Zero Feedback Impedance Amplifiers

[Susan-Parker]

Hi Eric,

Please excuse the brevity of this reply, I have been quite busy of late.

Originally posted by erictoucan
Hi Susan,

I find a thread www.bonavolta.ch/hobby/en/audio/2sk135se.htm

They call it SE Super Nemesis Compensated.

It has equivalent idea to my previously proposed magnetic balanced topology. But, I do not see the point of connecting the gate of balancing Mosfet to the collector output by 12K. What is the exact purpose in doing that? Do you see it?

The 12K connects the DC bias level to the capacitively coupled input giving the two mosfets the same bias levels.

BTW my understanding is that Jean Hiraga's original Nemesis had a transformer coupled input.

Originally posted by erictoucan
Thank you for your remind, I think you are correct, take the same bias as the amplifying Mosfet to have exact balance. But, do we need that exact? Would 20% deviation significant? I think not and we can just use resistor or lamp of suitable resistance(no need for heatsink) to balance it without the mosfet.

The circuit as shown is a quasi push-pull and the mosfet is dynamically counter-balancing the current in the active mosfet (the two mosfets should thus ideally be matched).

Replacing this with a resistor is not going to be optimum.

I have (in Spice simulation) reordered the circuit to have followers (and a transformer input) and one can see the drive and balance currents. The second mosfet is clearly working for a living.

To my mind, given the relative circuit complexities, using a single mosfet with a gapped transformer is easier to understand and prototype. Once one has the basic circuit working additional stuff can be added.

Using a follower and a suitably large ungapped transformer one can drive single ended even with core magnetization and although not ideal the low impedance of the mosfet will produce more acceptable results than with a tube circuit, as is shown in my Zeus SE page:

http://www.susan-parker.co.uk/zeus-se-amp.htm

I am always interested to hear about variations and inspirations derived from this thread and look forward to reading about your progress.

Best wishes,
Susan.

P.S. I am using B2Spice V4 Pro

[Susan-Parker]

Hi Oliver

Originally posted by el`Ol
I know this is the totally wrong thread, but I hoped to find some transformer experts here.
There is a Japanese Fullrange speaker with two voice coils and a transformer between, at least for low frequencies.
Interesting new fullrange drivers out of Japan
I ask myself whether a simple 1:1 transformer between the voice coils of a dual voice coil woofer would improve the performance in combination with a low dampening factor amplifier.
Regards, Oliver

The dual voice coil enables the additional components shown in the schematic to linearize the frequency response of this speaker.

I am not sure what benefit you would obtain with a conventional driver unless you were wanting to do something similar to match the driver's frequency response to a specific enclosure.

You can easily try this out with a toroid mains transformer using the dual secondaries as a 1:1 transformer - will need to be minimum of 300VA, probably higher - with a voltage that approximately matches the peak AC voltage output of the amplifier. Don't forget to insulate the primaries as they could give you a nasty shock otherwise.

Best wishes,
Susan.

[erictoucan]

Hi Susan,

Well, I see the 2 Mosfets in Super Nemesis is basically biased by the voltage between the 2.2K pot. The 12 K resistor can be taken as a decoupling resistor between the signal and the bais. But, it we take a lamp of the same conductance of baised 2sk135 (0.65A) would do the same thing. If you can simulate it easily by program. Would you be interested to do it?

The next interesting thing I find interested to research is the forward transconductance of Mosfet. Can we design a circuit to correct the non-linear transconductance of non-feedback SE? Not by feedback of course.

Eric

28 Jan 2006

[thorstenlarsen]

Hi erictoucan !

For linearization of output-devices you might look into the threads about "Lovoltech" J-FETs in the Pass forum. It should be possible to get some reduction of the second harmonic by using cascoded J-FETs.
As I said before: Your idea will work. Just go ahead.

Thorsten ( not that one )

[erictoucan]

Hi thorstenlarsen,

Thank you very much for your concern. Your idea and comment really helpful. I want to go ahead with my design. But, I am lazy and doing this part time. So, as long as I got opportunity to experiment it, I will let you know right away. So, many staff in my head Career, Finance, Philosophy. Audio development for good sounding world. As one of your interest, I always try to develope audio gear within my desired philosophy. Thank you very much for your support again.

Cheers

Eric

30 Jan 2006

[thorstenlarsen]

Hi Eric !

I can understand you. I`m lazy too. And it seems like we share some other interests too.
Hope to hear from you again.

Thorsten ( not that one )

[jkeny]

Hi all

I wonder if there could be a sonic comparison posted between this unusual design amp and some recognised reference amps, AKSA etc

I presume some people have now built a variant of this design and feedback on sonics would be helpful for others thinking of embarking on this path.

John

[jkeny]

bump

[Susan-Parker]

Hi John,

Thanks for your posts.

Originally posted by jkeny
Hi all

I wonder if there could be a sonic comparison posted between this unusual design amp and some recognized reference amps, AKSA etc

I presume some people have now built a variant of this design and feedback on sonics would be helpful for others thinking of embarking on this path.

John

Mike posted a review on this thread - Audition of the Zeus - Post #570

http://www.diyaudio.com/forums/showt...135#post528135

Originally posted by jkeny
bump

?

I have asked a friend knowledgeable in these matters, who listened to my system this December just passed, if he could write something up for me to post to this thread. This will hopefully be in a few days time.

Best wishes,
Susan.

[jkeny]

Thanks Susan,

I read the very positive review you referenced and was hoping that others who have built the amps would also post their experiences etc.

Maybe this will follow in time - I look forward to the new review you mentioned

John