January 30, 2010

What's Your Flavour? My DIY Tube Pre Amp.


The inspiration!

This project has been a long time coming. I began conceiving the idea building my own tube based pre amp to fill the tube void after my previous reference pre amp, the Sonic Frontiers Line 2 departed from my system. But first, I had to scout around for a great schematic to implement. The design brief was simple, the finished pre amp must sound better than the Sonic Frontiers, well, at the very least sonically match my Pass Labs X2.5 in the audiophile technical parameters. Other design brief must include a high end remote controlled volume kit, and the overall build quality must at the very least, match the current standards, set by the high end audio products. Lastly, as a firm believer of balanced designed audio circuits, the design must have full balanced topology. The Audio Research LS26 naturally came to mind, after I heard them on a couple of occasions. The Audio Research LS26 was one of the most refined, not to mention, transparent sounding pre amp I've ever heard. It only losses out to the bigger, meaner brother, the ARC Reference 3 in terms of macro dynamics.

The insides of the ARC LS26, note there's only 2 x 6H30 tubes, that's because the input stage uses J-FETs for lower noise floor. The 6H30 tubes are pretty microphonic. They ring like a bell as the volume kit's relay switches on and off. The speaker tweeters go "Ding......, Ding......., Ding......., Ding......" A tube damper over each tube solved that problem mostly. And yes, this even happens on the ARC LS26!

My DIY efforts, see the similarities? Note, all pictures shown are the final MKII board version, as I did not document the project's early stages.

The A-28 case kit arrived flat packed to save shipping cost.

It was some time in April 2007, when I started hunting around for the schematic of the ARC LS26, but since it's still the current model, the schematics are not web published just yet. I took the earlier schematic available in the form of the ARC LS25. I then set about acquiring parts for the project. I came across this Hong Kong based website, http://diyclub.biz/ selling DIY kits and various parts for hifi DIYers. From there, I sourced the A-28 casing and the V-03 balanced volume control kit. Next is to mate the V-03 kit on to the audio, and power supply design boards. The boards are designed and CNC manufactured with the aid of CAD/CAM industrial system, thanks to a friend in the electronics industry.


The V-03 volume control kit, it also does input selector via remote! 2 pcs for stereo, 4 pcs for stereo in balanced mode. Note the relay switching resistor step ladder design.

Also included in V-03 kit is the display board, power supply board(not shown) and remote.

Once the main boards are ready, it was time to assemble a test board to check for circuit noise and operational stability. This is necessary, due to the use of the 6H30 tubes in the input and buffer stages of the pre amp design. The 6H30 tubes requires very beefy power supply and gain stage voltage regulator heat sinking of ample size, to maintain it's operational consistency. Too large a heat sink, the sound will not be good, too small the heat sink, the voltage regulators will destabilize, and at worst, burn them selves out. And during the testing stages, many other ideas started to creep in like, how about an auto mute system for starting up? This allows the tubes in the pre amp to warm up, stabilize, before turning the pre amp out puts on. And during these very infant stages of testing, I felt the sound of the DIY pre amp to sound about 65% of the inspirational ARC LS26, which I borrowed from a friend to use as bench mark. The DIY pre amp sounded some what less transparent, less harmonically rich, and that trade off in the macro dynamics was something that I just could not live with, in the long term. Not bad for an alpha board build however. By now, it was past Chinese New Year 2008.



The power supply section, the heat sinks are hot, really hot! Noted the auto mute circuit timer, towering above all. I set it up for 2 minutes warm up time.

The V-03 volume kit implemented in 4 stacked configuration to save real estate. The empty resistor slots are for fine tuning, just in case I need to use them.

I then commission a MKII version board to be drawn up and made, based on the experiences gain from the alpha board and to include the new feature that I thought would be nice to have. I also allowed the MKII board to have various feed back points to be dialled in to the audio circuit design as and when required. Following ARC's board designs, only gentle right angle curves are used on the main audio board for best sound quality. At this point of re-design, it was deemed best to separate the power supply boards from the main audio section. The power supply transformer would be housed in a separated chassis, so only filtered DC voltages would be allowed in to the main chassis, another trick adopted from the high end designs.






The custom wound transformer for the required voltages.
The back panel with fused IEC input and out put umbilical cord.

The front aluminium face plate for the power supply box.

