Thursday, January 28, 2010

Preamp power supply rebuilt

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.

Friday, January 8, 2010

Mighty Midget KIVed again

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.

Revisiting the 5687 Preamp

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.

Bass panels are in place at last

Finally mounted, after some trial and error. I had to drill the bolts on the ceiling rather than the wall on the rear as the "wall" was really false wall. It's just a box meant to flush the 8 pieces of diffusor panels. Drilling on the ceiling was tough. The front wall was much easier to drill.

An extra wire had to be used to secure the bottom of the panel so that it can be pulled up into the corner, otherwise it will just hang down like a painting on the wall. I used my manfrotto tripod to help prop up the panel while I adjust the bottom wire as tight as I could. An extra pair of hands would be very useful, but I don't think I can find someone crazy enough to help me do this. ;)

After mounting the panels, it was off to vacuum the room and to shampoo the carpet. I used a Scotchgard High Traffic Foaming Cleaner from 3M for the carpet. Quite a good product.

Some pics here.