F5 power amplifier

[rubydac]

Could I increase bias from 1.3A to 2A for F5 ?

Because now I had compared with my Aleph J cloned, I loved Aleph J more than F5 because its close to my 2A3 but in my remember I had adjusted bias of F5 from 1.3A to 1.75A ! I loved it !

What's problem if I change bias of F5 to 2A ? (don't tell me it's so hot !!!)

Thanks
Rubydac

[Zen Mod]

Originally posted by rubydac
Could I increase bias from 1.3A to 2A for F5 ?

...........

why not - if you keep mosfets cool enough (Papa's usual test - hand on 'sink)

[Rob Alexander]

I'm not sure if this has been already answered but I've seen a few references to a balanced version of the F5 but don't recall a schematic from Nelson, what the gain would be compared to the srandard version, whether he built one and if so how it sounds in comparison. Just curious.

[EUVL]

> Could I increase bias from 1.3A to 2A for F5 ?

If you are still using +/-24V power supply, you would be running each MOSFET at 48W. Even if you have a large enough heatsink (say 0.4K/W per channel), your MOSFET junction temperature would rise to almost 100 degC. Unless of course you use water cooling to run the MOSFETs at case temperature of about 30 degC (essentially room temperature in the summer in Hong Kong).

You may ask what's wrong with 100 degC junction temperature.
Nothing, except the realibility of the MOSFETs goes down significantly. So if you are prepared to accept that the MOSFETs wil fail now and then, and you have speaker protection installed, by all means.

Of course you are also changing the "sweet spot" at which the original circuit is set by going to 2A, as the drain resistors (trimpot) of the JFets would have to be increased to increase the bias of the MOSFETs. You may of course also achieve the same by increasing JFet bias instead, by using devices with an Idss of say 10mA and keeping the drain resistors as they are (at around 700 ohm).

Or you may wish to consider using the balanced circuit I published, which runs at 16V 2A per MOSFETs, thus still keeping dissipation per MOSFET at about 32W.

I have only built and listened to my all-Toshiba balanced version, so I cannot comment on how different it may sound to single ended Fairchild. But I believe Nelson has both single ended and balanced versions, so perhaps he might care to say a few words on that.


Patrick

[Nelson Pass]

Originally posted by Rob Alexander
I'm not sure if this has been already answered but I've seen a few references to a balanced version of the F5 but don't recall a schematic from Nelson, what the gain would be compared to the srandard version, whether he built one and if so how it sounds in comparison. Just curious.

Ordinarily the gain would be the same.

Balanced versions would come in various flavors.

The primary advantage of balanced output operation is more power,
since the push-pull nature of the single-ended version already has
a third harmonic character at higher power levels. Assuming that
you want to maximize the power, the first thing you are likely to do
is to double the number of output devices (2 in parallel), although
this is not essential.

Then you consider whether you simply want to "bridge" the
amplifier by taking to F5 channels and operating them across the
load out-of-phase.

Lastly, assuming that you want an input stage which integrates
what would otherwise be two separate independent input stages,
you have a couple of obvious alternatives.

The most obvious is to simply take the 10 ohm resistors to ground
and connect them to each other instead.

The other is to make an X circuit out of it.

[Variac]

The other is to make an X circuit out of it.

oooh!! ahhhh!

[Nelson Pass]

Originally posted by Variac
oooh!! ahhhh!

Doing the time warp again?

[c2cthomas]

Originally posted by Variac
The other is to make an X circuit out of it.

oooh!! ahhhh!

Work'n on a Chain Gang?

http://music.yahoo.com/Sam-Cooke/Cha...5102#lyricstop

[EUVL]

> Then you consider whether you simply want to "bridge" the amplifier by taking to F5 channels and operating them across the load out-of-phase.

> Lastly, assuming that you want an input stage which integrates what would otherwise be two separate independent input stages, you have a couple of obvious alternatives. The most obvious is to simply take the 10 ohm resistors to ground and connect them to each other instead. The other is to make an X circuit out of it.


Referring to Klaus's post earlier regarding these 3 variants :

http://www.diyaudio.com/forums/showt...33#post1538833

" I simulated all three variants, the simple Bridge (the nodes in question grounded), Zinsula (nodes connected directly) and yours (nodes cross-connected only). But I couldn't find any significant change in distortion, both looking at the single-ended outputs (ref'd to GND) and the differential ouput, the third harmonic always dominated THD with its stable value. "

I wonder if you would care to comment on the merits of each of the three ?


Patrick

[Nelson Pass]

Originally posted by EUVL
" I simulated all three variants, the simple Bridge (the nodes in question grounded), Zinsula (nodes connected directly) and yours (nodes cross-connected only). But I couldn't find any significant change in distortion, both looking at the single-ended outputs (ref'd to GND) and the differential ouput, the third harmonic always dominated THD with its stable value. "

I wonder if you would care to comment on the merits of each of the three ?

Depends partly on the device models. If the N devices and
(separately) the P devices are literally identical which is likely the
case in your simulation, then the even harmonics cancel for similarly
and the odd harmonics are reduced by the proportion of feedback
which can also be made similar.