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The Opamp thread

Sep 17, 2010 at 1:32 AM Post #3,136 of 7,456
only OPA228, OPA627, OPA637 and AD744 of those respond to class A bypassing IMO (try out THS4031 as well). some of the others actually are harmed by it, as they mostly operate in class A anyway and I really dont see how there can be such a thing as a class A biasing socket, surely the amount of resistance/bias needed should be modified for each chip you use....
 
AD797 is tricky, you cannot just drop it into a circuit and expect it to do well, because of its very wide bandwidth and ultrafast nature, it needs bandwidth limiting, compensation and very good supply decoupling to work well in the audio band and to judge it without doing these things really isnt all that valid IMO. if in a circuit well suited to it, the AD797 is one of the best sounding opamps out there IMO, its one that I keep on coming back to to revisit. 
 
you should definitely add OPA1641 and OPA827 to your list. these babies are pretty much perfect IMO and I am also of the jfet input opamp loving club. along with some of the CMOS opamps, but watch the bandwidth
 
and if you are up for a bit of balanced opamp love, the newish THS4150 is sweet
 
Sep 17, 2010 at 10:01 AM Post #3,137 of 7,456
As you say - I have seen substantial reports about the AD797 and it not being a drop in replacement.... Hence my question as to whether it will work in the Matrix M-Stage circuit.
 
The Class A mod has been documented in a number of places around the web, and involves soldering 2 resistors between 2 sets of 2 pins...
 
Like this:

By doing it on a socket -  you reduce the risk of damage to board or opamp - and it is easily swapped in and out
You can then also have seperate sockets for different biasing currents - so you can try things out easily.
 
Finally for those who haven't touched a soldering iron in 15 years (like me), some of the vendors are providing the mod sockets at minimal cost (like coolfungadget.com )
 
I have a spreadsheet in which I have been recording peoples comments about different opamps for reference - this will help me to choose likely ones to try first, and possibly also which ones will work best with the Class A mod and which biasing level may work best.
 
Recomendations for OPA627 seem to be minimum additional biasing of 2mA and no more than 7mA - primarily due to heat (duh... class A right!)
 
But if the opamps are singles rather than duals and mounted on a side by side adapter - a heatsink may allow to go 7mA safely.....although I don't know whether there is anything to be gained thereby.
 
I've had reports that the HA3-2525-5 is particularly good in the M-Stage
Also reports that the LME4562 (and relatives) get some benefit from the class A mod, but minor
I've noted the OPA1641/827 on my list..... lets see how I go with my current collection though.
 
My objective is to achieve a "Wow that's a real improvement" effect over my current options. (Onkyo receiver onboard NE5532+buffer headamp, Firestone Cute Curve+OBH2 PSU, ART Headtap connected to Onkyo speaker outs)
 
 
 
 
Sep 17, 2010 at 10:48 AM Post #3,138 of 7,456
Class A biasing with a resistor is silly IMO.  The current draw with a resistor across the output of the opamp will only remain constant if the voltage output is constant.  Because music consists of rapidly changing voltages, than you will have constantly CHANGING current draw.  Not only that, but you're also wrecking the output impedance of the op-amp.  It works (sorta), but there are better ways (though none as simple).  Tangent's website has information about biasing opamps into Class A.
 
EDIT:
Here it is: http://tangentsoft.net/audio/opamp-bias.html
 
Sep 17, 2010 at 11:25 AM Post #3,139 of 7,456
Yep read that a few days back....
 
obviously in a perfect world I would study electrical engineering and design my own perfect amp..... but in my imperfect world, with pockets of limited depth, and a lack of soldering skills, the trick is to find a good base design, and then optimise it as best I can...
 
Concensus is that with some opamps (OPA627 being a prominent example!) there are tangible improvements to be gained from the crude resistor method (notwithstanding the flaws)
 
So my choice is to select a good basic design (Matrix M-Stage / Lehmann BCL Clone) and use the optimisations different people have tried to achieve something closer to my own interpretation of perfection.
 
I'm hotrodding a sedan, not creating a F1 racing car....
 
Sep 17, 2010 at 11:54 AM Post #3,140 of 7,456
You could do that same trick with the IC socket but use a CDR instead of a resistor.
 
Quote:
dlaloum said:
Yep read that a few days back....
 
obviously in a perfect world I would study electrical engineering and design my own perfect amp..... but in my imperfect world, with pockets of limited depth, and a lack of soldering skills, the trick is to find a good base design, and then optimise it as best I can...
 
Concensus is that with some opamps (OPA627 being a prominent example!) there are tangible improvements to be gained from the crude resistor method (notwithstanding the flaws)
 
So my choice is to select a good basic design (Matrix M-Stage / Lehmann BCL Clone) and use the optimisations different people have tried to achieve something closer to my own interpretation of perfection.
 
I'm hotrodding a sedan, not creating a F1 racing car....
Click to expand...



 
Sep 17, 2010 at 10:30 PM Post #3,144 of 7,456
Sep 17, 2010 at 10:58 PM Post #3,145 of 7,456
Hah I should have read further in the Tangensoft article - I stopped at the initial JFET stage..... looked at it - thought too hard, too much soldering involved and got no further!
 
But as you say CRD's (see how easily that rolls off the keys now I know what it means!) would be no more work than resistors....
 
There is a comment in there about input capacitance - anyone know what the impact of the input capacitance on sound would be?
 
So searching through google for CRD info...
 
What CRD's would allow for Biasing currents of between 2mA and 7mA .... @ 15V - target is likely around 4mA but I would like to be able to compare various options...
 
I am seeing various devices - and there is talk of "max limiting voltages" fwd current, etc... (gobbledygook to me)
 
It looks like the Vishay Siliconix J508(2.4mA), J510(3.6mA), J511(4.7mA) might fit the bill - but they are also marked as max limiting voltage of around 4V - what does this mean
 
Had anyone tried this approach?
 
Sep 19, 2010 at 7:44 AM Post #3,146 of 7,456
I wondred how LT1364s would sound with my AKG 702, So I pop them in my I/Vs, Sound good execpt The Piece Highs ruin it. But other then that I liked them. Just wish they didnt make my highs sound like that thru my AKG702s. So I in up poping 2x OPA2137P's back in.
 
I really wanna get ahold of 2 Sets of AD797s in the future and try them with my AKG 702s.
 
Sep 19, 2010 at 6:27 PM Post #3,147 of 7,456
You probably won't find the highs so piercing with 797 indeed.
 
Sep 21, 2010 at 6:17 PM Post #3,148 of 7,456
First post on the Opamp here. I need some help here i have a choice of 
 
(2 single Opamps) opa134, Ne5534
 
(Dual Opamp) 
 
ad823, ad827aq, opa2604, ne5532 
 
 
 
thanks a lot
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Sep 21, 2010 at 6:56 PM Post #3,149 of 7,456
Not that I am an expert but...
 
All things being equal 2x singles are reputed to give better sound.... they also have potentially better heat dissipation if you are running in class A - allowing them to be biased further into class A potentially.
 
On the other hand if you are swapping into a pre-existing circuit with a spot for a dual, then consider that putting a dual in avoids additional adapters/sockets and extended circuit traces with possible resulting reductions in audio fidelity.
 
Personally I would opt for the twin singles - I think that the gain might be greater than the loss..... but results do vary and the "suck it and see" method is always best
 
Sep 24, 2010 at 4:00 PM Post #3,150 of 7,456
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