Tuesday, 17 January 2017

A fast 12dB gain current-mode buffer for my HP3325B function generator

I'm sprucing up my HP3325B, after it stopped working (shorted out tantalum).

One of the options for the HP3325B is the "high voltage" output, which allows up to 40Vpk-pk, at up to 1 MHz, into 500 Ohms. Usual output is 10Vpk-pk, at up to 21 MHz into 50 Ohms.

Anyway, I don't have that. I was wondering if I could perhaps build a buffer-amp that beats their spec. Ideally it'd do 40Vpk-pk at 21 MHz into 50 Ohms, without adding appreciable distortion (the HP3325 is only good for perhaps 0.01% at audio, so that's not a difficult number to reach).

I have an old design for a current-mode headphone amp that I thought might be suitable, so I spruced it up a little, thought in terms of HF rather than audio, and I think I've come reasonably close.

It's actually a challenging amp - it's gotta be DC coupled, stable with a gain of just 4, and have reasonably low offset and gobs of speed, with tolerable distortion driving 40Vpk-pk, and really good distortion at lower levels. Here's the schematic:

At the hard end of the spec it manages 0.08% THD pushing a 1 MHz 40Vpk-pk sine wave into 50 Ohms. Relax any of that and it gets a lot better. For example at 1Vpk-pk, 1MHz, 50 Ohms it does 0.002%. At the "High Voltage option" max spec (500 Ohm load), it manages 0.05%, not a lot different from the 50 Ohm case, due mainly to the heavy feedback current.

At audio frequency it's kinda cool, and would make a neat uber headphone amp. (0.001%, 10KHz, 40Vpk-pk into 50 Ohms). And it's got gain that just keeps going. Yeah, 12dB gain at 100 MHz:

Thursday, 15 December 2016

A new extension to David Tilbrook's AEM6000 design

Adding current mirror loads to the AEM6000 design is something of an improve, I think.

Fourier components of V(out) DC component:0.000917462 Harmonic Frequency Fourier Normalized Phase Normalized Number [Hz] Component Component [degree] Phase [deg] 1 1.000e+03 2.828e+01 1.000e+00 -0.04° 0.00° 2 2.000e+03 2.333e-05 8.249e-07 79.25° 79.28° 3 3.000e+03 2.341e-05 8.276e-07 -108.35° -108.31° 4 4.000e+03 2.028e-06 7.169e-08 144.80° 144.84° 5 5.000e+03 1.475e-05 5.214e-07 -95.81° -95.78° 6 6.000e+03 1.319e-06 4.663e-08 162.22° 162.25° 7 7.000e+03 8.930e-06 3.157e-07 -87.34° -87.30° 8 8.000e+03 1.163e-06 4.112e-08 178.86° 178.90° 9 9.000e+03 5.607e-06 1.982e-07 -81.01° -80.98° Total Harmonic Distortion: 0.000134%(0.000000%)

Wednesday, 23 December 2015


Varnishing isn't easy. It takes a while to get good results. The good thing about it is you can always just apply another coat. I think I'm getting better at this, after (thinks!) six coats on my rowing thwart, I've got a reasonably good recipe that looks set to provide a tolerable finish, at least after a few more coats...

My ingredients are Feast Watson spar varnish, Penetrol and real gum turpentine to help the stuff flow, and a proper varnish brush, which is wide and very thin, so it doesn't hold too much varnish, with super smooth bristles, so it doesn't leave great big ugly brush marks. I'm thinning the varnish out with ~15 percent penetrol and a further 5 odd percent turpentine. That gives me a mix that flows out nicely. Of course that's a recipe that's highly dependent on environment, brush, technique...

I started sanding with 180 grit, but found 400 works better in the latest coats. Here's the rowing thwart thus far. There's a bit of general lumpiness but the gloss level I'm getting is fairly good:

My rowlock bases and tabernacle have had a couple fewer coats. In the case of the rowlock base that doesn't seem to be an issue, but the tabernacle still needs some love.

I find when I'm putting it on it's good to keep a little container with mixed varnish+penetrol+turps, plus one with half an inch of pure turps in it handy, so I can thin out and clean the brush periodically, to keep it from sticking to everything.

Sunday, 20 December 2015

Camouflaged cat is camouflaged.

See if you can spot the cat hiding in this picture:

Of course like all good cats, Mogget's goal in life is to ensure his paw prints are in all varnish.

Saturday, 12 December 2015

Finished the coamings

After gluing the coamings in place, the next step involved trimming them so they were the correct size. This involved a process not-unlike trimming my fringe. Take a little off one side, look at it from afar, take a little off the other side, look at it from a distance, take some more off... Luckily I managed to stop myself before I reached the deck.

Then I sanded things smooth, coated with epoxy + filler (this 4mm ply has a pretty crap open-grained face ply, which swallows epoxy), then a couple of coats of unthickened epoxy, then sand down to 180 grit, and finally toplac paint.

Here's what it looks like tonight.

