F5 class AB mode?

[FYC]

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?

 

[Zen Mod]

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 ?

 

[FYC]

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?

 

[Zen Mod]

I believe something around 350-400mA

try it , if you can without too much expenses

if you like it , then good

 

[Dennis Hui]

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

 

[FYC]

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

 

[Dennis Hui]

Cool. Please keep us post on how it turns out.

Cheers,
Dennis

 

[Bigun]

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.

 

[FYC]

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.

 

[indra1]

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.

 

[Russellc]

AB100 Class AB Power Amplifier

Do this?

Russellc

 

[OnAudio]

Thanks Zen Mod

Does this imply you can hear the difference between a switching mosfet and one that’s not switching

That comes to the issue of a particular signature associated with the amplifying device. So we can say mosfet signature , tube signature, nutube signature, bipolar signature , jfet signature, SIT signature etc

Would you say mosfets sound good only with class A operation or when switched at frequencies outside the audio range such as in class D ? Have you ever fancied any class AB amplifier with mosfet outputs ? Have you build any of these 2DIFFET amplifier , POC amplifier , SF Amplifier ? Note the last two are not fully tested but were designed for experimentation.

Would you say the forte of class AB is the BJT? which happens to be risky in class A due to heat.

What would you say about SITs ?

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 ?

 

[indra1]

...What would you say about SITs ?

If you think you need of a bit more grunt, get the Sissy one.

 

[Zen Mod]

Thanks Zen Mod

Does this imply you can hear the difference between a switching mosfet and one that’s not switching

That comes to the issue of a particular signature associated with the amplifying device. So we can say mosfet signature , tube signature, nutube signature, bipolar signature , jfet signature, SIT signature etc

Would you say mosfets sound good only with class A operation or when switched at frequencies outside the audio range such as in class D ? Have you ever fancied any class AB amplifier with mosfet outputs ? Have you build any of these 2DIFFET amplifier , POC amplifier , SF Amplifier ? Note the last two are not fully tested but were designed for experimentation.

Would you say the forte of class AB is the BJT? which happens to be risky in class A due to heat.

What would you say about SITs ?

old farts don't think too much



just one thing - THD spectra is main thing which is differentiating amplifiers

besides that - good amp is good amp ...... you can make one from snails , if you are able ..... and it'll be good as any other good one

 

[brian.goodman]

Mosfet Citation 12 - Pass AB Design

The Mosfet Citation 12 is worth consideration for your project given your parts constraints. It is a Pass AB design - low parts count, compact board available from Ingvar Ahlberg for the complementary version, article at PassDiy, lots of posts about it on the Pass forum.

Regards,

Brian