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F4 power amplifier
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Old 15th January 2020, 12:51 AM   #4491
rgregg3 is offline rgregg3
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Default Adding Switchable Bias

Wanted to see if I had a reasonable idea to allow for variable bias settings on my F4 build Iím planning. The basic idea is to have a low bias setting for a low power setting to keep circuits warm, a mid bias for normal listening, and a high bias room heater mode for getting the most outta the amp.

I didnít want to add a mechanical switch to the basic F4 circuit, so I used LDR optocouplers as the means to change resistance of R9. I also wanted to make sure DC offset stayed at 0 for each bias level so I used another LDR to change the R22 resistance. The specific LDR is the NSL-32SR2.

I will need to add a regulated 5V power source for this circuit and need a shorting 4-pole 3 position selector switch (based on a comment from Nelson to an earlier post trying to do variable bias as well). Sorry for the handwritten description... I drew this at breakfast while on a trip. What an opening for comments about a different type of trip... :cool

The target bias settings on the note attached are not finalized until I go thru the biasing of my build, but gives the basic idea of where Iím headed. Iíve borrowed ideas on the LDRs from posts on the Lightspeed volume control and the Zen Mod approach to use LDRs. The F4 circuit is not included and is only indicated by a hard to read F4* indication. And finally, the ideal resistance range for R9 and R22 has not optimized until Iíve been through biasing the basic circuit. For the R9 position, the LDR and a companion resistor is in series to get the approximate resistance range while for R22, the LDR has a companion resistor in parallel.

Let me know if this will work and any suggestions for improvements...

Cheers
Rob
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Old 15th January 2020, 12:58 AM   #4492
Zen Mod is offline Zen Mod  Serbia
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it's trivial thing making selection of appropriate resistor around TL431 , once you establish appropriate values ; I'll do that with mini relays (whatever I can find being smallest and cheapest) in a way that one resistor is fixed and 2 additional ones for additional Iq selection are switched in parallel

using LDRs is simply overkill and I don't trust them enough for that position , even in case that I organize it that thingie is going in underbias if something goes south
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Old 15th January 2020, 01:09 AM   #4493
rgregg3 is offline rgregg3
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Thanks Zen Mod. I suspected you moved away from using LDRs because off your experience using them. If I understand, you would have a R9 satisfying the lowest bias setting and add the appropriate resistor in parallel with relays to achieve the mid and high bias settings. Do I have that correct?

Would you do something similar for the DC offset control R22?
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Old 15th January 2020, 01:23 AM   #4494
Zen Mod is offline Zen Mod  Serbia
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sleepy now , so no sketch this time

if you're not in a hurry , I will post a sketch, but not before tomorrow evening

LDR attenuator - it's still nice thing, no disappointment at all! , but I moved personally to solution which interest me more and was much bigger thing to accomplish ....... while with LDRs sole thing I could improve is uproc environment for LDRs , enabling total calibration etc.

not so puzzled with digital side , so didn't pursue it

though , there is a need for some Arduino work , for next iteration of Iron Pumpkins

edit : in a meantime , just point me to your ref. schm , so I can use it
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Old 15th January 2020, 01:28 AM   #4495
rgregg3 is offline rgregg3
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No hurry Zen Mod. I just wanted to know variable resistor idea could work so I could settle on the holes needed in front panel.

I’d appreciate any knowledge on what a typical resistance range is needed to change bias by 100 to 150 mv...

Rob
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Old 15th January 2020, 06:20 AM   #4496
valvesound is offline valvesound  South Africa
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I also itched to play around with the PS on my F4 over the holiday period. I removed the usual CCRCC and replaced it with two 100,000 uF big cans. At first I thought the mid-range was lacking but after some 20 hours burn-in it all came to life and the overall sound is more lively and dynamic sounding than before. I am very happy with the mod.
Click the image to open in full size.
The two 10uF Solens was a test run and I subsequently removed them as the change was very subtle on the high frequencies and I preferred it without them.

Last edited by valvesound; 15th January 2020 at 06:22 AM.
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Old 15th January 2020, 07:44 AM   #4497
Andrzej Sochon is offline Andrzej Sochon  Poland
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Quote:
Originally Posted by Norgaard View Post
- recommendations and experiences would be most welcome
Pumpkin, Pumpkin, Pumpkin...
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Old 15th January 2020, 05:02 PM   #4498
cal3713 is offline cal3713
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Quote:
Originally Posted by valvesound View Post
I also itched to play around with the PS on my F4 over the holiday period. I removed the usual CCRCC and replaced it with two 100,000 uF big cans. At first I thought the mid-range was lacking but after some 20 hours burn-in it all came to life and the overall sound is more lively and dynamic sounding than before. I am very happy with the mod.
Click the image to open in full size.
The two 10uF Solens was a test run and I subsequently removed them as the change was very subtle on the high frequencies and I preferred it without them.
What brand/type of capacitors are those big cans? Thanks...
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Old 15th January 2020, 08:41 PM   #4499
Zen Mod is offline Zen Mod  Serbia
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Quote:
Originally Posted by rgregg3 View Post
No hurry Zen Mod. I just wanted to know variable resistor idea could work so I could settle on the holes needed in front panel.

Iíd appreciate any knowledge on what a typical resistance range is needed to change bias by 100 to 150 mv...

Rob
S* and S** - whatever you fancy - nice switch(es) or relays controlled with switch(es)

include R* - first lower Iq step

add R** - second lower Iq step

you must get values experimentally - per your wish

with relays - you can wire them Normally On , so lowest Iq if relays aren't energized ........ or the other way

I'm not commenting do you need entire thingie or not , even if I'm using my Papa's Koan almost every day in my workshop , rarely going above 500mW

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Old 15th January 2020, 08:57 PM   #4500
rgregg3 is offline rgregg3
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That seems easy. Now to find the correct relays.

Put a little iron in your F4!

Cheers
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