The MKII board is then assembled for testing again, before work on the chassis begin. Only when the heat sink sizes and final board layout was confirmed, only can work on the chassis begin. I used the A-28 casing with the bottom panel cut and re drilled to suit my board lay out, and the top panel fabricated from steel mesh welded frame, with the whole casing powder coated again for consistency of finishing. The aluminium front face plate was re-polished for better shine. This was all done sometime in October 2008.



With the assembly work completed, it's time to final tweak the sound. All the gain stage power supply regulator capacitors are white grade Mundorf for very tight tolerances(+/-5%). Post rectifier storage capacitors(reservoirs) are Panasonic KAP series. And in the audio section, the DC filter and by pass capacitors are Mundorf Supreme, and Silver/Oil grade(+/- 3% tolerances). These caps are truly potent stuff, once heard, you'll only want more!



The Mundorf Supreme DC filter caps, and Silver/Oil by pass caps. They're simply magical sounding!

All power supply resistors are 1% metal film grade with all other audio section using TKD audiophile grade resistors. The V-03 volume control kit comes with Soshin audiophile grade resistors. All internal audio related wiring used vdH SCS18, and all rear panel audio in and out puts are Neutrik sourced.



The back panel of the main chassis, all Neutrik audio connectors.

The final assembly completed and finely polished front aluminium face plate. Rubber gaskets are inserted behind the buttons for that tactile feel, when in use.

It was during final testing during September 2009 that I discovered that while the pre amp circuit was specified for use of 2 pairs of 6H30 tubes, in reality the pre amp circuit is very chameleon like and can accommodate the use of most other 6 volt audio tubes too. These include the ever popular 6DJ8, ECC88, 6922, 6FQ7, 61N1P and quite possibly 6H6P tubes too.



The see thru custom fabricated top plate, for maximum ventilation and a chance to show off the insides!



Testing was done with the Copland CDA-822 CD player.

I only used XLR connections in my hifi, but I've included single ended RCA connectors just in case some one borrows the pre amp.

During audition, I tried the pre amp using 6922 NOS Mullard tubes and it truly reminded me the sound of the Sonic Frontiers when I last used them with the same tubes. I also tried using 6FQ7(so favoured by Conrad Johnson in their driver stages) and noticed a shade of that rich, lush yet muscular sound of CJ in my pre amp. I have so far, not tried any other tubes in this pre amp just yet. This pre amp is certain capable of changing it's tune to suit your flavour of the day, depending on the tubes inserted. This pre amp project was officially completed sometime in October 2009.

An update 05/02/2010, since this article, I have received some enquiries about the cost to build this pre-amp. Many were horrified that parts alone, had cost me RM$4,600 or so to build it. Well I guess if one can do away with the luxurious Mundorf Caps, TKD resistors and the V-03 Remote Volume kit, and possibly a cheaper case then I goes one can build this pre amp for very much less than RM$2,000. Some have question my financial senses for pursuing such an extravagant DIY project, and others may think it's not worth while, or better to just buy a used pre amp? Well, I'd say the choice is for each one to make.

For me pursuing this project was to see how far I could go(sound and build quality wise), without cost constraints. A sonic success or white elephant? I've never thought of it that way. It's the thrill of the build, if there's such a thing!

Next up, I would aim to build a power amp to pair with my pre amp.

3 comments:

Gary said...

Bro..my hat's off to you on your AR-LS26 clone D-I-Y build. I stumbled on your article as I am also scouting around for a balanced tube pre-amp diy project.
As for a good power amp, I can recommend a 20-watt pure Class A John Linsley Hood design which I built, but its single-ended. Puts the big boys to shame really! Let me know if you are interested. Regards...

Big E said...

Gary,

Thanks for your kind words. My buddy is enjoying the pre amp now. He likes it so much that he is considering keeping it for good.

However, I am also considering building a solid state muscle power amp in the ball park of 200W or so.

At this point, nothing is finalised or concrete yet, but the idea is there.

Again, resources like time and effort are hard to come by these days. My soldering iron is probably missing me a bit now.

Thanks for your suggestion.

Arlerole Copkings said...

Man, you've done that pre-amp in such a clean way! You got a nice, expensive-looking case to go with it, as well. When it comes to reliability though, does a tube pre-amp last longer than your average, market-grade solid-state amp?

Arlerole Copkings