I am fairly certain I have the PCF board stuffed correctly and installed in the F6 correctly as well. I sent the input from the PCF board to the input of the R3 resistor where it connects to the input transformer. I also tried the input signal at the input transformer 1 and 2 pins. Nothing evil happened and the amplifier behaved normal when I turned the pot. I was using a cheap test speaker and I experimented with putting the signal in and taking it out. Using this test speaker I could not really tell any difference in the sound with the pot maxed out. I was using a 150 ohm pot and turning the pot did change the resistance measured going to the input of the amplifier but if there is any perceivable sound difference I could not detect it. Maybe others need to build and try this circuit themselves on their F6 and post their results.
I made the above crude and ugly change to the official F6 schematic.
It is the PCF in its most simplistic form.
0,5 ohm sensing resistor (the one between the speaker and the ground) might be way to low value.
The F6 has a very low open loop gain. You might need 1,0 ohm or even as much as 1,5 ohm to get enough signal back into the transformer to make a difference.
I prefer to be a little too cautious, and slowly increase the PCF when everything else seems to work properly.
The F6 has very little open loop gain and therefore very little negative feedback.
The PCF will increase gain by bootstrapping the input and increasing the ability of the jfet-buffer to drive the transformer.
You are using some of the reserve power from the output stage to drive the input stage and the transformer.
Since you greatly increase the openloop gain with PCF you will also increase the negative feedback. This will lower distortion down to a certain point. If it makes the amp sound better or not is something I can't say.
This is why you should use a potentiometer and tune the PCF to your speakers and your preferred taste.
Cheers,
Johannes
Your diagram helps and I did try injecting the signal at that point along with pin #1 and between pins #3 and #4. I could not tell any difference but this observation was with cheap test speakers that do not show a lot of detail and as you say I need to do some tweaking. Having very little feedback the PCF may not have as much effect as with the F5. It has been fun trying something new. Posting a F5 circuit using PCF with values is taboo and justifiable so. Let the Master reap something for designing this amazing circuit.
I remember the old saying "If you do not feed your hound dogs they will not hunt."
I remember the old saying "If you do not feed your hound dogs they will not hunt."
Well I did try it and wow! I've never heard my speakers like this before. The sound is hard to describe, instruments are more defined,bass much more definition,sounds came out of my speakers last night that I've never heard before (New Large Advent). Used a 1.5 ohm 10W in series with the speaker and tapped a 2.2k 1/2W from the spk negative/1.5 ohm point to the Jfet gates.
I measured the damping factor at 1khz to be 80.With a little higher (say 2.4k) resistor and a decade box across it you could fine tune your setup. I may try this but I was too excited to listen to do this. I went by my LTSpice sim results as a first go. No oscillation or fried components. I did notice that you have to get the F6 up to temp before it sounds good. When I fired it up after bench testing it sounded thin and lo-fi. I thought rats, have to undo my handywork but I let it play and after a while the magic came. All I can say is try it, its an easy mod and from what I've heard it is well worth the trouble,at least with my speakers. One last note, I listened to Jimi Hendrix "are you experienced" and the guitar solo near the end was never like this before.
I do have to give a big thank you to Mr.Pass for getting this knowledge out there (F7) and for fanning the flames of curiosity by mentioning that you can do this with his other amps.
I am just a "backyard mechanic" and would never had thought of this in my lifetime
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Well I have to report after listening this afternoon that this is the best sounding amp I have heard, hugely better than the F6 before this mod. Individual instruments heard are not smeared when you have more than one playing.Bass is like never before,multi layered.Vocals are not smeared like before,I can now actually pick out what Adele is saying in her song "Hello".Musical and alive comes to mind. Whoever has built an F6 has to try this, I'm not kidding do yourself a favor and then write back.I like some others was not impressed by the amp as built,this is another level. I think it has to do with the control the PFB mod exhibits on the speakers.Please try this.
If I see the schematic right the PCF consist of R17 and R19 with R8 being the speaker. I appears you have lowered the values of C1 and C2 also and changed their connection point as others have said to do. Is this correct? And thanks for your experimenting. I have been looking for improvements to try with some of my amps. When I tried PCF I connected it into the circuit after the jfets and I see you have injected it before the jfets.
The F6 I built is a very very good amp, maybe too good. It will make poor recordings sound bad and good recordings sound great. If you want to hear detail like you have never heard before the F6 is a good amp.
The F6 I built is a very very good amp, maybe too good. It will make poor recordings sound bad and good recordings sound great. If you want to hear detail like you have never heard before the F6 is a good amp.
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Actually the cap values are 1000uF on my amp,the lower values is just the sim.That doesn't seem to make a differece.I have changed the connection point as in the schematic(sim) to the bottom of the source resistors,that was an improvement over the original in my opinion.Yes r17 and r19 are the feedback with the values as in the sim. R8 is the speaker. I did a sim with a 1mH and 50uF in series with the 8 ohm speaker resistor as per what Nelson had shown as a "difficult load" in one of his writings. I fed a 150hz square wave at 1 volt and the output was very well behaved,much more so than without the PCF. I think this is the speaker control Nelson had referred to and if so I can attest to its effectiveness. As I said you have to try this and report.
The 2.2 k is 1/2 watt metal film, the 1.5 ohm is 10w wirewound (both NTE as they are locally available).
Thats what I thought, 2 components. Didn't even have to adjust bias or offset,checked them and they were fine (using 1.3A bias).
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I have not built and tested the F6 with PCF, but I know what it can do for simple single ended class A amps like the Zen amp.
I agree with your "Musical and alive" statement. The right amount of PCF seems to bring out the best of single stage class A amplification.
Thanks for experimenting with your F6 and sharing your results and impressions.
Cheers,
Johannes
I have not built and tested the F6 with PCF, but I know what it can do for simple single ended class A amps like the Zen amp.
I agree with your "Musical and alive" statement. The right amount of PCF seems to bring out the best of single stage class A amplification.
Thanks for experimenting with your F6 and sharing your results and impressions.
Cheers,
Johannes
How cool is this forum, guy from Canada, U.S. and Sweden working together on a project.
I think it would be neat to put some PCF into the F6.
The schematic already has PCF. Current across the source restore increases
the voltage across it capacitive couple to the transformer windings at the gates of the output mosfets. Out current iss
The schematic already has PCF. Current across the source restore increases
the voltage across it capacitive couple to the transformer windings at the gates of the output mosfets. Out current iss
The impedance of the capacitor changes with frequency, at 5khz it is about .03 ohms so its just an ac short. It provides a common for the gate drive coil.The PCF I have makes a significant difference in the sound of this amp.
I installed the PCF circuit in my F6 and the bass was way too boomy. When I turned the volume up it blew one of my speaker fuses. My thoughts are too much PCF and I need to lower the amount going back into the input. What are your thoughts on how much more resistance do I need to install in the feedback. I am thinking of maybe double 4.4K instead of 2.2k to start with and see what happens.
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