ppl
Building amps and assuring water resistance.
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- Jun 21, 2001
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I have been an advocate of using Constant current Sources to bias an Opamp driving a Buffer into Class A for over a decade. The Advantages is to remove the Possibility of Crossover Distortion From the Class b or Class ab Opamp output stage. Some Opamps like The AD-845 Have a Class A output stage and as sutch will not be improved by this technique. Some Opamps notably all the Most used Burr Brown Types used by Head-Fi'ers show a dramatic improvement with this method. My most often used method of suppling this Constant current is via one Of the Current regulator Diodes. or using a Jfet with the Gate and Source connected together and this connecting Between the Vee (Neg. Rail) and the Opamp's output. This works alot better than the simple resistor most folks use to do this as the CRD will not load the Opamp's output stage with a low resistence from a Resistor and thus removing the Dynamic Benifits of using a Buffer. This method while Great is still not the Best way. The Nonlinear capacitence of the CRD or j-FET will have a negitiive impact upon the sound quality of the Opamp. I have got around this in the Past by Placing a resistor in series with the CRD and the Opamps output to Buffer this capacitence. Since most of my recent amps have been battery powered The Possibility of the CRD droping out of regulation on large signal swings is a possibility. I have thus been testing Current sources made from cascoding Two J-FET's. A cascode circuit has a constant capacitence and Leakage current with voltage. This may at first seem a minor Point unless like Me you Think god is in the details. and if your search for the Holy grail in sound quality from an Opamp based Headphone amp I cay say without reservation The Cascode Current source is so mutch of an improvement over a Current regulator diode in sound quality that I was Most impressed. The most notable improvement is in the Midrange upward. My first Impresions using an OPA-637 Driving a stack of EL-2002's were staggering almost like another Otave was added to the Reproduction and this changed what is normaly a slightly dark sounding Opamp with a Normal CRD into a High and Mid frequency sonic Wonder. All BB OPA's without any Resistor or CRD sound Dark and vailed to me. using a 5K resistor strips all the dynamics away, while still only providing trivial improvements. replacing the resistor with a resistor Bufferd CRD brings the dynamics back while almost Compleatly removing the vail over the Sound image. Replacing This with a cascoded low capacitence jfet pair made this dark sounding opamp into a nice and detailed Performer. This same improvement happens with AD Opamps like the AD-825 & AD-823 also. Infact the sibelense sometimes present on the AD-825 is dramiticly reduced.
In conclusion If your going to get the best sound out of your Opamp Buffer Combo cascode a Pair of jfets like the MPF-103 or a Lower capacitence type and enjoy a sonic improvement You will find hard to resist.
In conclusion If your going to get the best sound out of your Opamp Buffer Combo cascode a Pair of jfets like the MPF-103 or a Lower capacitence type and enjoy a sonic improvement You will find hard to resist.