Earspeakers

Correct, my vendor (BayAreaCircuits) didn't offer better pricing for a group over the 5 board store special they have, so I gave up trying to get a decent quote.

Anybody know of a 2" or bigger volume knob? Old school style preferred.

How about a differential (EDIT and high gain) version as an input to a electrostatic output circuit? That is, a Megatron without the coupling capacitors. Coupling caps are evil, I use teflon and that costs a fortune for the megatron.

LM317 supplies, I love it!

I got the boards back from Bay Area Circuits and so here's my report Turnaround was about a week kgstmini05 and kgstpsu21 For the KGST mini boards the best deal is the "Golden Gate". You can get custom quotes, but people here were squirreling and they needed a number, so gave up and just ordered one special. For up to 15-20 boards it seemed like getting multiple specials was better than a custom quote. $300 for up to 4 board types maximum 352 sq inches, you can pick color I received 5 PS boards and 10 Amp boards, for a total of 5 amplifiers, which works out to be $60/set The online software is first rate. Upload your files over the website and you can view them before submitting. Note that oddly Kevin has the PS board in .rar format and the amp in zip, but the ordering process wants all zip so I converted it. They are US based and nice folks, I talked to them on the phone (see below) to get some special help Last minute I found that there was some opinion I should be getting 2oz, where the Golden Gate special is 1 oz. I called last minute and they were nice enough to give me 2 oz for free (I recommend specifying it in the comments or contacting them beforehand) The boards look professional and first rate. Blister packed and look great So I recommend trying BAC, highly recommended. You might pay a less going with a Chinese manufacturer, but the service and quality of BAC looks worth it to me. I'm not selling any extra boards at the moment, I'm going to build two amps at least, so I recommend interested parties in clubbing together and buying the Golden Gate 5 board special.

Looks great eggil. Also tell us where you got the chassis sides. Looks like a great solution, if you don't mind I'll probably do that. With thinner panels though, don't need ones that thick.

Looks beautiful, and looks like a design I'd like to do. Please post your FPE files for us.

Can't tell you if they're OK if you don't have the part number and the specs. Probably OK, but who knows for sure.

Hi Craig, What setting do you use on your screwdriver?

Greenwire is pure bush league ladies ... no self respecting engineer would touch it (well, if they can help it )

My memory from the last time I used these (some years ago) was that the bushing additionally went through the heatsink. Combined with a washer on the other side, the screw was insulated from both the silicon and the heatsink. Extra degree of protection possibly. I could be misremembering but I believe I did this at least once.

Realized I could figure this out as I have a caliper and an example bushing. TO-200 tab is about 1.33mm Ceramic oxide insulator is 1.78mm 3.11mm to get you to the heat sink The ceramic insulator and TO-220 hole diameter is about 3.68mm, but the heatsink diameter is less than that, so the bushing will stop at the heatsink. So length of bushing should be about 3.11mm, that part is 3.18 which is about right, so yes that part should work. Oddly the bushing is not meant to go through the heatsink hole, so the screw (if metal) will be at heatsink potential.

Can Kevin or somebody verify this part number? Thanks

Yeah exactly, the bushing just makes everything line up nicely.

Hm, yeah searching isn't bringing up anything obvious. However the solution my buddy uses is to use PEEK screws. With that I assume any old plastic bushing would be fine.

I'd recommend CNC only if you are willing to take up learning CNC. It's a lot of setup, most all of them (except the million dollar professional machines) use a special linux distribution, and you need to deal with all the tweaky little issues to get it to work. Then you need to learn how to program it, and how to solve problems with making three dimensional surfaces. Gabe Newall (CEO of Steam) said he got one of those million dollar machines to relax from work, only to find he was solving the same problems as at work all over again (making 3D surfaces in metal versus pixels). So unless you want to make a hobby of it you should sub it out.

I'm not saying otherwise and am definitely not pointing fingers, also not doing an appeal to authority here. Just wanting to get clarity on some points, and highlight these safety points if they're important. Good information otherwise, thanks. Both points make perfect sense. Specs are one thing, practice is the final arbiter. However what is the "special feedthru that goes along with it"? In my lab stock at work we have plastic bushings for TO-220 screws, do you mean those or something different? Agree, good points. Along these lines, these are the safety points I've picked up so far, any others to put on the list?

