(download)

Click here to download:

i-v-for-sabre32-jAdjxmiftamqwyuEwfyB.zip (5.67 MB)

Anyways, there is the need to do up an I/V stage for this DAC. There's so many choices out there... and coincidentally, John Broskie had a newish Broskie Cathode Follower design on his site. I then built the BCF for 6H30 tubes. The tubes will be powered by a Salas High Voltage Shunt regulator, which would be recitified by my Telefunken RGN1064 mesh plate globe tube. All great stuff in their own right. :)

I was checking TubeCad today and Broskie had an update to the BCF in the form of The Unbalancer. An input differential voltage gain stage is added to convert the lowish voltage swing into something more usable for an amplifier. Well... this made sense. I was just hoping that the BCF alone will be able to do the job... Seems I will need to put something here. I've got 5 stars GE 6072, but the gain (44, i think. The Unbalancer circuit used a 12AX7 - gain of 100) may not be enough. I'm thinking of using my red base 6SL7 (gain of 70), which would be a better candidate. Or maybe I should buy some 12AX7...

Just over the weekend, we had a Sabre32 mini shootout with 3 different implementation of this chip. My EE MiniMax DAC, Nick's Amplidac, William's UTS. We played some very nice cds. I especially liked the Sara K & Chris Jones - Are we there yet? and a Koji Tamaki cd.

You're gonna ask me which implementation won... Well... it's down to personal preference. The Sabre32 signatures of better resolution, imaging and soundstage are definitely there. For DIY guys, this is an avenue to custom build a Sabre32 DAC to their liking that matches their system.

Guess I can only pursue this after my reservist which starts tomorrow... In the meantime, enjoy your music!

(download)

Click here to download:

rca-834-tubes-FHdDdxDIbokAebFdhcAu.zip (11.47 MB)

Went to the post office to collect the 2 pairs of RCA 834 tubes. While collecting the B4 sockets for the Telefunken RGN1064 from Joh, he mentioned that the top pins would need to be "heat sinked" in order to dissipate the heat otherwise the glass would crack. One of his friends encountered this issue with a similar tube. I remembered coming across a site somewhere on the web where a DIY guy can help machine those top caps meant for the 833 tube. Need to do some searching...

Anyway... this will be a build that I'd attempt only after I managed the RCA 808, which has been in the pipeline for a long time. I need to finish up the 1626 first (some adjustments need to be done on the aluminum plate so that the top plate can be accommodated. need to get a few short standoffs as well), then build the Sabre32 DAC with Broskie Cathode Follower with a Salas High Voltage Shunt regulator, before the 808 project starts.

(download)

Click here to download:

some-updates-on-the-1626-build-aiqmwxwlAlCeelwmHJHg.zip (13.37 MB)

I could not find suitable 5k 10W resistors for cathode duties at the audio stores. Both RS and element14 (Farnell) did not have stocks for the Dale aluminium power resistors. I looked around for a while, and eventually ordered Caddock MP820 series from RS. They were extended availability items and would have to be shipped from elsewhere. Total cost for 2 of these babies is around SGD25. I sure hope they would do the job well.

The MP820 resistors dissipates a bit of heat. Mounting the cathode capacitor on the aluminium plate is not ideal. I then looked around for suitable smaller sized capacitors that can be directly soldered onto the resistor leads rather than mounting the large Nippon Chemi-con. Anyways, there is not that much headroom for the Chemi-cons. It's a tall order to fulfill since the voltage would need to be around 200v, and yet be of a manageable size.

John Broskie happens to have an old post about calculating cathode capacitor value and the corresponding capacitor value to inject the right amount of power supply noise to counteract output noise in the circuit. The formula to calculate cathode capacitor is Ck = 1 / (2pFRk) or Ck = 159155 / F / Rk. I think this formula is pretty much standard. For my circuit, with a cathode resistor of 5k, this works out to 2uF for a 16Hz cutoff. Standard capacitor values guided me to this cutoff frequency... not that I play pipe organs that much in the system. There surely must be some mistake I thought. Is a mere 2uF enough?

Well... I tested with a 1uF capacitor that I had on hand and it worked pretty well to replace the 220uF Nippon Chemi-con that I used in my tests so far. It turned out to be quite an advantage for a direct coupled circuit as this is a small enough value to consider a quality polypropene capacitor in this location. The same post also mentioned a formula to calculate a corresponding capacitor (ultrapath capacitor) to inject ac signal to the cathode, thereby canceling the power supply output noise. The 1626 has a gain of about 5. This results in 1/6 of the 2uF Ck, giving 0.33uF.

