a preamp with cathode follower output I modified the gyrator load by replacing the DN2540 by a LND150 and 2SK170 which have a lower capacitance and will improve the performance. Likewise, the tail CCS now has low-noise audio transistors such as the KSC1845 and KSC3503. Bias point is roughly the same, a bit lower than before. Overall distortion in the simulation is great: THD<0.005% @ Vo=16Vpp and loading 100K and 330pF (cable load representation).

U2 could be 6Z52P, D3a or PC86. The latter will require and adjustment as the filament is 3.8V and not 6.3V.

I think is time for building and listening to a potential great preamp…

01a DHT preamp updated

Updated this superb preamp. See here for details
Enjoy
Ale

26 Preamp Gen2 completed

Finally, here it is.

Improved sound compared to LL1660 version. This OT (LL2745/8mA)is better suited for the 26 in this configuration. Sacrificed output impedance a tad to get more gain. Filaments starved at 760mA. Distortion is as low as 0.02% @ 3Vrms output.

First listening session with Mingus, exceeded my expectations. Rounder bass and more space. Like this sound!
Will do some proper measurements but for now will enjoy and listen to it…