DIY Audio Again

I finally got the Wima capacitors from 8audio in Hong Kong. After soldering these to the TA2024 amplifier PCB (and adding a S$ 24 power adapter and some really old Aiwa speakers I got on ebay.com.sg) I now have sound!

The Tripath TA2024 is considered to be a very nice piece of audio gear, some people claim it's better than the GainClone (and all that for US $25 plus minimal shipping). And my DIY GainClone back in Manila sounds better than my Cambridge Audio amplifier, except for having a really ugly case, no volume control, and thumps on power-up and power-down.

I was really thinking of buying the Philips mini-component with a tube front-end, but it's gotten bad reviews and costs S$ 399 or thereabouts. Besides it's no secret that consumer electronic gear is as bad as the manufacturers can make it without pissing off customers. So for much less money (I did have to buy a power supply, soldering iron and a large roll of solder - they don't sell small quantities unlike at Alexan) I had half an hour of DIY fun and the knowledge that the amplifier I'm using is sonically superior to consumer gear that costs five times as much.

I can't really tell if this Tripath sounds fantastic because it's connected to cheap, used speakers (well S$ 3.75 for each two-way speaker is really cheap, so I had no expectations). Also the Nokia E51 is not a good audio source: there is audible hiss during low-volume passages. But then again I also heard hiss from Lalai's iPod Touch 3G during low-volume passages..

In any case (pun intended) all I need now is a good case for the amplifier, then I can go looking for Wharfedale Diamond 10's or something..

Finally Some Photos from the ZenithStar 70 ED

I was able to order a "store demo" William Optics ZenithStar 70mm ED refractor from the W-O online store for $338. This included free shipping worldwide!

I had it shipped to Singapore and paid the 7% GST. At least no unpredictable customs fees depending on the phase of the moon and the arbitrary-ness of Philippines bureaucrats..



The thing has amazing fit and finish. The 10:1 2-inch Crayford focuser is really smooth, and yet does not suffer from creep. The entire focuser also rotates, although as reported by others on the internet, there is a "catch" at one point of the rotation which causes the 360-degree rotation to be not entirely smooth. Not a huge problem however as rotating the entire focuser is not something one does very often..



I also got an EQ-1M quartz-controlled motor drive for the EQ-1 mount which I bought at True Value. Actually I purchased an Orion mini-EQ mount and the motor for S$100 from someone on the SingAstro forum.



Here's an afocal Jupiter at about 140X (12mm Plossl, 3X Meade Barlow). It's not as good as it could be, I've seen much better with this type of equipment. The EQ-1 mount is really horrible, with a lot of backlash that needs to be taken up before the system starts tracking properly.

Also even with good equatorial alignment, the EQ-1M does not track perfectly. I would expect that Jupiter can only remain in the field at 140X (0.35-degree) for about 2 - 3 minutes.



And here's the moon, with 32mm Plossl and 3X barlow.

For The Birds

Went to Jurong Bird Park yesterday (by myself, unfortunately).

I'd told myself I would try using my old Pentacon 135mm f/2.8 "bokeh monster" but manual focus just is so clunky. I ended up using the 100mm f/2.8 D-FA Macro, which, while a macro lens, works just fine, although it is a bit too short.

Big Bird:



Small Bird (lorikeet, about 4 inches long):



I think it's safe to say only a macro lens could take both these photos!

But that said, the K20D + 100mm Macro has got no chance at all of tracking birds in flight. And 100mm is too short for any except the biggest birds. So for captive birds the ideal lens would be a 70-200mm f/2.8 zoom. Which Pentax doesn't make, and the Tamron and Sigma offerings don't focus that quickly on Pentax bodies..

CMoy Headphone Amp

I've decided to build a headphone amp (blame Dennis!) so that I can maximize my almost ten-year old Sennheiser HD495 phones.

Anyway Alexan doesn't have the OPA2134 but they do have the much worse (but still orders of magnitude better than the LM358!) TL072. About $1.50 apiece (67 pesos). Not so bad..

I use the "Gwado" design which is very similar to the CMoy but with lower gain and a series resistor to drive low-impedance cans. The Sennheiser is 32 ohms which is difficult to drive, more so with a lowly TL072. But, the TL072 is pin-compatible with the OPA2134, so if this is a success, I can get some of those and drop them in.

So far all I have is a protoboard layout. Because I don't have 3.5mm jacks, I can't pipe in a signal or pull out any output. All I can do so far is measure the DC offset. It's about 11mV on one channel, and 20mV on the other channel. Not small, but not fatal.



I'm using the virtual ground design with two 470uF capacitors, as in the original CMoy design. The clerk at Alexan failed to include any 0.1uF capacitors (which are power supply snubbers) and I didn't check. So I used some humongous 0.047uF, 600V (!) capacitors from one of my past photo-flash projects. These caps are even bigger than the 470uF electrolytics!



Once this build is validated, I will transfer it to an Alexan PC201 veroboard, which I've cut down to fit inside an HC101 plastic case.

Pentax A Mod

Old Pentax manual-focus bayonet lenses are divided into K, M, and A types. The K and M only have mechanical aperture coupling, so on the "crippled KAF2" or "crippled KAF3" mounts, which are the mounts of modern Pentax SLR's and DSLR's, K and M lenses won't meter except in stop-down mode.

"A" lenses have a pattern of contacts on their bayonet which communicate their maximum aperture, which allows full program operation, P-TTL flash mode, and so on (basically everything except autofocus).

There's a mod floating around which supposedly tricks the camera into thinking any lens mounted on it is an "A" lens. This is easy to do because "A" lenses have a recessed pin which pops out when the aperture ring is moved to the "A" position. The moving pin is called the "A" pin. So to dupe the Pentax DSLR into thinking any lens mounted on it, is an "A" lens, all we have to do is short the "A" pin on the camera mount.

Here's your typical crippled KAF3 mount:



The "A" pin is the semi-recessed one that is third in line from the left along the rank of pins next to the lens release button. It's distinctive because it's flat and recessed, unlike the rest of the pins which stick out and are round. We need to short this pin in order to "tell" the camera that the lens attached is an A lens.

After getting the metal bayonet off. Note that there's a very thin metal ring which must not be molested or removed; there is a thin wire soldered to this ring and that wire had better not get cut!



This is the metal bayonet, stuff a piece of aluminum foil into the "A" pin hole.



Another approach is to put a tiny aluminum foil "cap" on top of the "A" pin itself:



Put the metal bayonet back (don't force it if it doesn't go in nicely, the metal washer underneath must fit neatly in for the bayonet mount to go down nicely). Torque in the screws as if it were a tire, opposite screws at a time, finger-tight only.

This is what the mod looks like:



An ancient K-mount Vivitar 90mm macro:



And as we can see, the camera thinks it's an "A" lens!



Now for a few caveats: fixing the lens at "A" doesn't really do anything. P-TTL metering results in massive over-exposure. The camera thinks that the lens is an f/1.2 lens (the fastest K-mount built) because your SMC-K or SMC-M doesn't have the aperture-setting pins, so program modes also result in massive metering errors.

Green button no longer works because the camera doesn't try to stop-down the lens anymore (it's an "A" lens, right?)

So the only benefit I can think of, is that aperture data is now recorded in the EXIF. But you have to make sure that your aperture set on the camera, matches the aperture ring on the lens. Also, since "A" lenses have a differently-designed diaphragm from K- and M-lenses, there will be exposure errors.

Overall, not a worthwhile mod but heck, it takes ten minutes.

Update: after a bit more testing, it seems that P-TTL flash does work, but only with wider lenses. I'll try to see if this mod is actually worth keeping, before I dismantle it.