Pars

Here I go quoting myself again From Kevin's post: if you go with fets you can multiply both Rf and Ri by 10 to increase the input impedance. 1) Is there a reason I would want to increase the input impedance even more by going 50K for Ri and 250K for Rf? I wasn't sure whether your statement really does not matter for this applied to the ubal to bal, or to the multi amp.

Thanks. I just looked at the gerbers and the 4.7uf are indeed bypass caps. With 100uf electros on the board, any reason to choose 4.7uf as a bypass? I'm not really up on the latest thoughts regarding bypassing and whether it is well thought of or not. I have in the past seen some pretty good reasons not to do it.

Good point Marc. I hadn't thought of that, though I am not sure it is an issue. From what I gather, using the FET input instead of bipolar already increases the input impedance considerably just by itself? Looking at the original Dynalo schematic, with the SJ109/SK389 input, there is no series resistance; my stuff has no issue with driving that.

If I am using FET inputs instead of the THAT340, is there a reason I would want to increase the input impedance even more by going 50K for Ri and 250K for Rf? I wasn't sure whether your statement really does not matter for this applied to the ubal to bal, or to the multi amp. Also, a couple of part questions (trying to do this with what I have around): 1) The 1uf integrator caps... any reason to get ceramic X7Rs if I have some polyester film laying around? 2) The 4.7uf film caps... I presume these are rail bypass caps? So value isn't particularly important? 3) If I do raise the impedance, does the compensation 5pf cap need to be changed? Thanks!

You should see if he can use Royal Mail or something that would hook up with USPS. Not likely, but with UPS you could get hit with customs. Of course, I could be wrong

Finally got around to measuring 200 transistors: MPSW-06 NPNs from Mouser and MPSW-56 PNPs from Avnet. FIrst off, these seem to be tighter in spec than the Toshibas from the original Dynalo. The NPNs overall were a bit higher than PNPs. I used a base resistance of 221K ohms with power at 16V for these. I had a DMM in the base and collector legs set for uA and mA, respectively. Ic ranged from 15mA to 22mA or so; most fell within 19-21mA. I waited until the DMM measuring Ib hit a particular value and then recorded the Ic. Subjective, but it seemed like a good way to do it. I already had a home etched board with SIP sockets and posts on it for test leads, resistor, etc. I put resistor leads in the DUT SIP socket and tested the devices while still on the tape; didn't seem to affect the result. Should be enough close matches for a multiamp I think.

campagnolo... campagnolo... you know you want it Sorry, I've been a Campy-phile since the '70s. It hasn't always been the best choice over the years (I feel like the fonze trying to say sorry), but it always has worked for me. Of course I am stuck back in friction mode, but I did like the Ergos on my brother's Bianchi. The shit just doesn't break. Decades.

Looks very nice Jose! What is the knob to the left of the volume knob for? And, how does it sound?

Happy Birthday! Have a great one Nate!

^ Yes, as far as joining two wires from a dual secondary transformer to form a center tap. Pay attention to the phase markings. For example, look at amb's instructions for the sigma22. The schematic shows two of the secondaries connected together. http://www.amb.org/audio/sigma22/

Happy Birthday Craig!

I usually crimp with something similar to what UFN posted, shape with needle nose pliers, check the fit, then solder them. Once done, with the keyed housings, it is impossible to fuck up when plugging in. Of course if you use 3 pin for both power and signal or something else, you need to watch that or mark them somehow.

Not to mention that the sand in a lot of these amps is getting very difficult and expensive to find. No sense in saving a couple of $ in hardware when it could cost you big $$$ if something catastrophic occurs, not to mention the probably huge amount of time finding the replacements and verifying that they are genuine.

I've never built a Stax or electrostat amp, but I'll take a swing at a couple of things you bring up: HV resistors: I think some people had problems with leakage in particularly the battery section of the T2. They found that the xicon 273 series, raised off the board alleviated that. From what I recall, PRPs exhibited this problem as well as the seemingly beloved Dales. Raising them above the board would seem to indicate symptoms of breakdown to the ever present ground plane below them. They are cheap (the xicons), so if that is what works, it's not like a Black Gate mantra... Thick traces: not sure where the now std. 2 oz copper came from. Back when I was more involved in board group buys, usually from Imagineering, 1 oz. was standard. Insulating pads and fasteners (PEEK screws) seems to have come about from a what works perspective. Realize that people like Kevin Gilmore, Craig Sawyers, etc. that are instrumental in these threads are also engineers.

One reason I never use those terminal blocks, much preferring these

Good 1st post diyampli

Yeah, you might get a couple of good pairs outta that

You make a good point as always I was planning on running @ 16V or so. I'll see what happens, but will probably stay around 16-18mA. I don't really want to use OPA445s for the servos, so need to stay below 18V.

Anyone have the latest DynaFET schematic, preferably showing the feedback options? The only one I am finding is the .png one that j4cbo had... Nevermind... I found the balanced DynaFet schematic so can discern what I might be looking for from that.

Kevin, After looking at the datasheets for the BJTs, 16mA seems rather low for these devices. Even the 18mA you noted on the previous page seems low. These seem pretty linear all the way out to 70-100mA. Any reason not to go higher than 18mA on these? I am currently measuring hFE on the PNPs (from Avnet). Still waiting for the NPNs from Mouser. At ~16mA, using this circuit, with 100K Rb and my Fluke in series on the base in uA mode, I am not experiencing the transistors even getting warm after 2-3 minutes. Current parameters I am planning on using for matching: V = 7.5Vdc Rb = 100K Ib = ~70uA Ic = ~16-17mA The original 2SA1015 / 2SC1815 in the Dynalo are considerably lower power handling. Looking at the datasheet, I can see where 16mA was a good operating point for these. Thanks. BTW, anyone still looking for thru hole devices: Avnet has the MPSW56s (around 1500 still left). Mouser still has the MPSW06.

Happy Birthday Naaman!

Don't you need to ground the - inputs to the board when using the RCAs?

Chinese trimpots? Has anyone tried these Bourns "clones" on ebay, such as this one? Funny, they use "Burans" but same font more or less as Bourns uses. I have a few of these and haven't noticed anything peculiar about them as compared to an actual Bourns, but wondered if there are any hidden gotchas, such as abnormal drift, etc.? EDIT: I see some posts on diyaudio about these, one in particular stating that the fakes are carbon vs. the cermet of the actual bourns. I'll buy the bourns...

How is the toggle switch on the back wired? It only affects input selection?

Too small is definitely worse. I normally ride the large end of the scale according to the numbers I got from Competitive Cyclists fitting tool.