PCF in F4-amp ?

You could implement PCF in an F4, but it is not as simple as in other amps.
You need some kind of voltage-gain. You could use a step-up transformer between the output and input to increase the voltage sensed by r-sense.
I would consider it better to build another amp or add a voltage gain stage to the F4, but this will make it into a whole other amp.

Cheers,
Johannes
 

wdecho

Member
2014-10-28 11:45 am
It is amazing to me how you can confuse me with such a short sentence.

Wikipedia

Unity Gain: When a signal passes through a system or a device without changing level (amplify or attenuate) it is said to be at “Unity Gain”. Unity Gain is fundamental in setting up any audio chain or signal path.
 

Nelson Pass

The one and only
Paid Member
2001-03-29 12:38 am
It is amazing to me how you can confuse me with such a short sentence.

I was not clear?

You can set up a unity gain follower for less than unity gain using an input
voltage divider, and then bootstrap the bottom of that divider to the resistor
which is sensing output current to ground.

This will lower the output impedance of the follower.

:cool:
 
yeah
 

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Well... I think it is not a good idea to try pcf in an F4 nonfeedback amp but I had to ask...
/ tord

It's not necessarily a bad idea it's just not the best use of pcf since the circuit is already quite respectable and can easily be further improved simply by increasing the bias as high as your heatsinks will allow eg 2A instead of 1.3A.
 
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2 picoDumbs

Member
2013-09-06 9:35 am
Yes good point.
Some people like to warm their house in winter and make full use of the performance of their existing heatsinks. :)

You can also achieve the same sought of improvement at 1.3A by increasing the number of output pairs from 3 to 6 pairs.

Many ways to skin the cat.

Bear in mind though going from a damping factor of 40 to 100 is not as Audible as going from say 4 to 100. So it really depends on the performance of the circuit in stock configuration and your type of speakers whether it's worthwhile.
 
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wdecho

Member
2014-10-28 11:45 am
I was not clear?

You can set up a unity gain follower for less than unity gain using an input
voltage divider, and then bootstrap the bottom of that divider to the resistor
which is sensing output current to ground.

This will lower the output impedance of the follower.

:cool:

I just meant I am not knowledgeable enough to keep up. I am trying to learn. No doubt I have the best teacher there is.

:)
 
Skärmbild från 2016-04-19 16-25-13.png

This is not meant as a serious proposal, just some LT-spice play with PCF in a simplified F4 source follower amp.

With these resistor values the gain actually increases from 0,8 times the input voltage to 1,08 times the input voltage while the simulated distortion drops by almost an order of magnitude - and this for a higher power output.

Cheers,
Johannes
 
F4-PCF.png

Here is a much better version. Less unnecessary stuff leftover from fooling around with Nelson Pass suggestion of bootstrapping a voltage-divider.

Nevertheless, that was a useful and imaginative example.

Thanks Nelson Pass! I can't take any credit from this though. It was your idea. I just fooled around with some variations on how to bootstrap the voltage divider when it occurred to me that I could bootstrap the signal source with positive feedback from the R-sense (R8 in this example) instead.

I must thank you for this idea. I am currently playing with an IRLB3813 as a source follower with an autotransformer as source-load and output "transformer". I will test this idea with bootstrapping the PCF through the signal source tomorrow.

Cheers,
Johannes