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Old 16th June 2002, 08:20 AM   #21
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Default Re: The Ear Brain is far more Accurate than Given credit

Excepting Low impedance loads within the Audio range like Headphones But It is my posistion that Opamps should be Buffered by a Dedicated Buffer IC like the BUF-634 or a Discreet component Alternative, However since the gain stage is Monolithic then why not Keep the concept similar in the output stage. [/B][/QUOTE]

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With BUF634/AD825 combination in a DAC output stage (Assemblage DAC3), I have found it very desirable to load the 825 at 2 mA rather than 1 mA. The treatment removes a long tyerm problem with the hf side but surprisingly also improved bass resolution.

I intend to gradually increase current with future mods whilst keeping thermal problems under review to see what happens.

Incidentally, Vishay Siliconix FETs have mildly magnetic leads. Some recommended in AN103 note seem to be rather temperature sensitive.
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Old 22nd June 2002, 09:24 AM   #22
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Default current loading etc

Hi fmak,

Sorry if I mis-expressed myself. I do not mistrust hearing! What I mistrust is the reliability of doing listening tests to determine optimum opamp current loading by varying it from 1 through 1.1, 1.2, 1.3 etc and at each step trying to decide whether it is better or worse than the previous step. I was offering a shortcut, assuming that you want to run the opamp in its linear mode, ie class A, which you actually confirmed.

Your explanation why 2mA would be best really doesn't mean anything to me, so I can't comment on that.

Cheers, Jan Didden
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Old 22nd June 2002, 03:57 PM   #23
ppl is offline ppl  United States
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Default i'll Tell Ya what I Think!

When loading an opamp with an active Current source I use from 0.5-1.5 Ma on battery Operated Headphone Amps in order to get as mutch of that Class A Sound as possible while still maintaining a Resonable ampunt of battery Life. I mean you just can't use A battery Amp that will run for only 3 or 4 Hours it's just not Practical, but then again you still want as Cleen of sound as possible so a lightly loaded Opamp sounds alot Better than No loading and futher Increces in Current Improve the sound up to about 1/10 Th the Nominal Output Current of the Opamp so in the case of an opamp spec'ed at 50 Ma of output Current Improvement is possible up to 5 Ma of Curent load. above that the opamp is starting to strain and more often than not start to sound worse at currents above 1/10.



Resistor Loading Only is not what I recomend because the Amount of current is Dependent upon signal. this is going to create Thermal Distortion in some Older or less than Perfect Opamps. In addition the Resistor method kind of defeats the porpose of Current loading and Active Buffering because of the Changing Curent Demands placed upon the Opamp with Signal. I can fully understand why fmak could tell the Improvement offered by the CRD over just resistor loading.

Buffering the CRD with a Resistor removes the nonlinear capacitence of the CRD whitch will Change by the amount of rev. bias placed upon it by the Signal. The sonic effect of the resistor is an apparent improvement in speed and delineation of Detail. this resistor should be as large of value as possible consistent with Voltage drops you dont want the CRD to drop out of Regulation. (See min Voltage complience spec of CRD then add the drop from the resistor at max. signal) A good way to tell if the crd is maintaining a constant curent under all output levels up to the Clipping point is to monitor the DC voltage across the Buffering resistor with a DMM able to read DC in the Presence of AC. if the Voltage remans constant the Design is good.
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Old 23rd June 2002, 06:46 PM   #24
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Default current loading etc

Right on, PPL!

Cheers, jan Didden
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