F6 Amplifier

Official Court Jester
Joined 2003
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tnx JG

(and yes , Buzz - he certainly did listening test :clown: )

few yrs ago we had some chat with Pa regarding cascode (voltage level) biasing - to put their reference to gnd or to source of Jfet ...

he said - put them to gnd and that way there will be some distortion cancellation , but that was the case for Jfet used as gain stage (common source ) .....

funny - concept of your Hawksford buffer is known for me from work of Great Ozzy - Allen Wright ; his SLCF is taken/derived from Tektronix tube scopes ;

slightly different in execution, but same concept - swinging cascodes
 
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Official Court Jester
Joined 2003
Paid Member

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Yes, i designed it. It is a driven cascode but "the other way around". Hawksford´s original
circuit had a driven cascode in the second stage. There is a paper on his website about it.
Later i heard that Borbely did the same 7 years before and that it was known to designers of Oszilloscopes.
People that build it responded that the bass was sounding better compared to the ground referenced version. It should improve distortion into low impedances.
Joachim Gerhard. My face appears to look like yours. Pls check it out in the thread "Does anyone make their own headphones?" I'm wearing a diy helmet headphone. Best regards
 
The difference is big....unfortunately it is difficult to discuss this point because the circuit is not published.
Maybe I make things wrong... But I really tried hard..... having an original open.

As ZenMod stated I see the greatest problem in the values of the Semiconductors and that there is not much published knowledge how to calculate a Mufollower really, for instance.

It would be very interesting to hear from other people having heard original First Watt amps and clones in comparison, let us say F5 or F4.
Unfortunately there are often two separated groups, one group can afford the FW amps and have no intention to hear clones and the other group building clones does not have the money to buy the originals.

On one side I was really really frustrated about the difference between my clone and the original J2
On the other side I was fascinated what Nelson can do with the same circuit to let it sound so wonderful! I take my hat off!

Nevertheless I go on building, decision to buy a First Watt would stop this process.......
Why should I do this?

And I hope that Papa takes into account our smaller possibilities to calculate exact values and getting completely matched parts.. when writing his instructions and developing the circuit for F6 making it more foolproof...:D

I believe Jameshillj has both commercial and clone F3 & F5's.
http://www.diyaudio.com/forums/pass-labs/121228-f5-power-amplifier-364.html#post1807961
 
Here is my attempt at the F6 design

This is a Spice simulation that looks pretty good, whatever that means. I do not have a lot of faith in the R100 models or the transformer model.

I have most of the parts needed to breadboard this circuit. In about 6 days the remaining parts will arrive and I can do the smoke test.
 

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Re: Here is my attempt at the F6 design

An explanation of the bias circuit:

Nelson actually suggested (perhaps in jest) using batteries. I did the following calculation: battery-life-years= battery-milliamp-hours/(milliamp-drain*hours-per-year). A typical AA battery has more than 1000 mah. 24*365=8760 hours/year. A 1.5 volt battery with a 250k ohm pot has a 6uA current drain.

battery-life-years=1000mah/(6ua*8760) = 19 years.

Thus, the issue with the battery is selecting a battery type with constant voltage over a long shelf life. The now unavailable mercury battery would have been a good choice. AA lithium batteries claim 10 year shelf life, but I do not know about their voltage constancy. AA alkaline batteries rarely are advertised to have more than 4 years shelf life.

The schematic contains ".param Rs=.01 vbatt=1.23 wiper=.0001". I do not suggest that you will find a 1.23 volt battery. If you have a 1.55 volt battery, the wiper must be adjusted for 1.23 volts bias.

The input jfets can be matched for identical IDDS, or a source resistor (R17) added the the JFET with higher IDDS in order to zero the output voltage of the input source follower. This is necessary to prevent DC voltage flow through the transformer primary.
 
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Thus, the issue with the battery is selecting a battery type with constant voltage over a long shelf life. The now unavailable mercury battery would have been a good choice. AA lithium batteries claim 10 year shelf life, but I do not know about their voltage constancy. AA alkaline batteries rarely are advertised to have more than 4 years shelf life.

I think a Lithium Manganese Dioxide battery is what you are looking for.
They have a flat discharge rate, maintaining their voltage consistently through out their life. They have a low internal discharge rate, comes in a 1.5 volt AA package. See Radio Shack link: Lithium Manganese Dioxide Batteries
I used the 3 volt version in a Vacuum tube preamp for years, trouble is you forget about them after a while. As the voltage drops the current goes up and the heat, etc.

Rush
 
even if you choose battery for bias ,Papa's regular solution will be , ahem , more - regular :clown:

regarding DC through primary - remember that feedback path ; it depends also on output offset , and how patient you'll be regarding setting it
I am sure Papa will have a better, "more regular" solution. I went for a simple design with small parts count.

Yes, the output offset is also a problem. I did some simulations with a capacitor in series with the primary windings. The problem (perhaps a Spice artefact) was a low frequency resonance involving the capacitor - inductor circuit. I do not have enough experience using transformers in the signal path to know how to best approach it.
 
An explanation of the bias circuit:

This is necessary to prevent DC voltage flow through the transformer primary.
Is capacitor coupling acceptable? The direct coupling you show at the primary will not influence potential DC offset at the output.

I am not familiar with LTSPICE. Please jot down: voltage of power rails, current flowing through each JFET, and the maximum power output into 8 and 4 Ohm loads.

Can one have multiple upper and multiple lower JFETs to increase power?