I got some GAD-viva capacitors from Koba since they had these values (and others too... their GAD 0.1uF Silver Gold is around SGD105! ). Managed to wire them up tonight, together with the MP820. Dialed in the operating points and finalized the design. 6N6P (gain of 20) is running at 110v, -2v bias (actually the yellow LED drops 1.95v). 1626 is running slightly hot, at 185v, -18v, 26mA.

The final schematic is also in the pictures above. The schematic for the CCS used is the HV PNP CCS from diyaudio.com (pdf can be found here). I used an additional red LED for the reference voltage string (LED2). This increases the reference voltage, causing the emitter resistor to be larger, and raising the CCS impedance. The pdf for the Janus Shunt Regulator would provide detailed information on how to build it. My implementation used a Ohmite rheostat for R5, allowing me to adjust the voltage, and a JJ ECC832. C1 used is 22uF, C3 & C4 are 220uF. Bypass capacitors are GAD-viva 2.2uF ones.R4 and R7 are Kiwames.

I just need to finish up the IEC inlet and switch, plus the top aluminium plate. I guess these can be done at a more leisurely pace since music is flowing now. ;)

The PS-2 shunt regulator was nicely fitted into the preamp. I contemplated fitting it into the power supply chassis, but gave up thinking of how to mount it. When I built the PS-2 on the heatsink, I was thinking of mounting it via a screw onto the back of the preamp. That's how it was done in the end.

But looking at the power supply, I wanted to rebuild it with a proper transformer as the one being used now outputs a B+ of 350vdc. The shunt regulator will be very hot dropping 200+ volts. And it turned out that I had one odd R-core transformer that was shipped wrongly to me when I ordered transformers for the 6T10. This also presents the opportunity to build a chassis that looks similar to the preamp.

I ordered the aluminium from Superfix as usual. Collection was done on the same day I placed the order. When I got home, I checked the dimensions and to my dismay, there is some excess on the metal. 3mm is not easy to file off... especially if there is a long length. I tried to do so initially and it was a tough job just to file off 2 surfaces. I then decided that I should get a Jig saw.

It's the first time I went to Homely Hardware Superstore at Ubi Cresent. It's almost like Home Depot and I had much fun there, picking stuff to buy. I ended up with a Makita Jig Saw and other stuff.

I also got some e-nuts, screw covers and stuff from Yong Seng Screws at French Road. The most I ever spent on screws... The screw cover set is not cheap. $4 for a set of 4 screw covers and mounting flange. I bought 10 sets of 4. The e-nuts were $20 for 100 pieces. I think I'm nuts... but I have a total of 3 chassis to build. One for power supply and 2 for the 6T10. My previous builds used wall plugs instead. Not very tight after may rounds of opening the panels.

The 3/4" MDF were bought from Ban Heng Long Trading at 11 Syed Alwi Road. They cut to the mm. Nice. ;) The jig saw was really nice. Opening the IEC inlet on the 3/4" MDF never was so easy. It's like a sewing machine for men! Trimming the excess off the aluminium was also effortless. Next time I should order my aluminium with at least 5mm excess so that trimming is easier.

I made sure that MDF was drilled carefully when putting the e-nuts in. First a pilot drill with 3.5mm, then 6.5mm. 6.5mm is the best diameter to use at the sides of the MDF. I tested it for grip and made sure the MDF does not split when the e-nut is screwed in place. Alignment is important. So I drilled the holes on the aluminium first, and then overlay it on the MDF and used a dot punch to punch the mark on the MDF for the e-nut position. The aluminium pieces were drilled 1mm larger so that I have some freedom to align the MDF and top and bottom aluminium. I got an auto dot punch when at Homely, and it is very accurate. It is spring actuated and is much more expensive than a normal dot punch. $16 each I think, but well worth the investment.

Finishing spray on the top and sides are Mr Hobby Super Clear lacquer. The front plate and rear MDF are sprayed with Tamiya Metallic Mica Blue to match the preamp.

For the power supply, I reused the 50H chokes and the current regulator. Topology used is the RCA83 -> 50H choke -> 56uF GE oil capacitor -> 50H choke -> Mundorf 47uf+47uF capacitor. This then goes to individual PS-2 shunt regulators for each 5687 tube at the preamp. I changed the 2 LM317 on the regulator to a LT1085 in TO-3P package. The output of the regulator has a OS-Con 100uF 20V capacitor. The series resistor is connected by a pair of screw connectors. This is done so that if I use my power supply for other preamp builds, all I need to do is vary the current to the tubes being used. Of course, a PS-2 would be in future preamp builds to vary the B+. ;)

I learnt a lot from this rebuild especially in the chassis construction portion. It was a fun build, and now I will be ready for a preamp shoot out with 9 other preamps after Chinese New Year. This would be fun.

