Another Zero Feedback Amplifier

[Charles Hansen]

Hello -

I found another detailed schematic for a zero feedback power amp. This was the Wingate that enjoyed a brief run in the late '80s. It was advertised as a class-A, all MOSFET, zero-feedback design. It had all of the buzzwords of the day and, if I recall correctly, received a very enthusiastic review from The Absolute Sound. However, it was only sold direct and the company failed after a short time (as all such companies tend to do).

Since the original thread was so cluttered, I chose to post this separately. It looks like a nice design. Certainly very original and not at all like the "app-note specials" you see from some companies. I don't think I'd agree with his use of the term "current mirrors" in the first stage, but I don't really want to start another argument about semantics!!

http://patft.uspto.gov/netacgi/nph-P...&RS=PN/4688002

Enjoy,
Charles Hansen

[dimitri]

amplifier without dc servo and output relay

[JonMarsh]

Can't say as I know any IC designers that would call that a current mirror...

This configuration is not so different from the Hawk Audio A18 non loop/interstage feedback design, which has been cited in other posts at this forum, including a recent one in the DC Servo thread.

The block diagram labelling is curious, because what's called the voltage gain block is where it should appear, from the output of the OTA formed by the complementary input/level shift transistor pairs, but what isn't shown is a gain set resistor to ground. A little obfuscation, perhaps? Otherwise that block appears to merely be the bias stage, though this makes me wonder what FETs they were using on the output, as the thermal feedback mechanism necessary for VMOS appear to be lacking. But with lateral style FETs like Hitachi or Semelab, they'd be pretty much unnecessary.

BTW, the A18 is a pretty nice sounding little amp, if a bit low powered.

Best regards,

Jon

[JonMarsh]

[Steve Eddy]

That current gain stage looks suspiciously like a pair of followers to me.

I guess the search for a zero feedback amplifier continues.

se

[johnferrier]

Originally posted by Steve Eddy
I guess the search for a zero feedback amplifier continues.

With absolutely no feedback, don't think your imagination will take you far...

Square two involves adding Re (feedback).


JF

[Charles Hansen]

Steve, nobody wants to have stupid semantic arguments with you. You already have hijacked one thread with your silly arguments. If you want to argue about the definition of feedback, please go back to your other silly argument thread. You have already been warned about this once:

http://www.diyaudio.com/forums/showt...964#post332964

Now kindly go away.

[Charles Hansen]

Originally posted by JonMarsh
but what isn't shown is a gain set resistor to ground. A little obfuscation, perhaps? Otherwise that block appears to merely be the bias stage, though this makes me wonder what FETs they were using on the output, as the thermal feedback mechanism necessary for VMOS appear to be lacking. But with lateral style FETs like Hitachi or Semelab, they'd be pretty much unnecessary.

Hello Jon,

As the mandrill says in "Lion King", loook haardeer....

R23 and R24 are the gain set resistors. I agree that the terminology is confusing. He uses resistors as current-to-voltage converters and calls it a "voltage gain" stage! Again, I wouldn't call it that, but I don't want to start another semantic argument.

He says in the text of the patent that all the MOSFETs are Hitachis. We can assume the outputs are the TO-3 package parts, both from the mechanical drawings of oval output devices and from the power requirements. He explicitly uses the smaller TO-220 parts on the voltage regulators, but we are left to guess which ones he uses on the input transistors. I would guess the smaller TO-220 parts there as well. His text also describes how the heat from the output devices will heat up the input devices, reducing current there and hence to the output to provide a very slow thermal feedback mechanism.

Ciao,
Chas

[Charles Hansen]

Originally posted by JonMarsh
This configuration is not so different from the Hawk Audio A18

Wow, you can say that again! Since the Wingate presumably came first, I'd be surprised if the people at Hawk hadn't studied that patent *very* closely!

[JonMarsh]

Ah, OK! I didn't notice the ground tap between R23 and R24, and I haven't read the patent text yet, just flipped through it and printed out the pretty pictures.

Mea culpa!

But it's a goofy way to draw the schematic!

Voltage gain, schmoltage gain, OTA's gotta have an current to voltage conversion somewhere...

This is the first time I've heard of the Wingate amplifier, and this patent. But then I've not had the interest in researching prior art that someone involved in commercial design should have. You might be right that Hawk Audio had a close look to this before developing the A18, but I suppose only Jacco Dekker can say for certain...

There was another guy who did a number of amplifier designs in the early 80's, reviewed frequently at Absolute Sound, probably Sterehpile, too, who's last design was supposedly NFB, but ran hotter than a pistol, Rappaport was his last name, I'm thinking Andy was his first, but that could be wrong. Any idea what that one was about? Also did the Amp-1 and Pre-1.

~Jon