halcro amplifier

I had a chance 2 weeks ago to bench test one of these
units. On my equipment they do not quite meet their
specs, yet .0008 thd at full power at 20khz is not an
easy thing to do.

On my wilsons, they have a lack of bass. Partially due
to the current limiting.

They have 6 p channel and 6 n channel vertical fets for
the output transistors. They are uniquely built to say the
least. The universal input power factor corrected high
frequency supply is also interesting.

The add/subtract around the output stage looks like a
more complicated version of the seal electronics amp.

The stereophile and absolute sound reviews of this amp
both have discrepancys, like the fact that the amp has
no overall feedback. The amp does in fact have global feedback
in addition to local feedback loops.

I don't like Bruce Candy's attempted secrecy about various
circuits, never did like high falutin mumbo jumbo. The patent
however tells most of the tricks...

Doing the add/subtract around the output stage with
bipolar transistors would be much more work.

I would take a krell or levinson monoblock over these anyday.

How about some discussion about the design aspects...

Hei mikeke

Now we are two with the same feelings;) ...

When i see the patent i also nailed the output stage with error correction...of M.J.Hawksford in his work(Distortion Correction in Audio Power Amplifiers...65thConvention of The Audio Eng. Soc.)..
And the idea of bootstraped power suplyes .is also used in the Audio Analog Amplifiers....

But!!!...maybe i have only a souspicios mind!...


Jorge Santos
Re: Hi jorge..

mikek said:
You are absolutely correct.....amazing how people can get away with patenting someone elses idea that has been public domain for near 20 years...:eek:

That what's it all about, to apply for a patent. If somebody feel that someone has stolen ideas then you call for lawyers but haven't you got a patent I suspect you can't stop someone to get it.

When I see (or saw) Nelson's patents I thought "is this patentable"? The difference between Nelson and I is that he had made an effort to apply for patents (and also got it), but not I.

Circlotrones class A amp is surely patentable!
Re: Halcro!!

Tube_Dude said:
Hei mikeke

Now we are two with the same feelings;) ...

When i see the patent i also nailed the output stage with error correction...of M.J.Hawksford in his work(Distortion Correction in Audio Power Amplifiers...65thConvention of The Audio Eng. Soc.)..
And the idea of bootstraped power suplyes .is also used in the Audio Analog Amplifiers....

But!!!...maybe i have only a souspicios mind!...

Have you checked JAES from the 80's with the article about "Amp with error correction"? This amp looks very much alike the one in the article. Do we talk about the same thing or similar? I have a vague memory of the word "error" in the title.
peranders said:

can someone explain the Hawksford error correction with this picture, if possible using the part numbers used in the schematic ?

What I detected was a copy of Lavardin input stage configuration (patented by Mr Perrot). Mr Perrot also described the floating power supply for power amps in the 80ies, and I guess, there are even some guys who had built such amps even before.

What I like are the diodes making the current mirror work more linear, part numbers 17 and 24.

Apart from generous usage of cascoding and the quadruple darlington output configuration, what is really special with Halcro, if at all ?

Even the oldest Mission Cyrus I/II and the cheapest Cambridge power amp use input differentials built from complementary darlingtons ...

Is it old or new?

I had looked at the design of the Halcro power amplifier patent application earlier this year. I may be wrong, but to me the circuit looked like it used quite a bit of circuitry that looked somewhat familiar to me. Thus I assume that I have seen some of it used in the past. As I recall the patent application did refer to Douglas Self and Nelson Pass designs. Perhaps there where others as well.

I do not care for the current limiting the Halcro power amplifier has. I suspect that this current limiting is required due to the likely hood that the error correction circuits (feedback) around the output stage may be able produce significant amounts of positive feedback when the output stage is heavily loaded. Perhaps a second reason may be that the switching power supply cannot charge its filter capacitors rapidly enough without excessive line current demand when high output drive currents are required.

In a way the distortion reducing circuit around the output stage it reminded me of the Burr Brown OPA 604 OP amps when they came out. These also have a distortion reducing circuit around the output stage. I have no documentation on this, but suspect that they used a similar method.

I’m not familiar with the AES articles mentioned by others in this thread, but I sure would be interested in looking at these. If anyone can tell me how I can get a reprint of these it would be greatly appreciated.

John Fassotte
Alaskan Audio


Paid Member
2002-03-29 5:19 am
To view patent pages requires a special viewing plugin program. Some help section (FAQ?) on the USPTO web site tells you about
several you can buy and/or download. I picked the "more
feature-rich but harder to install" one and had no problem
with installing or using.

If you read all the low-level circuitry "features" on the Halcro
patent and compare against Doug Self's articles/books, you
will find that most of them are actually shown by Doug, usually
under the category of "if you really wanted to 'improve' things
more, but I don't think it's worth the effort..." Halcro takes
these all and throws them together, so you get a bootstrapped
cascoded darlington VAS with emitter follower, etc.

As to the output stage, it does differ from Hawksford's circuit.
Hawksford's circuit is essentially a feedforward type circuit.
Halcro's is really a buffered op-amp. Take a standard
op-amp-with-current-buffer topology where the current buffer is
a pair of p-p complementary emitter followers and then throw
away one of them since that "half" of the output stage is
only one "polarity"; then use one on top and another on bottom
-- and you have Halcro's circuit. Then you have some tricky work
to create floating power supplies for each op-amp, make sure
they can't get fried when things go into overload and flail
about wildly, etc.

(Kevin, I think it would be EASIER, not harder to do the Hawksford-style error correction than all the push-ups
required by the op-amps)

The current limiting issue is an interesting one. Per the
audio rags, Mr. Candy (as in Bruce Halcro Candy) believes
that potentially huge output currents is a safety hazzard.
But maybe it's a way to avoid the "flails about wildly" business.

I could see maybe, possibly allowing a patent on the output stage
since it is different than Hawksfords (and the patent does
point that out), but did the patent examiner actually dig
out the Doug Self articles referenced by Halcro and compare?
Surely he/she/it couldn't have. And I would think that even
the output stage would have prior art. (Nelson, I'll have
to go check out your patent; I usually search all the way
up the patent food chain, but in this case I haven't gotten
that far yet)

Note that there are two or three Halcro patents. One has
to do with a hybrid switching/linear supply, again nothing
that would appear patentable.

I really get annoyed with Stereophile, TAS and some of the
web-based reviewers. "secret patented circuitry" my
big toe... Maybe they're just too stupid to
realize that "patented" stuff is not secret but is purposely
in the public domain -- you can look it up. But it's obvious
that the readers of this forum are smart -- we know how
to go to the USPTO web site, do a search, and check it out.
(even if we have a few problems with obscure viewer programs...)
And thanks to the web you don't need to mail $3.00/patent
to Washington DC and wait several weeks to get your copy!
Of course would those HiFi reviewers understand it if
they read it?

Man, I wasted too much time typing this -- time I could
be devoting to writing up some patents of my own. ;-)


p.s. repeating a question I asked months ago, has anyone
(successfully or not) built the Cordell amp design that
used the Hawksford circuit? It's far less complex than
the Halcro and has some amazing ("objective") specs
of its own. (but what does it sound like?)
I think this same Kind of Patent **** has been going on along time. if my memory is corect did not Marconi take the Patent for radio From Tesla? Since tesla Did not use the patent the Patent office let this happen. regarding Nealsons Stasis output stage Patent I have seen Circuits in IC's using what appears to me the same Topology.