luvdunhill

See Kevin's post above. Nothing has changed. Four FETs @ 150mA or 8 FETs @ 75mA. Same heat. I ran them harder just as a proof of concept, but this has always been the recommended current.

Would it help if I sent you a 160VA 2x30 toroid? Probably could drop it in the cheapest flat rate USPS box.

Ok, I see. I thought you recommending 75mA for the 4 FET version. In which case, nothing changes it's just as hot as we thought. So balanced would be 72W. My recommendation of heat sink still stands.

COBOL absolutely should fulfill a foreign language requirement

well, okay, but I for one interpreted your comment in post #19 as designing towards 150mA: HeadWize: DIY Workshop > My newest in the dyna series "If you run the default of 150 ma per fet, the 2 ohm resistors are fine." In hindsight, I see how that comment could have been misconstrued in a number of different ways. All that said, I think more than 75mA is better. I know there are quotes from Borbely who uses these FETs in one of his design that many of his customers preferred them at > 100mA, and I tend to agree. Here's a quote from his old website: "More is Better (sometimes)! Although the fact is well known that MOSFETS require a lot of current to sound super, I believed that the suggested 40mA or so in the output stage was optimal. Until one of our customers suggested to try more. Mind you the tiny heatsink we are supplying with the Kit can

Well, 2.5dB +/- 2dB

3 minutes? heh. Try the other feedback setting as well, once you get accustomed to this version. Need to recruit more advocates for adding a switch here

She'd probably just take it to Ray to fix anyways Oh, pabbi1, so far things have been pretty boring, except for this little "incident":

Has anyone ever looked at, or used these On Semi parts? Look interesting to me. (Mouser part numbers below): 863-MPSW56RLRAG 863-MPSA56RLRAG 863-MPSW06RLRAG 863-MPSA06RLRAG The "W" devices are rated to 1W @ 25C in free air.

I think that the MF30-100 is a solid recommendation for the DynaFET at this point. This is based on the results of my heat sink and some crude math. This heat sink is 300mm wide, 100mm tall with 30 48mm fins. The base is 8mm thick. It is rated as 0.32 C/W @ 80C, or around 0.35 @ 60C. That results in 171W at @60C. Given two boards of 4 devices and 30V rails, that right at Kevin's recommendation of 150mA, at a conservative die temperature. I wouldn't even recommend using 8 devices with these. This is my standard recommendation, if anyone is looking for one. If you're using another heat sink that you don't trust the C/W rating for one reason or another, you can use a crude method of comparing the two based on dimensions alone.

This is the prefect chance to do a little perf board project.

They are the .1%, 25 PPM/C versions: http://www.prpinc.com/pdf/Audio_PR9372_Series.pdf

I wish I could buy those PRP resistors

By "unmodded" do you mean pre-production?

I did testing specifically for your case. Just crank the pot all the way to it's maximum resistance (assuming that the series resistor I'm suggesting is added to the PCB) and pray that things don't get too hot From my testing, this will put you at nearly exactly the same current as the Dynahi output stage uses, and you should be fine. You'll have a bit more current in the PSU (you said you could dissipate a bit more heat here, so again, you should be fine), but the output stage should flow roughly the same. We can go from there.

Well, I would recommend helping out the servo somewhat, otherwise it's a bit slow as it has to overcome the fact that the MOSFETs aren't so complimentary, so it would depend on your definition of warm up. From what I've seen, it looks like 5 minutes is about the minimum for things to really settle down and the bias to drop down from the turn-on peak to steady state. During this first 5 minutes, 200mA extra current isn't all that uncommon (worse case), before the negative tempco of the MOSFETs starts to work its magic. This will also depend on how large your heat sinks are, I'd presume. During the next 5 minutes after that, it will hit the point where it becomes rock solid, only varying around 10mA during this period. Anyways, I have written down 478R / 518R for one test and 478R / 523R for another. Worse case, just install one pot... I think with 470 / 510 it will work just fine.

I dunno, it's hard to tell if there is a solder mask when FR4 isn't used, which seems likely here. I'm assuming stencils were used for the SMT stuff, so I'm betting there is some sort of solder resist layer.

Well, I finished up testing the 8 FET version and mailed off the board to digger945. Once you get it Scott, can you do me one quick favor? Measure the values of the two "500 ohm" pots.. I think I'm going to try on my final build using a 470R resistor for one and a 510R resistor for the other, but want to verify which is higher value, as it's not clear from my notes. I think I'd like to remove the trim pots, and let the servo can keep things in check, per the original schematic. The final recommendation for the bias pot, is a 20K pot with 3K3 in series, as well as halving the two 20K resistors. In the final version, I used 1 ohm ballast resistors and they worked great. Truth be told, I would have liked to try 0.47R, but I think 1 ohm is good. I know the original target was 125mA per device. I think this is a good number, but felt like I preferred even more current, as the old addage that MOSFETs love current seems to be true. With the 8 FET configuration, the lowest bias you can really get, without further modifications (i.e. trial and error) to the circuit was 165mA per FET, given the values I used. If you don't have the heat sink to handle this, I'd recommend sticking with four FETs. Most people should plan on the four FET version, in my opinion. I maximum bias tried in the 8 FET configuration was 315mA per FET, for a total draw on the negative rail (where my ammeter was) of 1.3A. Things were rock stable, and in fact I didn't have the servo opamp installed. This thing does indeed scale quite nicely. I cannot wait to try it with speakers Anyways, good stuff. Let's start working on the modifications to the boards and get these off to production as soon as possible. I'll work on drawing up a final BOM by Monday and we can start firming up those numbers and working the logistics. Sooner we get cracking on this stuff, the sooner everyone can start building

eh, try using one of these: OTTO 2:1 Switching Module rated only to 3A though, but might do the trick.

Is it possible that the free air impedance could be different from the impedance when loaded with the dummy head, or whatever Headroom uses? Did he measure them while they were on his head?

ask "opc" at DiyA, he has files that he will (usually) send out with decent documentation, albeit not fully sanctioned, IIRC.

I thought I saw a PPTC somewhere in there?

more likely the temperatures weren't calculated properly. Proper design should not assume any air flow (unless a fan is included, of course).

Dusty, can you post the two graphs in question, preferably including an impedance plot?

It probably runs more stable warm. There seems to be a fine line between too much ventilation, which can cause temperature gradients in the case, and not enough ventilation which can cause stability and longevity issues.