An illustrated guide to building an F5

Finished mine yesterday. Now playing in my main stereo. Used the same chassis and power supply as my Aleph J (from a couple of weeks ago). Just ordered an extra pair of the 4U UMS heat sinks and swapped out the heat-sink/amplifier-board assembly. Semi easy-peasy. I have noticed a very faint mechanical hum from the chassis (transformer) and a very faint hum from the speakers, but they're imperceptible when playing music.
Thank you to all the contributors here for making this so educational and fun.
 
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Warning - Over sharing Alert

This particular thread is very, very long with a large trove of information to sieve through. I am almost done with my F5 that I started two years ago (don't ask) and was quite lost going through the postings related to BOMs, changes in board versions, resistor changes and the occasional upgrade ideas. Whew.

My challenge now is addressing the biasing of the F5 V3.0 board. I had a heck of a time understanding the biasing procedures and using the trimpots.
I am using the V3.0 board with the BOM posted in #2339. I do not recall if I purchase a kit from the diyAudio store or not.
*Perhaps someone can add the LED and its resistor to the original circuit diagram?
The Bourns trimpot seems to create quite a bit of confusion. The BOM mentioned uses the model number 3296Y-1-502 product

I have attached a technical document for the trimpot.

https://www.bourns.com/docs/technic...brary/trimmers/technical-articles/ap_proc.pdf

From what I have learned recently:
Clockwise and Counterclockwise - If one is looking down on the F5 board (see attached photo), the #1 pin (input) is on the right side under the adjustment screw, #2 is the wiper, and #3 is the terminal end (output). You will notice the small extensions on the corners of the trimpot and the inscription with "Bourns" is on the vertical face, which would be directed downward towards the bottom of the photograph. According to the technical document, rotating the adjustment screw set the direction of travel "Clockwise (CW) or counterclockwise (CCW) rotation when viewing the adjustment end of the potentiometer". According to the diagram, clockwise rotation will move the wiper toward the #3 pin and away from the #1 pin. In reference to the attached photo, rotating the adjustment screw clockwise until one hears a "click" will produce a 0 ohm resistance across pins #1 and #3.
This is the proscribed starting point for the bias. Rotating CCW then increases the measured VOLTAGE value when power is applied to the board.
1710877368284.png
??? I want to make sure I am setting up the DMMs appropriately for the bias adjustment. I have leads attached across the two 0.45 ohm 3 Watt resistors, R7 and R8. I also have leads attached to the output wire for the speaker terminals. I have biased one board with ~0.600mV at the resistors and ~0.0mV dc offset at the speaker wires. I have let the amp warm up for a few hours and the heat sinks are about 45 degrees C, according to the IR thermometer I have. The rectifiers and MOSFETs are in the 20-30s. This is in a disassembled chassis. I'll get the other board biases and the unit put together and recheck with the lid on the system. Does this sound about right???

Please let me know if I am off track anywhere in this posting. I often fear that I am the lead lemming.

At some point I want to better understand what is happening along the circuit as the trimpots are adjusted.

JCB
 
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^ Best I can tell, you've done it perfectly. Nicely done.

An easy way to verify that the pots are set correctly after installation is by checking resistance across R5 and R6. If they're close to 0, you're good.

Now... I am a complete amateur, and this may be wrong, but this is how I have understood what's happening.

As the pots are being adjusted, you are increasing the voltage "seen at the gates" of the corresponding MOSFET's relative to the source. Thus when you reach a "threshold" voltage or Vgs(th), the MOSFETs begin to conduct. You play some Barry White and gradually turn them on, get them warmed up. If you had started with the pots turned 'the other way', then both MOSFETs would be 'wide open', and thus... poof.

As you tweak one pot vs. the other, you notice that when you have one MOSFET allowing more current through from Drain to Source (seen as evidence by measuring the voltage across the source resistors), the offset increases. We don't like a lot of DC going to our speakers. We like it to be nice and even. So, we adjust the N-channel and the P-channel MOSFETs (which inherently tend to have different behavior at the same Vgs when it comes to the amount of current D to S) to be evenly matched and null the offset.

tl;dr - you're making the N-side and the P-side behave as similarly as possible and controlling the overall current, Iq / bias.

I hope I got that even close... and that it makes some sense.

Edited to (hopefully) have it make a bit more sense.
 
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