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Old 15th September 2003, 09:26 AM   #1
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Default Opamp suggestion for DC-servo in an Alexander amp

Hi!

I'm planning to go ahead with my Alexander since I have got green light from Analog Devices. The thing I wonder about is which opamp I should use.
http://www.analog.com/UploadedFiles/...17334AN211.pdf
http://home.swipnet.se/~w-50719/hifi/qrv03

OP97 is the original.

Offset less than 1 mV
Small bias currents
Current consumption less than 1 mA

Any good alternatives?
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Old 15th September 2003, 02:44 PM   #2
DrG is offline DrG  South Africa
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This looks like a great amp! Unique design approach. 2 questions though...

I believe the SSM2131 is discontinued... what will you be using instead?

Are you doing your own PCB's?
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Old 15th September 2003, 03:40 PM   #3
Electrons are yellow and more is better!
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I'll guess you don't have seen the present amp? Check the qrv-03 which a korean guy has built.

The opamps in mind are "any" modern.

Pcbs, yes, but this design is protected by a patent so I have had a low profile until I have got green light from AD.
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Old 15th September 2003, 04:16 PM   #4
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Default Integrators low offset

Peranders

I done several integrators for commercial application zero out offset. The opamp used is a OP07A. It not fast and has very low offset.
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Old 15th September 2003, 05:20 PM   #5
Electrons are yellow and more is better!
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One of the main parameters is current consumption because of the current feedback thing. 0.1-0.5 mA would have been nice.

OP07 is OK if you have a "normal" DC-servo application, but this is rather unusual.
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Old 15th September 2003, 06:25 PM   #6
Henrik is offline Henrik  Denmark
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Hi Peranders

The danish HI-FI magazine Highfidelity once had an project on this amp, the people who builded it all complained about lack of deep bass but were much impressed at it`s hights. I wewre close to build it but didn´t. I am very interested in your oppinion on the sound on this one. Mark Alexanders article is very interesing indeed.

Regards
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Old 16th September 2003, 09:10 AM   #7
Electrons are yellow and more is better!
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I helped a friend in the 80's and the output stage used MOSFET's but I don't remember which type, could have been IRF some type which ELFA had.

I never got a chance to really test this amp but it measured and behaved good.

When somebody is used to certian sound and them tries something new it's hard to say what the problem really is.

Thin base and good high range may also be an indicator of that the normal amp has bad treble performance.

Deep bass (technically) isn't a problem because the amp is totally DC connected. Bass down to 0 Hz is possible without a problem.

So at the moment I have a neutral attitude towards the Alexander amp. Current feedback is interesting and I will dive down more into the subject. I have found lot's of info on the net.

BTW: I have also got the real original article from the AES.
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Old 16th September 2003, 12:10 PM   #8
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opa-277? very low offset, Iq < 1 mA , Ib quite small.

elfa. 73-387-59

I was also considering building this amp (according to the HF articles) until I heard about how many who had problems with the output stage oscillating and sourcing the output transistors.
could also have been due to the fqact that the output stage was connected to the main board with wires e t c...

Anyway, I have also seen a 200 W version of this amp with IRF 640/9640 in the output somewhere on the net, some years ago. ¨
I am almost 100 % certain I have the schematic printed (IRL ) on paper...

/rickard
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Old 16th September 2003, 12:49 PM   #9
Electrons are yellow and more is better!
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Do you mean this?
http://homepages.strath.ac.uk/~cnbp111/amp1.html

OPA277 can be something, not so expensive either.

The Alexander amp is a high speed design so I think the physical design is really important. No hard wiring!
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Old 19th September 2003, 05:21 AM   #10
BrianL is offline BrianL  United States
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Wow! Someone actually building the Alexander amp.
I'm waiting with baited breath to hear your reactions.

Patent: You need no permission from ADI to build this
amp as long as it's not "commercial". I believe that they
expressly give permission for any person to build one
for their own hobby use.

Servo amp: Low quiescent current is important as you
want the main op-amp to modulate as much of the current
in the current mirrors as possible. That's why the OP97 was
chosen.

Main op-amp: The SSM2131 was SSM's "improved" version
of the NE5534, designed for high end pro audio equipment.
If I were to build one of these, I'd consider an OPA627 or
AD8610, etc.

PCB layout: Alexander's paper has a very small and tidy
PCB layout; one board for the low-level stuff and the other
for the output devices. Also, good hints for physical layout
and routing of power wires -- same stuff that Doug Self
recommends.

BTW, "SSM" stands for "Solid State Music", a small audio-
oriented semiconductor company that PMI ("Precision
Monolithics, Incorporated", for you youngsters) purchased
shortly before ADI purchased them.

Does anyone know the wereabouts of Mark Alexander?
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