The shaped bit in the bow is to allow clearance so I can flip the forward thwart hatches over. The coaming is about 65mm above the deck at the bow, and 22mm above deck where I'm likely to sit on it.

Next job is to complete sanding the decks out to 180 grit and then paint them with top coat. No, I'm not using undercoat. I really dislike the stuff - it clogs emery way too fast for my liking.

Wednesday, 2 December 2015


Here's a padeye for my mainsheet. The usual arrangement for a dinghy is that one end of the mainsheet is terminated at the end of the boom. The sheet then goes through a block attached to a traveller or bridle across the transom (I still haven't made my mind up which way I'll go), then back through a block at the end of the boom, along the boom to about half way, then via another block mounted to the boom down to a block mounted to the centreboard case.

I have concerns about pulling a padeye out of the back of the centreboard case, as the loads might be fairly high, especially given that Geraldton is a windy place. It's hard to get inside the centreboard case to add a plate inside do through-bolt, so instead I'll use screws but screw into a couple of different faces, so that the screws are at an angle to one another. So here's that custom padeye so far. It mounts to both the vertical rear of the centreboard case as well as the sloping part, and will be held in by half a dozen #8 screws:

After welding with oxy-acetylene and CIG "Com-weld" filler, it looks really ghastly. I'm calling this piece "snot on bronze".

The below picture shows it mostly finished in-situ, poking up at the top of the aft end of the centreboard case - note the angle isn't 90 degrees (are they ever?).

I'm probably 70 percent of the way through the finish work. I've got to cut down to the bottom of some pits and polish it properly. Even so, from the regulation ten feet I reckon it looks okay.

Sunday, 29 November 2015

Last coaming piece.

Today I put the very last coaming piece on the boat. Now that I think of it, it's also the last piece of plywood. All subsequent bits of wood (spars, rudder) will be solid.

The second layer of coaming progressed very slowly, as I needed lots and lots of clamps for each piece. While waiting for pieces to set up I shaped other areas of the coaming and made some mounts for my nice bronze row locks from Jarrah.

This was fun, as they're a complex shape (practically no 90 degree angles here) and necessitated lots of delicate plane work.

They're held down with two #10 x 50mm screws from under the deck, plus epoxy. When I put the rowlocks in, there's four #8 x 25mm screws through the coaming into these blocks, plus an additional #8 x 38mm screw from on-top. The rowlocks are thus very solidly located to the boat.

The angles are rounded out and bum friendly, and I've raised the rowlocks by about 8mm from our initial test with Perry pretending to row. I think the rowing ergonomics should be reasonably good. It's still a big (wide) boat to row, but I reckon I've done everything I can.

Doing the final shaping on the coaming is easy on top, as there's plenty of room to plane and my Stanley no. 4 and spokeshave work really well. Alas it's rather harder underneath, as there's not enough space between cockpit seats and coaming to get the no. 4 in. I very nearly pulled the pin on a low angle block plane, but figured for this job a simple piece of coarse emery on a wooden block will suffice. That leaves me a little more money for the enormous pile of blocks that I now have to buy.

Wednesday, 25 November 2015

Coamings and thinking about rigging

I've been on holidays from work for three weeks, and it's been an incredibly productive time for me. After painting the interior I went straight to work on the coamings. Once the coamings are done I can paint them and paint the decks and timber-work, and the hull is then essentially complete, leaving a much shorter list of things that need to be finished before launch.

Here's the first piece of coaming being glued on. I started with the hardest bit, as I figured this was where I was likely to run into trouble. I used 4mm ply, and oriented it so that the central ply had its grain running along the boat. This made it much easier to bend, which was necessary because that's a really tight radius bend around the front of the cockpit.

After this piece, I just kept putting pieces on until the cockpit was filled in. Then I went around for another layer. The second layer needs many more clamps than the first, as the glue area is much larger. Here's an example piece going into place.

Now I'm starting to turn my attention to all the little detail bits. One of the questions that I'm mulling over at the moment is of how to attach my main sheet. The only other Navigator sloop that I've seen any detail of is Dauntless, which goes for a high-zoot factor traveller across the transom. Wayfarers generally opt for a bridle across the transom, and I'm leaning towards this arrangement, as is seems simple and straightforward.

So now the options of how to locate the ends of the bridle. The photo below shows some of the options I'm mulling:

The join in the ply that the eyes are sitting on has a 19mm square Tassie Oak stringer underneath it. At a minimum, I could simply put a pair of #8 by 25mm countersunk screws into this stringer to hold the smaller eye down. For something stronger, I could make a bronze plate up to go under the deck, and use #8 bolts, sandwiching the deck in a similar fashion to what I did with the chainplates. Or I could do the same with the larger four-bolt eye. I suspect the last option is incredible overkill. Indeed I'm suspecting all three options are overkill, but I've sailed on a boat (albeit a 26' one with a cabin-top mounted traveller) watching the traveller coming off the top of the cabin in gusts...