Improving the Futterman
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 18th August 2003, 12:46 AM #1 Sch3mat1c   diyAudio Member     Join Date: Jan 2003 Location: Milwaukee, WI Improving the Futterman Disclaimer: I've made some assumptions here, since I don't know everything about this... I've tried to think it through though The main problem with the Futterman is that the NFB is imbalanced. The top tube is a cathode follower, yet the bottom is a standard plate follower with no more NFB than the triode used (and with a load 1/4 or less of the plate resistance, there isn't much internal NFB left!). As a result, there will be much 2nd harmonic in the output. This is usually fixed by using a cathodyne phase splitter grounded to the output, and inverting the drive phase to the output tubes. That helps a little but is only masking the deficiencies of the topology. I propose using shunt NFB applied to the bottom tube, as shown in the attached drawing (note: drawn for theory only). Say you drive it with 20V, and the tube produces a voltage gain of 5 in the circuit conditions. Then the plate signal output is 100V. Yet the same conditions with a CF produces an output of 16.6V, or 15.5dB of NFB. To balance the circuit requires the same amount of NFB applied to the bottom. But how? Driving the grid through a high resistance, and applying some of the plate signal back to the grid, we form a voltage divider from input to output, with the grid inbetween. To solve this, we assume the input and output - 20 and -16.6V (it's inverse polarity remember) respectively, and then since the tube still has a gain of 5, grid voltage must be 3.3V. So there are 16.7V across the input resistance, and 20V between grid and output. The scale of the resistance should depend on HF constants (capacitance, etc.) and the possible driver Zo (for instance if driving with a pentode, the Rp will be a significant portion of the 'input resistance'). The advantage is that for two extra resistors (maybe just one if driven by a pentode) you balance the output; however since a CF's NFB is applied in series, whereas this is shunted, the characteristics vis-a-vis changing Rl will unbalance it at anything other than a certain impedance. I think! Erm.. hope that makes sense Comments? Tim __________________ Seven Transistor Labs, LLC Projects and Resources / Electronic Design and Consultation
Sch3mat1c
diyAudio Member

Join Date: Jan 2003
Location: Milwaukee, WI
Attachment:

P.S. I was going to mess around with this a little on the breadboard... we'll see how that goes.

P.P.S. Dang, I posted this in the etc. forum? Whoops...
Attached Images
 shunt nfb otl.gif (1.1 KB, 333 views)
__________________
Seven Transistor Labs, LLC
Projects and Resources / Electronic Design and Consultation

 18th August 2003, 12:55 AM #3 SY   On Hiatus     Join Date: Oct 2002 Location: Chicagoland Moved. You might look at some of the old reviews of Futterman amps, look at the measurements and waveforms, and see if they bear out your hypothesis. __________________ "You tell me whar a man gits his corn pone, en I'll tell you what his 'pinions is."
fdegrove
diyAudio Senior Member

Join Date: Aug 2002
Location: Belgium
Hi,

Quote:
 Erm.. hope that makes sense Comments?
It does...
Only you seem to forget that your local NFB loop will also put a DC voltage on the output.

Cheers.
__________________
Frank

 18th August 2003, 08:06 PM #5 Sch3mat1c   diyAudio Member     Join Date: Jan 2003 Location: Milwaukee, WI I noted it's a theoretical diagram, for the AC signal path. I'd use a coupling cap there. Tim __________________ Seven Transistor Labs, LLC Projects and Resources / Electronic Design and Consultation
fdegrove
diyAudio Senior Member

Join Date: Aug 2002
Location: Belgium
Hi,

Seems we solves one problem just create another here but what the heck...

Just for the fun of it:

Cheers,
Attached Images
 shunt nfb otl_r.gif (2.0 KB, 226 views)
__________________
Frank

 18th August 2003, 10:35 PM #7 PRR   diyAudio Member     Join Date: Jun 2003 Location: Maine USA

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