in the solid state fourm, the question was presented about sound of bpt mosfets etc in class a.
it was pointed out again more current bias in a mosfet is good.
made me wonder. what is the point of no b getting better.
llike a dimmer switch you want it in the middle. so you can get brighter and dimmer.. to much bias and you wont get a full swing.
so my question is......what is the best.
if the fet is 10 amp should you aim for 5amps?
or is there a sweet spot with each brand, type, of fet. if so what is that called? is it in the spec sheet?
all of this is pretending heat etc dosent matter.
hope im not being to ignorant.
also please forgive spelling, i cant check using my phone.
it was pointed out again more current bias in a mosfet is good.
made me wonder. what is the point of no b getting better.
llike a dimmer switch you want it in the middle. so you can get brighter and dimmer.. to much bias and you wont get a full swing.
so my question is......what is the best.
if the fet is 10 amp should you aim for 5amps?
or is there a sweet spot with each brand, type, of fet. if so what is that called? is it in the spec sheet?
all of this is pretending heat etc dosent matter.
hope im not being to ignorant.
also please forgive spelling, i cant check using my phone.
Rather than said above raw current, the thing that counts when biasing a device is the power it can handle.
P= UI, so you have to be sure you don't break too much the SOA limit ( Safe Operating Area ). For current TO247 devices, Nelson states to get them running at about 30W max each.
This is close to the limit chosen by the master himself for the Aleph_J, good example; the last of the Aleph series, with a double jfet as front end. I'm currently upgrading my Aleph_4 to get this jfet build running. If you look closer, intended bias is 1,15A per device, on 24V rails it means roughly 28W per device. Nelson used 2 output devices / 2 CCS power mosfets IRFP240, each device handling about 30W.
After this limit, depending on heatsinking arrangement and thermal losses ( Mica / Sil Pads / Keratherm, torque applied to the Mosfets, thermal conductivity issues, etc) you can't be sure the inner temp of the device is cool enough. Too high temp reduces lifespan of your project. That said, the higher the bias, at the point your heatsinks can handle, often the better the sound is 😎
The sweep spot exists in all devices. Most of the time it is not predictable however, won't show up clearly in the datasheet, so you'll have to trial & error to find this particular point. Doing so, it is better to cross check measurements and sonic results. This is where mastering a design becomes difficult for most of us here
Best,
nAr
Last edited:
just go to Pass and/or First Watt site , download all pdfs and read
Papa already gave more details and insights in subject , than we can write here in a month
anyway - shortly from praxis here in Papaland :
more bias is better
dissipation per device in range of up to 25W ( usually ) to 50W ( for brave ones )
be sure that heatsinking is good/sufficient (interface between part and hsink also very imortant ) to keep things no hotter than 55C on hottest outer part of hsink
edit : Nar was faster .... but I'm still dumbest around
Papa already gave more details and insights in subject , than we can write here in a month
anyway - shortly from praxis here in Papaland :
more bias is better
dissipation per device in range of up to 25W ( usually ) to 50W ( for brave ones )
be sure that heatsinking is good/sufficient (interface between part and hsink also very imortant ) to keep things no hotter than 55C on hottest outer part of hsink
edit : Nar was faster .... but I'm still dumbest around
just go to Pass and/or First Watt site , download all pdfs and read
Papa already gave more details and insights in subject , than we can write here in a month
anyway - shortly from praxis here in Papaland :
more bias is better
dissipation per device in range of up to 25W ( usually ) to 50W ( for brave ones )
be sure that heatsinking is good/sufficient (interface between part and hsink also very imortant ) to keep things no hotter than 55C on hottest outer part of hsink
edit : Nar was faster .... but I'm still dumbest around
Faster, but edited his own post 😀
Reason : adding some more lines ... 🙄
Best,
nAr
I have not encountered a case where more bias did not improve Mosfet
performance, at least until the devices died...
😎
performance, at least until the devices died...
😎
We use them at PL. I use mica and grease on FW. The difference is that
PL uses tons of devices, and FW uses fewer and works them harder.
😎
PL uses tons of devices, and FW uses fewer and works them harder.
😎
thank you, all of you.
now, if i could ask why?
is it the heating up of the simiconductor stuff on the die or is it the extra current that it provides for the cir?
or somthing else i havent thought of.
for example 10 mosfets at .5A sound the same as 5 mosfets at 1A
is it the indaviduual devices or total?
thanks.
Fundamentally, the transconductance becomes higher with current and the
non-linearity of the transconductance becomes a lower percentage of
signal current. The gain or damping factor gets higher and the distortion
gets lower.
Also, chicks dig it.
😎
non-linearity of the transconductance becomes a lower percentage of
signal current. The gain or damping factor gets higher and the distortion
gets lower.
Also, chicks dig it.
😎
cool, i will do some research.
I'm not going to pretend i understand all of that..... i do understand "chicks dig it" just not my wife.
she loves your amps, don't get me wrong. she stands over them quite often in her pj's to warm up. she doesn't care about sound quality with what she calls music.
thank you.
I'm not going to pretend i understand all of that..... i do understand "chicks dig it" just not my wife.
she loves your amps, don't get me wrong. she stands over them quite often in her pj's to warm up. she doesn't care about sound quality with what she calls music.
thank you.
it's better without pj's
and big fur like carpet , surrounded with hot amps
..... speaking of that
- who needs a woman - just imagine lazy sunday , with pile of cartoons and bottle of wine
and big fur like carpet , surrounded with hot amps
..... speaking of that
- who needs a woman - just imagine lazy sunday , with pile of cartoons and bottle of wine
it's better without pj's
and big fur like carpet , surrounded with hot amps
..... speaking of that
Wine should be stocked outside the listening room, in order to have proper degustation temperature 🙂
Best,
nAr
Sorry for interruption, but I am going to try some laterals in the F4/BA today and have been thinking on the issue of current and voltage as it applies to distortion. From what has been said more current leads to more trans-conductance and less distortion, is the same true for voltage. from what i have seen from FW pdf's, both seem to play a role. Also, doesn't capacitance play a role and shouldn't we be looking for linearity there as well.
- Status
- Not open for further replies.