F5 power amplifier

[a.wayne]

Some members account that the amplifier sounds " bigger " more "dynamic" with out thermisters , could it be due to this "feedback" effect?

[AndrewT]

the thermistor feedback is very slow, many seconds of time constant, maybe minutes.
It will not be audible from millisecond to millisecond, nor even from second to second.

However, it achieves this stabilisation by changing the Vgs for different operating conditions.

It is quite likely that the amp will have a different bias current when the ambient conditions are different. This may be audible from morning to evening, or day to day, or season to season. It will depend on how sensitive the amplifier sound is, to changes in bias current.

[labjr]

Originally Posted by AndrewT

the thermistor feedback is very slow, many seconds of time constant, maybe minutes.
It will not be audible from millisecond to millisecond, nor even from second to second.

However, it achieves this stabilisation by changing the Vgs for different operating conditions.

It is quite likely that the amp will have a different bias current when the ambient conditions are different. This may be audible from morning to evening, or day to day, or season to season. It will depend on how sensitive the amplifier sound is, to changes in bias current.

Some have built the F5 without the thermal tracking and current limiting and say it's better sounding. I wonder how much of that difference is related to the thermal tracking and how much is the current foldback circuit.

[hags]

Originally Posted by AndrewT

at first start up from cold the temp comp thermistors are also cold. The have high resistance.

The thermistors are in parallel to the bias setting resistors.
The high resistance cold thermistor hardly changes the effective bias resistor value and this sets the gate source voltage (Vgs) applied to the output FETs.
As the heatsink heats up, the thermistor also heats up and it's resistance drops.
This reduces the effective bias resistance and thus reduces the Vgs of the output FET. Less bias current.
If the sink cools a little, lower ambient temperature, or a door/window left open, then the thermistor cools slightly and increases Vgs again. More bias current.

This is a feedback system that attempts to minimise the variation in heatsink temperature with changes in operating conditions.

That's what I was looking for, how, if in fact it does influence bias during actual operation. I understand the start up/warm up thermistor function.

My heatsinks run very hot, I can only touch them for 4-5 seconds, I guess I'm not missing anything not having used them????

My build seems to warm up fine and hold steady bias.

[hags]

Originally Posted by labjr

Some have built the F5 without the thermal tracking and current limiting and say it's better sounding. I wonder how much of that difference is related to the thermal tracking and how much is the current foldback circuit.

I have built mine without the thermal compensation and current limiting as well. I had no intention of using the current limiting but I misplaced the thermistors and was debating on whether or not to include them so............

My main concern is the heat, I don't know if the heatsinks I'm using are big enough.

[tinitus]

Originally Posted by hags

My heatsinks run very hot, I can only touch them for 4-5 seconds, I guess I'm not missing anything not having used them????

Try again
Im sure you can make the 6 seconds needed

Anyway, might be worth to consider how accurate measurements really are
I suppose you measure voltage drop, and not actual current flow

[kp93300]

Thank all for the replies.
I have the thermistor behind a soft start board that i bought from Connexelectronic.This is on the primary side of the transformer.
i have used this set up for the past one month .
I check that the maximum operating temperature of the CL 60 is 175 deg c .
I think CL 60 will be ok
I might try danny's soft start circuit one of this days.

BTW, my current set up with the F5, OB5 from GR research, Promitheus DAC and AMC CD 6 player as transport is the best in my 20 over years of hi fi journey.
This set up is good in all aspects and I think I am spending too much time listening to music !

thanks again to all the helpful people.

kp93300

[bobodioulasso]

When using a soft start, the CL60 becomes redundant...

[kp93300]

This is what I thought but I was advised by the soft start board manufacturer to use the CL 60 thermistor in addition .
The soft start board is a relay mechanism with the delay timing regulated by a cap.

kp93300

[bobodioulasso]

I would get rid of it.