Papa's article says to measure bias across R8 and R9, but the schematic shows R10 and R11. Is this a typo?
"P1 and P2 are adjusted so as to set the DC bias of Q3 and Q4. You will want to set them at zero when you first fire up the circuit, and increase their resistance to achieve the correct bias voltages across R8 and R9 (about 1 volt)"
"P1 and P2 are adjusted so as to set the DC bias of Q3 and Q4. You will want to set them at zero when you first fire up the circuit, and increase their resistance to achieve the correct bias voltages across R8 and R9 (about 1 volt)"
Everything is biased and offset, and the amp sounds really nice. The detail is breathtaking. The only downside is I can only run it for about 10 minutes before the sinks get to 60C, so I'm going to install the R219 resistors. Papa suggests starting at 47K and decreasing as needed. Does anyone have any experience with this? I'm going to order a few pairs, and I was wondering what constitutes a 10%, 20%, 30% decrease in output. Is it linear with resistance change? What is the percent wattage drop with the 47K? That's probably a dumb question, but I'm no EE. Heck, I'm barely an ME.
Although, do keep in mind what Dennis said earlier about power into 4 ohms. Without much bigger heat sinks, you’re going to have to limit current with 32V rails, and peak power is going to be limited by current. If you can, move to 24V rails so you don’t have to reduce current that much to get thermal under control.
Or maybe you don’t need much power…
Or maybe you don’t need much power…
Yeah, it sounds like it will be best to replace the 24V transformers with 18V. I have an extra FE and a set of BA-2 output boards. I suppose I could build a 32V complimentary output version with those, using the R219 to manage heat. Or I could just install them in the current build. Hmmm...
I've never tried it but maybe you could tie the primary taps in series in order to drop the secondary down to a more reasonable level. Then let the output stage current run at full throttle as intended without the limiting (no R219) since we're trying to run 4 ohm speakers in this case and you need small rails and big current, rather than the other way around. Just an idea. The single-ended OS is more true to the "Burning Amp" moniker than most, demanding some big hardware, maybe fan cooling, and so on. I'm not sure what the 4U can really handle dissipation wise, haven't looked into it..
I would try it and see what the temps are doing. The rails are obviously a little lower than we might like but you're limited by the hardware here. Set the bias at a level that runs the sinks at 50-55C, at equilibrium. Maybe the amp is acceptable that way, heat and sound. If not, well, it was a learning experience that didn't cost anything but a little time. 120/240 it's still a 300VA donut.
Ya I hate when you're right... 
We haven't magically made the wires bigger, so the secondary current rating does not magically increase. But with half the rail voltage we've halved the dissipation being asked of the PS assuming the Iq stays the same of course. I've seen Pa suggest this series primary thing before a few times to lower the secondary voltages.
We haven't magically made the wires bigger, so the secondary current rating does not magically increase. But with half the rail voltage we've halved the dissipation being asked of the PS assuming the Iq stays the same of course. I've seen Pa suggest this series primary thing before a few times to lower the secondary voltages.
I guess its a little late, I have similar BA3 with same rail voltage. The Deluxe 5U handles it no problem. Maybe easier to use one of Zenmods "babysitters" will also increase your heatsinks capabilities.
Russellc
Those chasiss’ were copies of Pa’s Ma’s old iron frying pan.
