F5 Turbo Builders Thread

[Mallard]

Sorry I meant: I don't understand why 0.5Ohm are NOT suitable to drive 2 Amps...

 

[AudioSan]

the voltage drop over the resistors get too high, and the diodes starts conducting. then you have a bias runnaway, and a amp on fire because of it.

 

[Mallard]

Ok.. any recommendation for a reasonable source resistor value?

the voltage drop over the resistors get too high, and the diodes starts conducting. then you have a bias runnaway, and a amp on fire because of it.

 

[AudioSan]

that need to be calculated from your bias goal.
you MUST know this before you make an attempt. othervise, stay within pass's recommendations in the article, or dont use the diodes.

 

[Mallard]

As I've said, actually the goal is 2Amp per channel. I understand it that way that for 2Amp I need to set Pot1 and Pot2 to ~1V, correct?
That's a lot in comparison to the recommended 0.4V in the documentation...

Therefore I'm asking if it#s reasonable to bias @ 2Amps!

that need to be calculated from your bias goal.
you MUST know this before you make an attempt. othervise, stay within pass's recommendations in the article, or dont use the diodes.

 

[AudioSan]

2A pr ch is 1A pr device. that is a voltage drop of 0.5V (as long as it is 2 output pairs)
as long as the voltage drop is below 0.4V, the bias can be as high as you like it. well. as high as the outputs can handle

 

[alazira]

As I've said, actually the goal is 2Amp per channel. I understand it that way that for 2Amp I need to set Pot1 and Pot2 to ~1V, correct?
That's a lot in comparison to the recommended 0.4V in the documentation...

Therefore I'm asking if it#s reasonable to bias @ 2Amps!

Ohm's Law: V=IR
For 0.4V and 2A then R=0.4V/2A=0.2ohm resistor
For "fraidy cat" 0.3V and 2A then R=0.15ohm resistor

The Toshiba devices seem to be stable within this source resistance range (see WalterW build). However, I am uncertain about other brands.

 

[AudioSan]

Ohm's Law: V=IR
For 0.4V and 2A then R=0.4V/2A=0.2ohm resistor
For "fraidy cat" 0.3V and 2A then R=0.15ohm resistor

The Toshiba devices seem to be stable within this source resistance range (see WalterW build). However, I am uncertain about other brands.

it's 2A pr ch. 1A pr device.
dual 0.68ohm resistors is just about right. then he will have 340mV at 2A pr ch.

 

[6L6]

Therefore I'm asking if it#s reasonable to bias @ 2Amps!

If you have the heatsink to keep the devices to 65C or below and the heatsinks to 55C or below, and you don't bias past 65-75% or your total transformer VA, you should be fine.

 

[Mallard]

I'm going to use 2 x 800VA transformer and the 5U chassis/ heatsink. Does this sound reasonable to achieve the 2Amp per channel?

If you have the heatsink to keep the devices to 65C or below and the heatsinks to 55C or below, and you don't bias past 65-75% or your total transformer VA, you should be fine.

 

[6L6]

You will probably run out of heatsink before you run out of transformer. You may need a fan.

 

[AudioSan]

2A bias and +/-32V rails = 128W dissipation pr ch. the 5U chassis should handle that.
transformerwise. 300VA pr ch will be minimum. so 800VA will for sure do the job

 

[Mallard]

Ok, thanks 6L6.
What are the typical bias values other F5T builders?

Most likely I will use the 1Ohm source resistors and try to bias as high as possible by beeing able to touch the heastsink for more than 10 sec.

And will see what the result is.

 

[6L6]

2A bias and +/-32V rails = 128W dissipation pr ch. the 5U chassis should handle that.

I thought we were talking about 2A/device (2A*32=64W, *4 =256W/channel)

Sorry.

 

[AudioSan]

that would have been hot as a dual mono amp 500+W.

 

[AudioSan]

Ok, thanks 6L6.
What are the typical bias values other F5T builders?

Most likely I will use the 1Ohm source resistors and try to bias as high as possible by beeing able to touch the heastsink for more than 10 sec.

And will see what the result is.

that WILL result in a burned up amp. it is NOT the heatsinks that limits your bias. it the point where the diodes starts cunducting.

 

[Mallard]

Sorry AudioSan, I just need to collect what I've learned from you

When I use two 0.68Ohm resistors in parallel and adjust a 340mV bias then I will end up at roughly 2Amps per channel? And in your point of view the 5U chassis can handle that?

it's 2A pr ch. 1A pr device.
dual 0.68ohm resistors is just about right. then he will have 340mV at 2A pr ch.

 

[AudioSan]

correct.

 

[aft]

Hello,

I'm planning to go for that f5T journey (V2, no cascode, stereo based on last board from diyaudio).
I'm already preparing and have collected usefull informations but still have doubt about matched components.
Can somebody explain :
- What active components need matching whithin a channel ?
- I can remember that the Aleph-x was requiring matched resistors, is that the same here ?
- Is there a need for matching between channels ?

Thanks. Raoul

 

[Zen Mod]

-if possible , input Jfets need to be of same Idss ; if going to multi output pairs route , all N need to be matched , and all P need to be matched
-if you buy Metal Oxide ones , they're already in enough low tolerance ; if using white ceramic ones (wirewounds) , buy more and measure them (with ccs , measure voltage drop)
-preferably , but not so important

see what's h_a is having to sell