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Originally Posted by ppl
First off the Diamond buffer and the PPA was not and still is not designed for these fast op amps.
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Well... unfortunately not, but if(in the future) changes are done to allow for fast opamp implementation, the PPA will be a totally new level of the headphone amplifier. A simple search on the net for 'voltage-feedback amplifiers for audio' returns proof why so many engineers already look at them as a highest-quality audio solutions possible. The sound difference is REAL and BIG compared to jfet audio opamps and you know it too, Phil
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After all, why the CMoy amp was initially designed for using fast opamps like LM6171 and the higher-end PPA is restricted to slower opamps, shouldn't be the other way around?
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Originally Posted by ppl
There is no ground plane and this is mandatory for these chips
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Right, it's part of the recommended design consideration for any fast opamp running full-speed, and would be ideal for PPA+fast opamp combo, of course. But in our case we don't and shouldn't use the opamp's full speed(175mHz for THS4081), the mica 100pF cap is placed right on the chip reducing it's bandwith to 1.6 as Tangent said earlier(f=1/(2*pi*R*C). Therefore the bandwidth reduction plays the key role in preventing the chip from oscillating at lower gains.
Quote:
Originally Posted by ppl
the resistor values around the op amp and the input are several times larger in value than recommended in the data sheet
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The values of 1.3k resistors in the datasheet are just given as an example to demonstrate how to balance the offsets on its two inputs of the opamp and avoid the high output DC offset, as again, proved by Tangent in the article called "Working with cranky op-amps". The datasheet doesn't prevent us from using the resistor values which are more applicable to our case with PPA(varying input, buffers, biasing to ClassA, etc).
Using the Tangent formula r2=((r3*r4)/(r3+r4)) and the standard values taken from PPA
AND the datasheet the equation works equally: 4.3=((4.32*1000)/(4.32+1000)) or 0.6=((1.3*1.3)/(1.3+1.3).
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Originally Posted by ppl
These issues are classic feedback theory and just because a few examples worked that are in contrast to accepted feedback theory, is in no way going to make me recommend these unscientific misapplication of a given op amp as clearly spelled out in the data sheet.
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A huge deal of brilliant scientific discoveries were brought into the world by unscientifically misapplied and often crazy experiments, those very ideas that were initially disapproved and critisized as deviation from the commonly accepted theories and dogmas.
Quote:
Originally Posted by ppl
So what would make the IC buffers work with a given device and not with the DB is again layout.
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The bandwidth reduction cap is a good workaround until the layout is different. It now works great with both DB and IC, not scientifically approved though
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Quote:
Originally Posted by ppl
The input transistors have a Fy of over 400 MHz
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The THS4081 is 175mHz which is below the buffer's speed. There are even faster opamp chips, but with more problems using them, like bigger DC change, more power consumption, that one is fast enough already and can be successfully implemented.
The DC changes with volume and there's probably no cure for that. I measured around 260mV at full volume with r4=6.64k, and around 15mV at 9 o'clock. No one will likely be listening to PPA at full volume for a long time, but how may such relatively higher voltage affect the operability of DB in the long run? The channel output is the same, of course, since it's regulated by the DB and unaffected by the opamp change.