Craig Sawyers

Hey man - have a great one!

"You looking at me?"

18k gold. Kevin - you sometimes leave me speechless

Looks like low voltage and low current. Which is fine for vanilla transistors, but no use at all for testing T2 transistors, which are all high voltage high current types. When I built my T2 with counterfeit 2SC3675's with low breakdown voltage (but still several hundred volts) I needed to throw a Tektronix 577 at them to diagnose why the T2 was in terminal destruct mode. And then to verify that the new ones were correct according to the specification. I just about had to replace every bit of silicon on the heatsinks (and as you know there is a lot of them!) and most of the LED's.

Heh. My wife has a Ford Fiesta. It has no point in common with that! Other than basic body shape. I reckon that I might have got into 5th gear by the time half that video was over.

Metrum DACs - I have a sneaking suspicion that this http://www.ti.com/lit/ds/symlink/dac714.pdf is what they are using. The only thing that it misses on is the supposed 15MHz operation - but that could apply to clock speed with a bit of hyperbole and misinformation thrown in.

I think that the first, crimped one might squeeze up to 4GHz, but the soldered ones I'm not so certain - if only because of a mistermination impedance of the coax when it is dressed to solder to the pins. I'm using that sort of course - but even with 192kHz sampling rate you only need 250MHz maximum to take up to the tenth harmonic of the serial SPDIF data.

I used TE CONNECTIVITY / GREENPAR 1-1478048-0. It is not the all time ultra-wideband BNC, but it handles (measured) 1ns rise times fine, so it is at least 300MHz bandwidth - amply enough. It has the benefit that (a) it fits the chassis hole in the metrum once you remove the BNC and insulating washers, And it is easy to solder up, and it is easy to return the Metrum to stock condition if you need to ever return it for repair, or sell it.

^ I started to watch this, cynically. Then OMG it was bloody brilliant - that guy has put together a great version, solo!

Good call, deep

Sealing wood is a dangerous thing. As a natural material it likes to breathe, and wax, shellac, danish or teak oil are fine - but polyeurethane is asking for trouble and does not look good either (IMNSHO). The only difficulty is that the wood needs to be well seasoned and without internal tension - if it has that you can actually hear the click or ping as it cracks as you work it. That is the precise instant before you swear like a marine and walk out of the shop to calm down.

In the UK we sit at the triple point of water - so a hell of a lot wetter than Texas. So you're right, maybe what happens to purpleheart is an environment thing.

That is a good thread - thanks for the link deep. Pity eric's TDR measurements are defunct, but the thread is 6 years old after all. To quote Borbely from AudioXpress 5/02, about his design for the all-fet line amp design: "My goal was to be able to drive loads down to about 100Ω in pure Class-A, also allowing it to be used as a headphone amp. To be able to drive 100Ω in Class-A with up to 10V RMS, you need to have a bias current of 70mA." So this would definitely be able to drive 50 ohm cable (so 100 ohm total load for 50 ohm cable) at up to a whopping 10V RMS. Note that to handle that the 50 ohm load resistors would have to be at least half watt - but since the typical maximum you would be putting into an amp is 1V, low power resistors are fine - or even surface mount. I've been intending to build this beast for a while, if only to test my belief (which I now realise that KG shares) that matching impedances should render the analogue link cable insensitive.

That is really interesting. Erno Borbely went down the same route with his designs and kits before he retired. Enough drive capability from FET output stage to drive from a 50 ohm source down a matched cable and load. I makes the connection almost totally impervious to cables - just high quality 50-ohm cable is needed.

That is the problem in a nutshell. RCA's are ubiquitous. And when BNC's are fitted, or cables are BNC fitted more often as not they are 50 ohms, not 75. The difference is easily seen - 50's have insulation around the pin and earth, so when they engage the whole connection is encased in insulation. With 75's there is no insulation at all - the pin and socket mate in air. It is only because I am anal about interfaces that I've gone to the lengths I have in changing things to be impedance matched throughout, and done some supportive measurements.

I'm not into turning - cabinetmaking (just recreational) is my thing. The cocobolo will probably be used in things like lids of jewelry boxes, storage boxes etc, or small cabinets. I'm a follower of Krenov's philosophy of buy wood. Then buy more wood. Then look at it for a long time until a piece suggests itself. Ebony and blackwood I use for contrasting wedges in through tenons. Against a paler wood (and most wood is paler than those!) it is a superb effect. One of the more disappointing woods to work with is purpleheart. Looks absolutely stunning bright purple when first cut, but oxidises slowly to a kind of muddy brown.

So I did some formal tests of impedance mismatch at BNC 75 ohm and RCA connectors. Measurement system was a Tektronix 7S12/S6/S52 time domain reflectometer - it measures reflected signals from impedance discontinuities. It has a resolution of about 10mm. Set up was a 50-75 ohm matching pad, 75 ohm BNC terminated cable. The other end of this cable was either terminated in a 75 ohm BNC plug/socket with a 75 ohm termination, or Neutrix RCA plug/socket with 75 ohm termination The pics show the results. BNC gave a small reflection blip 200 picoseconds long, corresponding to a -9 ohm drop. So for 200ps there is an impedance of 66 ohms. The shortness of this event means that it is entirely insignificant, even at 192kHz sampling. RCA however gave a much more significant 600ps reflection, corresponding to a -23 ohm drop. For 0.6ns there is an impedance of 52 ohms. In fact if you look at the curve in detail you can see a double dip, where the Tek gear picks up slightly different impedances for the plug and the socket. Since it is likely that both ends of the cable are misterminated in this way, several bounces back and forth will be necessary before the reflection dies away, and since the data stream is pulse length encoded, line resonances from these reflection transients will add a noise and jitter inducing effect. Craig

You betcha Grahame!

Ice cold in Ewok. Oh shit that dates me on so many levels.

Damn right

That'll be superb once turned. I'm always amazed when a grotty piece of wood completely changes character once cut. That is part of the joy and grief - what is inside? Make a guess and hope you get the cut right. I hit a goldmine recently at my local wood and tools shop. He'd just got a shipment of Cocobolo turning blanks, about 8-10 inches square and around 2 1/2 inches thick. I got to them first! I rooted around and picked up the best figured pieces - about 15 in all, some smaller, some larger. Cost me £140 for the lot, which is pretty fantastic value. Also picked up some ebony and african blackwood too.

^Jeez. Makes you blood run cold - and want to give the guy that caught him a big slap on the back!

Nice looking build.

That was awesome - watched the full 40 minutes of it. Filmed in IMAX, and would have been awesomer in that.