More pics here.

One of my KIVed project would be the Mighty Midget. I only got it working on one channel. The B+ was not enough when using the 6X5 rectifiers. Just a tad short for regulation. It was at around 290vdc. I need 300vdc for the Mighty Midget, and more if I want to use the Janus Shunt regulator. Out went the tube rectifier and in came some CREE CSD10120 babies. I'm using these  1200v 5A Schottky diodes from my parts bin.

I've also redid the Shunt Regulator onto a board and these are tested working.

The 6T10 really need a muting circuit, otherwise the amp will make some very nasty squealing noises upon power off. Tony recommended Rod Elliot's Soft-start circuit. Will have to order parts for this.

The time consuming portion is the need for a new chassis since the previous one had a hole cut for the tube rectifier already... Shucks... wonder if I should make this an all aluminium chassis... or stick with MDF and aluminium for quicker chassis construction.

Some pics here.

This preamp was working well since it was built. There were a couple of mods done since then, namely, the Feedforward Shunt Regulator, and also the DACT 50K Stepped Attenuators. I was re-looking the design especially from the perspective of my entire chain, and thought I should try out a different operating point.

What I had in mind was to do a -2v bias. This meant removing one LED from each section of the 5687 and also adjusting the B+ voltage. I also wanted to try John Broskie's PS-2 solo design. I intend to reuse the 10M45S on the Feedforward Shunt Regulator and also the heatsink for this modification.

I went to Koba and got some 1% 10R and 1R resistors, Wima and Rifa capacitors, a pair of LT1085, and a pair of BI Technologies 7276 - 10 turn 50k potentiometer to be used to set the output voltage. I also threw in my Mundorf 47+47uF capacitor and bought a pair of 240R Kiwame resistors from AHFartaudio for the PS-2. All these goodies would make any power supply green with envy. ;)

The PS-2 solo worked like a charm upon power up. With a 10 turn potentiometer, it was very easy to dial in the voltage on the plate of the 5687. With this potentiometer, the working output voltage range would be from 50vdc to 250vdc. Next steps are to mount the regulator into the chassis, along with some minor wiring.

You probably want to get John's PCB for the PS-2 instead of hardwiring.

More pics here.

Got some parts in the mail. They are GE 6T10 compactron tubes, Takman carbon and Mills wire wound resistors. Also in the picture is the Mundorf gold/silver wires. The wires are used as hookup in the multichannel UcD amp. Have to complete this before I can free up some space for either the Moskido or the Mighty Midget. I've already received the sockets for the 6T10. Got 12 of them from ebay... So if anyone needs a couple in Singapore, I am more than happy to sell some. ;)

I am more inclined to do the 6T10 at this moment. Possibly with a John Broskie Janus Shunt Regulator at the B+ as shown below.

Need to dig out some parts to see what I should use for the PSU. Cannot remember if I have the 6X5 or 6X4 rectifier tube around. If I am using a regulated B+, I may just use a CLC with the Nippon Chemi con 220uF capacitor.

Some pics here.

finally the feedforward shunt regulator design taken from John Broskie's blog is incorporated into the 5687 preamp. it is now providing dc with minimal noise to the circuit, biasing the tubes at 22.2mA per tube. will try to post a hand-drawn circuit of the implemented circuit later. more pics here.

i've actually got this board done up before i shifted. that must have been 3 or 4 weeks ago. i was even comtemplating putting it into my 5687 preamp the day before NoiSing07 so that i can demo the preamp with the regulator. good thing that i did not... it took many attempts before i got it to work. most of the time, i burnt resistors as i forgot where the ground of the board is cos it was sooo long and i did not mark the board with symbols. i also killed quite a couple of 10M45S chips. it seems that even though it is sourcing 10mA, heatsinks are still required on the chips. it seems that the regulator added a sense of scale and resolution to my soundstage. it is easy to pick out details... very resolving. also, the bass seems to have benefitted by being more controlled and detailed. not quite sure how to put it, but i would say that this is worth the effort. perhaps other HV regulator designs would produce the same effect? the circuit is based on Jon's feedforward shunt regulator. parts are swapped below as mentioned: 1. first dropping resistor after B+ as 1k2 10W instead of 309R 2. second dropping resistor after B+ as 2k2 10W instead of 614R 3. B+ is 290V, between dropping resistors is 228V and 136V (L channel)/140V (R channel)