26 DHT Preamp (by Barry French)
After owning several high quality pre-amps & modifying some of them to some degree or another, I decided to have a go at building a top quality unit, so here goes first I needed to build the power supplies for filaments & B+ in a separate case, I already had some idea of the look & design that I wanted to follow, so I sourced the following components from these suppliers:
- 2U Cases from Maplin Electronics
- Weiss Transformer from Thomas Meyer
- Lundahl Output Transformers Thomas Mayer
- Filament Transformers from RS Components
- Valve Sockets from Ebay
- Kiwame Resistors, Mundorf Capacitors & Resistors from Hifi Collective
- Russian k409Y PIO Capacitors from Ebay
- Jupiter & Neotech Wire from Hifi Collective
- Magnet Wire & variboards from Maplin Electronics
- Filament Regulators from Rod Coleman
- Hammond Chokes from Blue bell Audio
- Valves & Glow Tubes from Ebay
- Acrylic from a supplier on Ebay
- Maple Timber from Timbernation
- CMC RCA Sockets from Hifi Collective
- Valve Sockets from V4TC Hong Kong
- Salas Regulator Kits from DIY Audio
- Power Supply to Pre-amp 8 Pin Plugs from Maplin
I didn’t breadboard this Amp but I did draw out where things would be located, however if I was doing it again I would breadboard it for sure.
This design is basically taken from Ale’s design with exception to his CCS before the glow Valves, instead I used K&K Audio’s compact types, one for each channel, this can easily be adjusted using the on board pot, I also didn’t use Russian Glow Valves as they proved to be unreliable, as did using 2 Salas Regulators, I am pretty sure that using 2 drops the voltage too low coming from the raw power supply, making it difficult for both the SSHV2’s & the Glow Tubes to spark up properly, so I used just one, which works just fine. I am not running the 26 Valves at full spec, but I have found that running the filaments at 900mA has the best sound & lowest noise/ micro phonic’s for me, this gives a bias voltage of 9 volts exactly, the plate voltage is 147 Volts, noise is very low & sound is deep, open & detailed, the integration is excellent & up there with the very best at any price. I have owned & heard some very expensive gear & I feel that for my purposes it can’t really be bettered. The one thing I may do in future is modify it so it has 2 independent raw power supplies, which will probably improve it a couple of notches. I built mine slightly differently in that the Coleman Regulators are in the power supply case & as such I will probably separate the wiring for this & run it to the main Pre-amp Separately, to get the noise even lower. The 26 type Valve’s that I’m using are the Globe RCA 226’s which definitely sound better, I have used 4 x 10 ohm Mundorf Resistors in series parallel to spread the load a little on the filament supply, and this gives less drift on the current.
I used the 8 pin plug from Maplin as a supply cable between Power Supply & Pre-amp, which is the same type as used by Cary Audio & Supratek, it seems to be very good & has a screw lock.
Volume Pot is the very good TKD type; I did try a stepped attenuator but felt it wasn’t as good overall. I have also reinforced the cases top to bottom with Maple which gives the Valves a much more solid foundation to sit on, making the whole setup less microphonic & solid sounding top to bottom. All resistors used are either Kiwame or Mills for low noise; Capacitors are Panasonic low ESR type in the Regulator Supply, Mundorf in the B+ Supply & some Russian PIO types elsewhere. I used Hammond Chokes as well. I used good quality wire from Jupiter & Neotech, all is either Copper in Cotton or PTFE for higher voltage, and solder used was Audionote. That’s about it really; it is a working progress project & as such will probably go through one or two other mods as time moves on.
Barry French
Interested in the 26 preamp?
You can check the following designs here
Many thanks to Ale for the massive amount of help on this project & for posting it on His excellent website, Also thanks Rod Coleman for help on determining the correct resistors to use on His Boards.
I have done some mods to the wiring & power supply since this was written.
This Pre-amp will continue to be a work in progress project.
The Filament Raw Power Supply is now built on much more solid foundation with better capacitors also, although this made a smaller difference, but I’m happier with the build now.
Ohow!!!
Very Nice building, congratulations.
Cheers.
Thanks for your excellent input & wonderfully built Pre-amp, I have done a couple of modification to mine since it’s original build, I am now using a Telefunken AZ1 Mesh Plate Rectifier instead of Mullard AZ31, this made a very big diiference, as did changing the Russian PIO Capacitors for Jupiter HT’s, wide sound stage & more depth, I have built in reinforcement into the caseing structure using dried maple, I am also using RCA OA3’s instead of the Russian ones, everything is also quieter now. I love the sound this thing makes, I get a real kick out of it as well, I have heard nothing that betters it & I have heard a lot of good systems. The build quality of your Pre-amp is at the very top of the tree, impressive!!!!
Glad to hear you made some further improvements Barry. This is a great sounding preamp. I’m looking forward to rebuild my CX-301a shortly as sounded superb. Will do a proper side by side test when I get the chance!
Cheers
Ale
My findings after breaking this Preamp in are that the Mullard AZ31 is now sounding better than the Telefunken AZ1, also RCA 26’s ST’s now sound much better than the Globe type I was originally using, they did sound a little hard, but now sound sublime, much more detailed on top & tighter throughout, front to back they are more holographic.