F6 with PCF

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#81
Circlomanen said:
View attachment 542983

Here is how I would filter RF to avoid making the F6 into an AM-receiver.
The potentiometer is an easy way to adjust the PCF while playing music through the amp.
I would use a 100 ohm 2 watt potentiometer.

Cheers,
Johannes
Click to expand...

I did some sim's using your circuit with a 100 and 47 ohm (Zen suggestion) pot across the 1.5 ohm resistor.
The span in damping factor for the 100 ohm was -.07 to +.03 (max to min R)
and for the 47 ohm was -.01 to +.18 (max to min R).
It looks like the 47 ohm gives more of a range for adjustment.
 
#84
I notice this thread has gone very quiet. I recently finished an F6 using the Toshiba fets instead of the IRF's and the basic amp was similar to an F6 I bought last summer. The 1.5 ohm resister probably does change the stock sound for the worse. Using the 100ohm variable resister and a 1.2k set of resisters along with the 10 nF cap completely changes the amp. I have tried it on the lower midrange of a Linkwitz LX521 and is superb at about 1/2 point. Same with a Medalion lowther and a full turn for an Avantgarde Trio horn of a friend of mine. Not sure of the math to do the high frequency roll-off but to me looks like about a 13,263 rolloff which on the full range speakers was quite audible. I took it out and the sound became rather harsh in upper frequencies so went with a 750pF and sounds great. Putting some PVF results in a rather amazing tranformation of a pretty basic amp and I think it is such a simple modification many DIY's should try this.
The quality of the mid-range in particular is amazing.
 
#86
Putting some PVF results in a rather amazing tranformation of a pretty basic amp and I think it is such a simple modification many DIY's should try this.
Click to expand...

Yes, it seems the F6 really does like some PCF.

Based on what people have written about the F6 and PCF in this thread, I am getting very tempted to buy a pair of Jensen transformers to test it out.

Thanks for sharing your findings and you experience with the F6 and PCF. It is a very interesting subject.

Cheers,
Johannes
 
#88
Thanks TordNilsson! I have read and replied to your PM.

There might be a F6 with PCF on the horizon for me.... 😎

Has anyone tested Ixys Polar fets in a F6 amp? I would love to test some IXTH 140N10P or similar devices in the F6 amplifier. With a pair of IXFN200N10P (SOT227) it would be quite easy to build a 100 watt in 8 ohm F6.
Since the PCF seems to intensify the innate character of the amplifier, it is even more important to have a nice sounding amp to begin with.

I wonder how the F6 sounds with a pair of IRFP7430?

Cheers,
Johannes
 
#91
It should replace the speaker return to the board. If in parallel then it is effectively shorted out and will not do anything.

The resistor is there to sense the current flowing through the speaker and use this small signal for feedback.

Cheers,
Johannes
 
#93
oreo382 said:
I did some sim's using your circuit with a 100 and 47 ohm (Zen suggestion) pot across the 1.5 ohm resistor.
The span in damping factor for the 100 ohm was -.07 to +.03 (max to min R)
and for the 47 ohm was -.01 to +.18 (max to min R).
It looks like the 47 ohm gives more of a range for adjustment.
Click to expand...


needtubes said:
Maybe output impedance instead of DF?
Click to expand...

Are you measuring the impact on output impedance of the F6? Does this mean that PCF reduces the output impedance? The original F6 has an output impedance of 0.5 if I recall correctly. The damping factor will more than double (at 0.18)? What is the impact on overall gain?
 
#95
Any of you have some images of the changes you made to the boards and are you willing to share? Would love to see the work you did. I am waiting on a few more parts then will make my F6. This thread has me interested in the PCF difference. Not sure I will try it until I get the F6 stable for comparison.
 
#97
brianhiatt said:
Any of you have some images of the changes you made to the boards and are you willing to share? Would love to see the work you did. I am waiting on a few more parts then will make my F6. This thread has me interested in the PCF difference. Not sure I will try it until I get the F6 stable for comparison.
Click to expand...

I just bought the transformers and circuit boards from the store and stuffed them. Stability will not be a problem with the basic F6. PCF at least in my setting changes an already good amp for the much better and really a very simple mod as Circloman has suggested. The 1.5ohm with the 100 ohm variable resister allowed me to use this with 16 ohm horns at a full turn. The usual 8 ohm Lowthers took a 1/2 turn or essentially .75ohms and a 4 ohm speaker took a 1/4 turn so quite adaptable to almost any load. You know when you have gone too high as speakers tend to waterboard and the sound tells the tale. I have tried to follow the math for making Rsen insensitive to load as described on the F7 thread but not knowing exactly the gm makes it difficult. I have not played around much with different Rsen as 1.5 seems right for me. Also I feel the 1.5 will not be the same as .75 for PCF as regardless of PCF rsen is a damping resister to your speakers. Maybe others can explain this and the math better than I can as I don't have a solid grasp on it.
Tom
 
#99
Probably not the correct term but that is what I see, a slow excessive movement of the bass drivers, back off a bit and stops. I didn't waste any time stopping it and think the frequency was too low to hear. I was using an older set of Thors and the lower impedance is 4 ohms and forgot I had the variable resistor set for my 8 ohm Lowthers. I guess the rule is to start with feedback and increase slowly.
Tom
 
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