I'd like to call into question some do's and don't I've heard about building Stax amps. I'm an engineer who works on scientific instrumentation and have heard a few things I don't understand based on my experience, so thought it's worth a discussion here. Please correct me if my thoughts on these issues is incorrect, from an engineering perspective. High Voltage resistors are necessary In general HV boutique resistors are really only needed when there’s a large differential voltage. Let's take a high quality but not HV resistor, the KOA Speer which range from 250V-1kV working voltage http://www.koaspeer.com/catimages/Products/MF-MFS-RK/MF-MFS-RK.pdf Take the average as 350V. Looking at the Megatron schematic I’d have to calculate the voltages involved based on the currents running through these resistors (good old Ohms law), but the ones to look out for are like R12/R24 which go to ground. Those are coming off the cathodes and are biased from that and the 1M coming off the grid. Oh R8 might see something too. The rest of them are only going to see whatever voltage they drop which shouldn't be much. But the whole amp is at best 450V (less in reality), so what resistors in the amp are going to have to bear more than 350V working voltage? I'm not seeing a lot jumping out at me, but I wouldn't mind running a simulation of this circuit to see. The other concern might be insulation breakdown to a nearby ground plane. They list the insulation resistance it as not less than 10G Ohm@100V, and the important parameter we are looking for is Dielectric Withstanding Voltage ("The rated voltage that can be applied to a designated point between the resistive element and the outer coating, or the resistive element and the mounting surface, without causing dielectric breakdown.”), which is listed as no breakdown. Thick traces We had this discussion on another thread but I thought to include it here. 1Oz traces is standard in the business. 1/2 Oz is used too in the high frequency stuff I do. Now 450V isn't really considered high voltage, that's more when you get above 1kV, say 5kV which isn't uncommon in the days of picture tubes. I've worked at national labs in High Energy physics where we used many tens of kV for Silly/Gelly detectors and such. At any rate for those you have to start worrying about corona, so putting more height on your trace is undesirable. The important consideration here is margins and trace width. Otherwise I've heard it's easier for DIY work, and I'll start by saying I haven't compared a 1Oz to a 2Oz board back to back on this. But again I don't see why more is better. It's mouse nuts compared to the solder our ham fists are going to flow into the joint. The only reason I know of for thick traces is if your running a lot of current which isn't the case here. Ceramic Insulators Let's look at the data sheet for a typical TO-200 thermal pad commonly available. The Bond-Ply® LMS-HD is listed with a breakdown voltage of 5k VAC. Now the liquid version right below that is much less, 250V/mil, and it depends on how it's applied. The same goes for the silicon pad too, over tightening will reduce these margins, but as far as I can tell they're within spec for the voltages we're working with. On the ceramic the specs aren't so clear to me, "21.7 x 103 volts/mm". Not sure how to interpret that, if anybody knows I'm interested. But surely the answer is a very high voltage, in the kV range I'd expect. Conclusion While I'm an experienced engineer I'm new to building Stax amps so am open to missing some points here. In general I'd say it's not true that over-engineering doesn't hurt, many times it actually is counter productive. But it is generally true that having a large margin/spec is good engineering. In this case I think that over speccing these components isn't a bad thing, though possibly it has marginal benefit.

I checked with the hardware engineers on my team (I'm in R&D and we do the highest performance measurement instruments on the planet). These guys have worked in HV too, like 5kV! Anyhow none of them could think of why thicker traces are important, 1oz is standard and what you want most of the time. The only difference is that for some high frequency circuits you want 1/2 oz, but that's up in the GHz range. 500VDC is bush league The only rational reason I've heard so far is that it's easier for DIY work.

I've been working with Bay Area Circuits, and am not getting much of a discount over their specials (e.g. the "Golden Gate" for 15 boards/$300), so it seems to be better to stack up a number of specials, but then why do the group buy at all in that case? Basically I'm finding it more expensive to get a custom quote rather than one of the specials, which are geared toward smaller quantities. So the group buy isn't working out as well as smaller individual buys, at least with the route I'm taking. Also as this is the first time I've done a board run I'm cautious as to how well it would work out. Rather than risk unhappy group members I did a special one run of five sets. I'll get the boards back and report back here to how well it went, then people can decide if they want to club together in smaller groups to do a buy, or try another vendor. If it works out I'll have a few extra sets to sell. I should get these back next week so will report then as to how well it went.

These are the ones I ordered http://www.ebay.com/itm/181469359862 They've been selling for years as I found a link to them from a Birgir post some time ago.

I ordered some of those teflon 9 pin sockets off the eBay user many here have used. They look fine but I've not used 9 pin teflon before, at any rate getting a tube in one is extremely difficult. I can remove an individual pin and put it on pretty well, but leveraging all nine into the socket is a nightmare, I can't imagine trying this with a soldered socket. At least when unsoldered the pins have a little play to align themselves. Other experience? Do I actually need to pull all the pins, try to use and pre stretch them before use? Ugh ...

Yeah setting up and programming CNC isn't easy. It's a skill that takes practice and time like everything else. I have access to a full manual machine shop at work (Bridgeport, etc) and I prefer outsourcing to a professional when possible.

Hmm, no, but I'll check. Is a greater thickness preferred? Why?

That's another option, feel free to go with them if you like. I tried their web interface and found glitches, not sure how comfortable I am doing a HV board set. It'd probably be fine though. I've been working with BayAreaCircuits which is a top drawer manufacturer, but costs a bit more too. Does it make any difference? Don't know. Anyhow Remold did you want to organize a run of cheaper boards through them?