I should mention that with my very efficient over 100db speakers the F6 has developed a bit of hum mostly at the tweeters so it is a high frequency hum that the amplifier did not have before the PCF was added. Nothing that is alarming and you certainly do not hear it with music playing but it is there. The circuit does need some more tweaking to eliminate the hum for more efficient speakers. I am still amazed at the difference in the sound the PCF made.
Since the PCF bootstraps the gain of the amp and there is a transformer in the circuit, any hum it can pick up from power supplies or electrical fields will be greatly amplified compared to the very low gain standard version. If you push the PCF to a high percentage of the NFB the amp can start to oscillate at a very low level at RF frequencies. It will start to pick up radio transmissions from the speaker cables and demodulate and amplify them. It is a 100 watt regenerative receiver with several meter long antennas (speaker-cables). With the right crossover parts in the speakers you could probably receive and hear local AM radio stations.
This is why I recommend a small 10 (?) nF cap to ground at the PCF input to short out any RF content.
This is why I recommend a small 10 (?) nF cap to ground at the PCF input to short out any RF content.
I moved the position of the PCF resistors away from some AC power wires and it cured my hum. I also lowered the main resistance down to .8 ohm to see what effect it is going to have and see if enough PCF is being injected back into the input. The bass was still a little too flabby for my taste using 1 ohm resistors.
A quote from 1957 "Radio and TV News" article.
"an attempt has been made to explain positive current feedback from a different viewpoint than is normally used and it is hoped that this article will clear up some of the controversy surrounding the subject. Three high-fidelity systems have been in use for almost one year with the networks described and at no time have any unpleasant results been observed nor has any listening fatigue been felt. Using these circuits on the Klipschorn and "Rebel" series enclosures gives a life-like bass that is only equaled by the most elaborate systems that are available."
"an attempt has been made to explain positive current feedback from a different viewpoint than is normally used and it is hoped that this article will clear up some of the controversy surrounding the subject. Three high-fidelity systems have been in use for almost one year with the networks described and at no time have any unpleasant results been observed nor has any listening fatigue been felt. Using these circuits on the Klipschorn and "Rebel" series enclosures gives a life-like bass that is only equaled by the most elaborate systems that are available."
As a fellow owner of highly modified Klipsch La Scala speakers I will follow developments in this thread but I will be a little cautious until any modifications have been tested to work better and is safe
I would however like any observations posted to include the style of music played so I can judge based on my music style
One thing I love about the F6 is the black silent background with "fast" sharp seperation and 3D presentation on all instruments
I don't want to lose that signature trait I noted above - but if those aspects are enhanced then I would be keen
I would however like any observations posted to include the style of music played so I can judge based on my music style
One thing I love about the F6 is the black silent background with "fast" sharp seperation and 3D presentation on all instruments
I don't want to lose that signature trait I noted above - but if those aspects are enhanced then I would be keen
After some more experimenting I have settled on .8 ohm as the best for right now with my speakers. I had some .2 ohm and .1 ohm 3 watt resistors in my parts bin and used them along with the .5 ohm 10 watt in series to make .8 ohm. The 100 ohm resistor on the output on F6 is a 5 watt so it leads me to think that 5 watt resistors will be enough. My speakers are so efficient I probably never use more than a 1 watt or 2 so I feel safe using 3 watt resistors.
My listening impressions is that with PCF the amp takes on some soul and sounds more like a good SET with PCF. If you like that kind of sound you will like the amp better with some PCF. After repositioning some wires the amp is once again quite. I now know you have to be careful positioning the wires going back the jfets to be amplified again from picking up AC signals. Before the amp was a very very good amplifier but a little too sterile for my taste even using a tube pre with generous amounts of 2nd harmonic. It probably comes down to personal taste and the speakers you are using.
I found this at mouser a 5 ohm 5 watt potentiometer one could probably use but being 5 ohms you will have to be extremely careful to have the pot set on zero to begin with.
026TB32R5AOB1A1 CTS Electronic Components | Mouser
They have 1.5 ohm pots but with higher watt ratings and a much higher price.
My listening impressions is that with PCF the amp takes on some soul and sounds more like a good SET with PCF. If you like that kind of sound you will like the amp better with some PCF. After repositioning some wires the amp is once again quite. I now know you have to be careful positioning the wires going back the jfets to be amplified again from picking up AC signals. Before the amp was a very very good amplifier but a little too sterile for my taste even using a tube pre with generous amounts of 2nd harmonic. It probably comes down to personal taste and the speakers you are using.
I found this at mouser a 5 ohm 5 watt potentiometer one could probably use but being 5 ohms you will have to be extremely careful to have the pot set on zero to begin with.
026TB32R5AOB1A1 CTS Electronic Components | Mouser
They have 1.5 ohm pots but with higher watt ratings and a much higher price.
Interesting enough for Nelson to have a brainstorm about and implement in his new design the F7. We may not have the circuit tuned as well as the master for the amp but right now I like the amp much better with PCF. OREO382 and circlomanen took the first steps, I just followed their directions.
Hello Johannes. So true, and initially spooky. The system picked up [loud and clear] AM station KYW 1060 [>10 miles away] and a Ham Radio transmission regularly while experimenting with positive voltage feedback.
Thanks Zen. With the PCF in the circuit the amp has a totally different sound, nothing like it was before. I am sure I need to tweak the values some more but it sounds more to my liking now as is. On my next Mouser order I will get a pot for more adjustments. Anyone thinking of trying this would be wise to start with a 1 ohm resistor. With the 1.5 ohm resistor my speakers got a little wooly. I think a 1 ohm would be a good place to start and adjust up or down from there depending on your speakers. Right now .8 ohm seems to be the sweet spot for me. I look forward to hearing others opinion of PCF with their F6. It is definitely worth trying.
Zen, it sounds more like the M2. I have not compared the two directly yet but that is my first impressions. Maybe better maybe not so much.
Zen, it sounds more like the M2. I have not compared the two directly yet but that is my first impressions. Maybe better maybe not so much.
Have you moved the capacitor leads to include the resistors yet, or still use stock configuration?
Russellc
I used 1.5 ohms for New Large Advents(sealed 2 way) and it sounds great so experimenting is the key.With my LTSpice sim the 1.5 and 2.2k gave the highest damping factor. Simming with 0.8 actually reduced the damping factor(raised output impedance) from the stock configuration(with no PCF).
My cap leads have been moved as well.
I haven't started or changed anything from stock yet on the F6 Russell but with the carefully chosen components installed, it's produced one sweet amp
I do like the sound on my system
Also waiting on M2 group buy PCBs to build it
FR
You will love M2 as well! I will shortly move the cap connections on F6, but will wait to the smoke clears on using positive feedback.
Appears greatest unknown is how F7 works for all speakers, maybe some ideas in the F7 thread? A bit over my head.
I was thinking of building my other M2 kit into the F6 chassis, but will hold off...this positive feedback is interesting, I'm watching!
Russellc
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