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F5 class AB mode?
F5 class AB mode?
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Old 20th February 2019, 08:54 AM   #1
FYC is offline FYC  Hong Kong
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Default F5 class AB mode?

Recently I start to think about building a F5. I built a couple of NP designs already like the the PLH, Zen with Aleph CCS etc. I do think I need more wattage in my new setup but I do not want to run into huge Class A projects. I believe F5 as having a push-pull output stage should work pretty OK into Class AB mode. My current thought is to bias F5 to 0.25A but having a supply voltage of about 40V. So it should work under Class A till 1W and then enters into Class AB and go upto 60-70W for 8 Ohm. Anyone already try something similar?
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Old 20th February 2019, 09:39 AM   #2
Zen Mod is offline Zen Mod  Serbia
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F5 class AB mode?
I believe people tried

I tried it years ago , but difference (to full A class ) was too much for my liking

of course - if you're satisfied , who can argue ?
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Old 20th February 2019, 10:16 AM   #3
FYC is offline FYC  Hong Kong
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Quote:
Originally Posted by Zen Mod View Post
I believe people tried

I tried it years ago , but difference (to full A class ) was too much for my liking

of course - if you're satisfied , who can argue ?
Thanks, do you recall what is your bias setting at that time?
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Old 20th February 2019, 10:20 AM   #4
Zen Mod is offline Zen Mod  Serbia
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F5 class AB mode?
I believe something around 350-400mA

try it , if you can without too much expenses

if you like it , then good
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Old 20th February 2019, 11:03 AM   #5
Dennis Hui is online now Dennis Hui  Canada
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F5 class AB mode?
At those rail voltages you really should cascode the jfets so I suggest
looking at the F5 turbo document and see about scaling the bias down.

http://www.firstwatt.com/pdf/art_f5_turbo.pdf

Dennis
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Old 20th February 2019, 11:26 AM   #6
FYC is offline FYC  Hong Kong
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Quote:
Originally Posted by Dennis Hui View Post
At those rail voltages you really should cascode the jfets so I suggest
looking at the F5 turbo document and see about scaling the bias down.

http://www.firstwatt.com/pdf/art_f5_turbo.pdf

Dennis
Yes, I am prepared to use cascode for the front end FETs. Thanks
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Old 20th February 2019, 11:28 AM   #7
Dennis Hui is online now Dennis Hui  Canada
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F5 class AB mode?
Cool. Please keep us post on how it turns out.

Cheers,
Dennis
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Old 20th February 2019, 11:50 AM   #8
Bigun is offline Bigun  Canada
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F5 class AB mode?
Quote:
Originally Posted by FYC View Post
Recently I start to think about building a F5. I built a couple of NP designs already like the the PLH, Zen with Aleph CCS etc. I do think I need more wattage in my new setup but I do not want to run into huge Class A projects. I believe F5 as having a push-pull output stage should work pretty OK into Class AB mode. My current thought is to bias F5 to 0.25A but having a supply voltage of about 40V. So it should work under Class A till 1W and then enters into Class AB and go upto 60-70W for 8 Ohm. Anyone already try something similar?
F5 is not optimal for an AB amp. There are many many AB amp designs you can choose from, better to start with optimal AB amp design if that is your end goal. For a simple but good sounding design with an output that is a relative of the F5 go look up the "P3a" amplifier.
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Old 20th February 2019, 12:59 PM   #9
FYC is offline FYC  Hong Kong
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Quote:
Originally Posted by Bigun View Post
F5 is not optimal for an AB amp. There are many many AB amp designs you can choose from, better to start with optimal AB amp design if that is your end goal. For a simple but good sounding design with an output that is a relative of the F5 go look up the "P3a" amplifier.
I certainly agree with you, if I start from blank. I do like Class A but I am however bounded by the components on hand. My transformer give 40V instead of 24V, the heatsinks can handle upto about 80W dissipation (20W per device). Hence I am thinking of trimming down the bias of the output MOSFET to stay within the boundaries. It should still be a "heavy" Class AB as I can go upto 400-500mA in principle. Certainly I am still looking at other alternatives.
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Old 20th February 2019, 01:16 PM   #10
indra1 is offline indra1  Indonesia
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The BA3 front end is basically a low power F5. In principle you can use a single pair of output stage at AB bias with an option of no global feedback or use the F5 feedback scheme.
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