Bob,
Thanks for this whole post. It was very helpful for a Class A newbie like me. It looks like I should able to reject between 25 and 30W, but I won't know for sure until I build and measure it.
You mentioned that I might have to change my "Source resistors" for the higher bias. Are you referring to R16 - R19? Please help me understand the tweak you referred to.
Thanks in advance.
Jac
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Good point. I am already planning a CLC power supply and I had forgotten to take into account the voltage drop from the inductor series resistance. Looks like everything will work out in the end. Thanks.
Jac
The middle pin (drain) of the output mosfets is actually the metal face of the transistor. Measuring the temp of that pin (little IR thermometers are very inexpensive, and DMMs with thermocouples are also quite cheap...) will give you the temperature of the transistor.
The temp of the heatsink is, as you say, easy to measure. The trick is to get the thermal coupling of the transistor to heatsink as good as possible. Bergquist pink or Keratherm red sil-pads are great, as is mica and grease.
Yes, you could do that, or better measure the voltage drop across the source resistors and determine the bias via measured current. The Aleph J PCB from the store has the ability to place a pot on R27, which controls the gain of the Aleph CCS and thereby determines the output stage's bias.
So twiddle R27 to get the voltage measured across output fet source resistor, that's how you set bias. Then adjust R7 to zero your output DC offset.
You're welcome of course. I am just paraphrasing things that Papa taught in his many articles. 
Yes, R16-19 are the Source resistors. Higher values mean lower current, and vice versa. R27 fine tunes bias, the Source resistors can be used to set it in larger steps. Note that in the ACS (all the stuff around Q4-6) with no signal the Source resistors are effectively across the emitter-base junction of Q4. That means (ignoring the effects of R27 for the moment) you'll see ~0.6V across the Source resistors. At 0R47 that equates to ~1.2A per device. At 0R68 you get ~0.9A.
What about that pesky R27? Note that R27 and R15 form a voltage divider at the Base of Q4 between the top of the Source resistors and the Collector of Q4. The lower the value of R27 the more current flows through R15. If you set R27 to get 0.1V across R15 what happens? The emitter to base junction of Q4 is still going to be ~.6V. Therefore the Source resistors will have 0.5V across them, giving you ~ 1.06A with 0R47 or ~0.74A with 0R68.
Your 25-30W per device range is generally safe with good thermal connections to the heat sinks, whether liquid or air cooled.
Yes, R16-19 are the Source resistors. Higher values mean lower current, and vice versa. R27 fine tunes bias, the Source resistors can be used to set it in larger steps. Note that in the ACS (all the stuff around Q4-6) with no signal the Source resistors are effectively across the emitter-base junction of Q4. That means (ignoring the effects of R27 for the moment) you'll see ~0.6V across the Source resistors. At 0R47 that equates to ~1.2A per device. At 0R68 you get ~0.9A.
What about that pesky R27? Note that R27 and R15 form a voltage divider at the Base of Q4 between the top of the Source resistors and the Collector of Q4. The lower the value of R27 the more current flows through R15. If you set R27 to get 0.1V across R15 what happens? The emitter to base junction of Q4 is still going to be ~.6V. Therefore the Source resistors will have 0.5V across them, giving you ~ 1.06A with 0R47 or ~0.74A with 0R68.
Your 25-30W per device range is generally safe with good thermal connections to the heat sinks, whether liquid or air cooled.
Read https://www.passdiy.com/project/amplifiers/zen-variations-2 to help understand the active part - the Aleph current source.
You guys know how it goes. You put in your order to Mouser/Newark/Farnell/Digikey, and after a few days, you remember something else you need to order. Since you don't want to waste the shipping fee on the one item you forgot, you start shopping. That is where I am with output transistor insulator pads.
The Keratherm pads from the store are great, but if I can find something from the big mail order houses, I can save the shipping.
What is the word on these?
Bergquist pink. Are they the 900S series? They appear to have thermal conductivity of 1.6 W/m-K or about 1/4 of the Keratherm.
Bergquist Sil-Pad A2000 are at 3 W/m-K or about half the Keratherm. Not bad.
SPA2000-0.015-00-104 Bergquist Company | Mouser
Or how about Aluminum Oxide Ceramic? They have 15 W/m-K or about 3 times as good as Keratherm, but I'm guessing they would take thermal paste.
4180G - AAVID THERMALLOY - THERMAL INSULATOR, TO-218, TO | Newark
Any thoughts or experience with these? Thanks in advance.
