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Old 14th October 2021, 12:25 AM   #6731
TungstenAudio is online now TungstenAudio  United States
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I like my F6 as it is now, but it needed some modifications so the source resistance in the output stage to get there. There have been a number of proposed changes, including some by Papa, making the F6 a fun amp to tweak for the contentment of one’s ears.
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Old Today, 06:33 PM   #6732
strongbow60 is offline strongbow60  Sweden
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Tungsten, where did you land regarding the values for source resistors?

I'm asking because I have a perpetual test build of an F6. Very much how I did with my ACA but without the chassi. Easy enough to tweak.
Listens to it just now, when no one else is around.
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Old Today, 07:52 PM   #6733
strongbow60 is offline strongbow60  Sweden
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One tweak I did tonight was inspired by the Youtube video of BAF Online Nelson Pass and the outside the box presentation.
I adjusted the offset of both channels to be about +500 mV. A direct result of that, the 2nd harmonic distortion went up sightly to 0.25% and 3rd down to less that 0.1 (measured in REW). Previously, with zero offset (less that 10mV) was very even at approx 0.1%. This was stable between channels.
More offset did not do anything positive, less might work. There seems to be a sweet spot where the distortion figures is optimal and that is not with zero offset.

I think I can hear a positive difference, before it had a little bit of harshness, especially female voices and modern pop music.
I course, this might be me expecting this. On the other hand, this can be the thing.


Can anyone else try out this mod and see if this is for real or if it is just me imagining things?

Last edited by strongbow60; Today at 07:55 PM. Reason: clarification
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Old Today, 07:53 PM   #6734
TungstenAudio is online now TungstenAudio  United States
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This seems to be a popular question
The F6 is certainly good for plenty of DIY entertainment. Here is where I left off with my F6: F6 Illustrated Build Guide
Details are somewhat unfortunately scattered across a couple threads. Basically, I changed the way the C1 capacitor is connected to the upper Mosfet. So the upper source resistance is implemented in two parts, with the C1 connection at the node between those parts.
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Old Today, 07:58 PM   #6735
TungstenAudio is online now TungstenAudio  United States
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Bad Papa!
Yes, I watched the video on PassDIY channel.. Plenty of ideas to play with. The offset voltage seems almost too easy. I imagine that its perceived effect and value would depend on one's speakers. Given this tweak and the source resistane mod that we were just discussing, I might be tempted to connect a current source to the C1 junction of the source resistance of the upper Mosfet.
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Old Today, 08:21 PM   #6736
strongbow60 is offline strongbow60  Sweden
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Is there really a difference between using a current source and setting the offset on a class A amp?
For an AB amplifier this is the only way, but class A has already the current running through it.
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Old Today, 08:29 PM   #6737
strongbow60 is offline strongbow60  Sweden
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Quote:
Originally Posted by TungstenAudio View Post
...
Basically, I changed the way the C1 capacitor is connected to the upper Mosfet. So the upper source resistance is implemented in two parts, with the C1 connection at the node between those parts.
Yes, I read about that. But i'm lazy so just wondering if you came to a conclusion about a suitable value for a 1% distortion figure
I have a 20 mOhm resistor somewhere but that's it for low values.
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Old Today, 08:34 PM   #6738
TungstenAudio is online now TungstenAudio  United States
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I’m thinking about using the current source with little DC offset at the output. The idea would be to change the operating point of the upper Mosfets relative to the low ones. Another way of adding H2.
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Old Today, 09:09 PM   #6739
Zen Mod is online now Zen Mod  Serbia
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Pa clearly sez that for A Class amps trick with additional CS is not effective as with Sissy amps (which is only logical)

you have these 2 source resistors in F6, to play with........ next 2 years or so

speaker trick with CS, that's another trick - related with speaker, not amp
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