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BF862 based SE Class A Headamp without the HEAT
BF862 based SE Class A Headamp without the HEAT
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Old 10th April 2018, 01:05 AM   #511
pcgab is offline pcgab  United States
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BF862 based SE Class A Headamp without the HEAT
that is a VERY interesting read, thank you for posting that mlackey!
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Old 10th April 2018, 07:20 PM   #512
xrk971 is online now xrk971  United States
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BF862 based SE Class A Headamp without the HEAT
Quote:
Originally Posted by mlackey View Post
Mayday, XRK,

Be careful about adding a small resistor to the output of the cap. The output feeds back into a resistor divider on the input, which sets up the gain of the amp. Adding a small value resistor to the output will change the divider, and effectively reduce the gain to near zero. As designed, the 100K in the divider also functions as a pull-down, bleeding off the output cap's DC leakage. If you need a larger pull-down, scale the divider appropriately, but also be aware that doing so will decrease the input impedance.

On another note, I was intrigued that Mayday was actually building the year-old board, so I dug up my old boards out of the box. But after listening I made a change. I replaced the single 470uF blocking cap with 2 ea 1000uF bipolars (Nichicon UES 1000uF 25V). Its my opinion that the sound is much improved, and actually very, very good. It is generally accepted that when it comes to output caps, bigger is better. Two in parallel doubles the capacitance and halves the ESR. Also, here's a link to an interesting read:

Nichicon Muse ES bipolar caps measured: <-120dB THD, <-140dB IMD
I typically have 270R load resistor at the output cap to (1) serve as a dummy load when I take measurements, (2) bleeds off the cap. How is single high impedance resistor to ground changing the amp's gain?

So for my pocket amp (now I typically use 2 x1000uF OSCONs for the output coupler with 1uF MKS film bypass). Having RL of 270R doesn't change the gain as verified by LTSpice and by measurment.

Click the image to open in full size.

or are you talking about this circuit - which has feedback connected to the output load resistor:
Click the image to open in full size.

I don't use this amp anymore - much easier to just have a single N-ch MOSFET like ZVN4306GTA than match 8 or 10 BF862's.
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Last edited by xrk971; 10th April 2018 at 07:22 PM.
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Old 12th April 2018, 03:48 AM   #513
mlackey is offline mlackey  United States
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Quote:
Originally Posted by xrk971 View Post
...or are you talking about this circuit - which has feedback connected to the output load resistor...
I was thinking of the latter, the one with 10 ea. BF862s, as I think that is what mayday is/was building, and that is what piqued my interest.

But you are correct, adding a resistor to the output does not change the gain. I was thinking that R2-R3 formed a divider which set the gain, when in reality it is R1-R2 which set the gain. Connecting a load resistor, RL, effectively replaces R3 with (R3||RL), which changes only the input impedance. Adding headphones to the mix, RH, again changes the input impedance to (R3||RL||RH). Maybe that's where I got off track? To be honest, I'm not sure what I was thinking at the time.

As Gilda used to say... never mind.
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Old 12th April 2018, 08:44 AM   #514
Mayday is offline Mayday  Sweden
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BF862 based SE Class A Headamp without the HEAT
That project, along with the others, are moving at a snail's pace. Not only due to pain, we've gotten a puppy. So, when my fiance is taking care of our daughter, I make sure that the puppy isn't chewing up the place.
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