Tag: cx-301a

The low gain DHT pre-amp library

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.

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01a Gen2 Preamp Build from Barry French

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.”

01a Preamp (Gen2) Universal HT Supply

Introduction

I received many requests throughout this year of folks building the 01a preamp (Gen2) asking for advice on the HT supply design.  Well, I have my own incarnation which serves multiple purposes as is a shared supply, therefore isn’t useful to anyone. 

My friend Andy Evans came up with a supply using available components. This is exactly what you are looking for the HT supply.  I made some tweaks to Andy’s design, so all credit should go to him.

Design

The HT supply design is very simple. It’s mainly a choke-input valve rectifier supply. It has an additional LC smoothing stage. Here is the high-level circuit, see some notes

The transformer is a 250-0-250V / 50VA with an output current of 60mA. It has two windings for 6.3V AC heaters, but you only need one. Here is a great choice from a recommended seller. 

The valve rectifier is a double-diode damper. Of course you can use some other options, but I like the sound of them. Here are some you might want to consider in your build:

  • 6BY5G: a nice double-diode damper with lower filament requirements than the commonly used.  This is my first choice
  • EZ-80: the famous rectifier. This is what Andy used.
  • EY-91: you will need 2 valves as there is one diode per bottle.
  • 6C4P-EV:  a nice Russian small double kenotron rectifier. Very cheap alternative for European builders 
  • AZ1 / AZ11 mesh valves. Their sound is unique, however they are very expensive these days. You will have to add a pair of voltage dropping resistors to accommodate the lower filament voltage requirement. 

For the indirectly heated diodes, it will be better to connect one end of the filaments to the cathode. 

The chokes are commonly available. These are from Hammond (155J) and have 15H @ 30mA. The downside is the high resistance, over 1kΩ. This isn’t an issue here as the current consumption is low so the voltage drop is minimal.  With a choke input supply you need a minimum current to operate. In this case is about 15-16mA so a bleeder resistor is needed (29kΩ 5W wire wound) . 

C1 serves to equalise the output voltage. It pushes the supply to operate a bit more like cap-input (hybrid) by increasing the output voltage. I use it to tune the output voltage to 200V. 

C2 and C3 are classic motor run capacitors. I personally use ASC Oil ones 450VAC rated.  You can choose what you can get hold. You can use any good quality film capacitors. I like the WIMA DC-LINK ones, they are great.

The output ripple is about 7mV. This isn’t a problem as the gyrator load has a very high supply rejection (PSRR) so no need to go crazy on this. If you 

The design is so simple that anyone should be able to build this easily. 

Hope it works for you.

Merry Christmas!

 

 

 

 

 

CX-301a DHT Pre-amp Build from Malaysia

A fantastic build of the 01a preamp using the gyrator PCB from Cheah:

I have just completed my 01a preamp with STP3NK60ZFP output follower. The preamp sounded great !!

A great looking preamp using Rod Coleman’s regulators, output follower and V-Cap capacitors. The jFET is BF862. 

Well done Cheah!

01a Preamp Gen2 finished!

01a preamp gen2 finishedFirst build of 2015, just after the New Year. I’m pleased to listen to the 01a preamp gen2 finished. It’s a beauty, only the warm sound of the 01a thoriated tungsten filaments can provide. A perfect match to the Starlight CD player that was lacking of a bit of gain.

It’s dead quiet, the frequency response is superb and the dynamics are there. Very good bass that improves what I was getting out of the discrete DAC. I love listening to this preamp. A great pleasure. Thumbs up to the return of the 01a. My best Sylvania globe valves are now in use, yay!

Will soon write a proper blog entry for this ultimate preamp.

 

01a Preamp Gen2: Build (Part 3)

The preamp build is progressing, albeit slowly.  I tried a new breadboard construction approach. A nice Ikea chopping board was used to build the front and back wooden panels. A pair of aluminium squared section rails are used to provide support and hold a pair of top plates for the power and audio connectors. As a bonus given the low power consumption of the filaments, I will use these rails as the heat sink for the Rod Coleman filament regulators as well 🙂

All in a very compact design as I literally ran out of space for further equipment in my living room!

01a Preamp Gen2: Build (Part 2)

Spent this afternoon building a compact modular base for the UX-4 sockets, a custom tag strip and the filament bias resistor array. Given the very low power dissipation on the filament resistors they can be placed below the valve socket board to minimise space. The actual size of resistors is a killer, but I have a stock of these lovely wire-wound ones:

Also the gyrator boards were mounted on top of the output teflon capacitors, again to minimise space. The trimpots to set the anode voltage are located at the top for handy adjustment if necessary: