F6 Amplifier

[Antoinel]

Originally Posted by Rush

Just curious how many did you order?

Rush: Here is an option to operate lhquam's design without feedback as you asked earlier. Do not add the front end yet when you assemble it. Instead, drive the transformer's primary directly with the low impedance headphone output [volume controlled] of your CD player. It is a free front end. May also consider decreasing OLG [50s down to 10 or less] with higher source degeneration of the JFETs to lower Zout, and to simultaneously manage power output. Thus, you'll achieve the best gain structure, simplicity and quickly listen to your music.

[Rush]

Originally Posted by Antoinel

Rush: Here is an option to operate lhquam's design without feedback as you asked earlier. Do not add the front end yet when you assemble it. Instead, drive the transformer's primary directly with the low impedance headphone output [volume controlled] of your CD player. It is a free front end. May also consider decreasing OLG [50s down to 10 or less] with higher source degeneration of the JFETs to lower Zout, and to simultaneously manage power output. Thus, you'll achieve the best gain structure, and quickly listen to your music.

Thank for the suggestion,. I will try lots of things. I haven't had a CD player in years. I use a Mac Mini music server to a QB9 to my tube preamp to a 2 way 4th order crossover to the F5ish amps and Snell "E" speakers. Soon to have the preamp and crossover in the same box. B1 all the way through, so the B1 output of the crossover could drive the transformer. I will try it both top and bottom. I also have a full range version of my speakers with crossovers in the boxes, so I can try it every way.
Can't wait to get these projects done so I can listen. Actually that is a problem, my system sounds very good now, I find other things to do instead of finishing my audio projects. The F6 is a motivation though and by the time the boards get here I hope to have a working phono preamp/crossover.

Rush

[lhquam]

Originally Posted by Antoinel

Rush: Here is an option to operate lhquam's design without feedback as you asked earlier. Do not add the front end yet when you assemble it. Instead, drive the transformer's primary directly with the low impedance headphone output [volume controlled] of your CD player. It is a free front end. May also consider decreasing OLG [50s down to 10 or less] with higher source degeneration of the JFETs to lower Zout, and to simultaneously manage power output. Thus, you'll achieve the best gain structure, simplicity and quickly listen to your music.

My PCB board has the option for a pot on the bottom leg of the feedback network. With that you can easily adjust the closed-loop gain to a value up to the open-loop gain. I experimented with this today with my point-to-point breadboard and it works fine. Keep in mind that with low feedback factors, this amplifier has significant high frequency rolloff, primarily due to the inter-winding capacitances of the transformer. I measured it at -3.6dB at 20kHz open-loop.

[anilva]

Papa, can you please post the official F6 circuit? I am lot less gifted technically than many others on this forum who are able to create their variants on your conceptual schematic. I can build very well though....but trust your design.

TIA.

[triode_al]

Originally Posted by Antoinel

[U][B]Rush: Here is an option to operate lhquam's design without feedback as you asked earlier. Do not add the front end yet when you assemble it. Instead, drive the transformer's primary directly with the low impedance headphone output [volume controlled] of your CD player. It is a free front end. May also consider decreasing OLG [50s down to 10 or less] with higher source degeneration of the JFETs to lower Zout, and to simultaneously manage power output. Thus, you'll achieve the best gain structure, simplicity and quickly listen to your music.

When you do not use the buffer stage and input from headamp, there is still feedback! The feedback is, as I see it, the difference of the summing junction versus earth to the primary, the latter being 'fixed'. [Probably ZM can describe this better].

What is OLG - onlinegain?

[Zen Mod]

Open Loop Gain

Closed Loop Gain

he probably meant on varying just resistor coming from primary bottom to gnd

if that resistor is 0 , then there is no overall feedback

[wimdehaan]

I was a subrscriber of the Japanese MJ magazine for many years. The attatched amplifier design is taken from the MJ magazine. I can not recall which issue, but was published somewhere the 90s.

The Tango is 500 ohm to 2x 30 ohm. I believe this was not a standard product of Tango.

I build two interstage coupled amps, but with tubes and mosfets. The enclosed circuit I did not try, but at least it omits the large capacitors in the outputstage. These capacitors can be 'heard', that's why I moved on to a circlotron output circuit.

Wim

[Antoinel]

Originally Posted by Zen Mod

Open Loop Gain

Closed Loop Gain

he probably meant on varying just resistor coming from primary bottom to gnd

if that resistor is 0 , then there is no overall feedback

Thank you ZM. OLG = Open Loop Gain used by lhquam. Also, there not any electrical connection between the output of the amp and the primary winding of the transformer. This winding stands alone. Thus, this transformer operates like a ground loop isolator.

[Antoinel]

Originally Posted by wimdehaan

I was a subrscriber of the Japanese MJ magazine for many years. The attatched amplifier design is taken from the MJ magazine. I can not recall which issue, but was published somewhere the 90s.

The Tango is 500 ohm to 2x 30 ohm. I believe this was not a standard product of Tango.

I build two interstage coupled amps, but with tubes and mosfets. The enclosed circuit I did not try, but at least it omits the large capacitors in the outputstage. These capacitors can be 'heard', that's why I moved on to a circlotron output circuit.

Wim

wimdehaan: Thank you for this interesting schematic. buzzforb, ZM and lhquam were inadvertently discussing a schematic like this actual one yesterday.

Rush and triode_al: Take the subject schematic and mentally disconnect the connection between the power output node and the inverting input to the front end Op Amp. The ouput stage nows operates open loop; or without feedback as Rush would like to operate it. The Op Amp and its input volume control is [like] a headphone circuit of a typical CD player. It is a free front end. It simplifies Conceptual F6 if one operates the output stage open loop. The amp operating open loop is characterized as transconductance or current source compared to a voltage source in the presence of loop feedback to the inverting input of Op Amp.

[Zen Mod]

Originally Posted by Antoinel

....Thus, this transformer operates like a ground loop isolator.

some xformer based gadgets are made exactly for that ;

but , every other xformer based gadget (or part of bigger gadget,mix console for instance) is made with some other primary purpose and , depending how xformer is wired , can bring that virtue , or not .

so , in case of MJ schematic ...... it's not (real) gnd-breaker , because you have same gnd connected in circ on both sides of xformer