|Zarcon Dee Grissom's Idea Page|
|Original shot||Adjusted feedback loop||The M.B.||PCB sim|
|Further testing reveals that the 10pF capacitor reduces the audio gain starting at about 16kHz through about -0.00025db at 20kHz. Some audiophiles may complain about that, and in agreement with Tangentsoft a slightly smaller capacitor like 8.2pF through 4.7pF would remedy that. I am not concerned about such a minuscule high end reduction, given that most sound cards and radios are infinitely worse in there high end frequency characteristics. The smaller capacitor reduces the RF tolerance of the circuit, so if I ever reduce the capacitor, a 6.8pF capacitor is the best for my requirements.|
|The input pins on the OPA2134 have a nominal impedance of ten tera-ohms (10E13 ohms), the terminators are 470 kilohms (47E4 ohms). Guess which one would fare better at the ends of the trace, errr microscopic straight-wire antenna. With the lower impedance devices nailing down the ends of the trace, the ultra high impedance input pin is better protected from transient effects in between the other lower impedance devices. The same goes with the eight Meg-ohm input pin of the buffer on the Interstage trace.|
|Know thy circuit, before attaching guard rings. Guard Rings can limit stray voltage effects, provided the circuit there connected to is capable of handling the load. Ideally in a non-inverting amplifier, the guard ring would be connected to the negative input pin of the op-amp (The feedback loop). In this amplifier the feedback loop trace would get dragged all over the place by the guard ring, causing all kinds of adverse effects. In this amplifier, the ultimate place to connect the guard ring to is directly to that channel's Vref. Unfortunately there is a ground plane and/or a power trace in the way. So the only stable place to connect the guard ring to without killing the ground plane, is the output pin of the op-amp. It is not perfect, however it is far better then the alternative.|
|The non-reflective Grounded Coplanar Waveguide, can prevent waveform reflections from adding up at the device input. If implemented correctly with a slight gradual increase in trace capacitance and a slight gradual decrease in impedance at all the device pins. These spots in the length of the trace can absorb some RFI energy before it gets to the device input and prevent the RF from reflecting off the device pin back down the length of the trace.|
|I didn't have a choice with the input decoupling capacitors, given the source of power. I went with the ridiculously large farad values to get the majority of the capacitors frequency dependent phase delay out of the audio band. I decided to go with the decoupled output rather then a floating ground for many reasons. The doubled up output capacitors are to reduce there total ESL and ESR effects on the output signal.|
|The on board capacitors are more for decoupling and power reserve, then power supply filtering. With the large electrolytic capacitors on the board, the chips are not limited by what they can get through the inductance of the power wires going to the board. I wanted enough reserve power on the board to keep the voltage steady through whatever draw the buffers may pool. And again, the multiple electrolytic capacitors are to limit there total ESL and ESR effects.|
The forward diode between the P-amp Power and the Op-Amp Power (OAP+), is to limit any voltage fluctuation at the op-amps caused by the draw of the buffers (or bus fluctuations). The reverse diode is to prevent the Op-Amp from driving a voltage significantly higher then the buffers V+ into the buffers input pin.
|Remember, in this amplifiers world, everything is in reference to ground, from the input, to the output. This in combination with a lack of guarantee that the power voltage will remain the same under all circumstances, a voltage divider is the last thing I want in this world. I don't want a mid point, I need a fixed voltage reference (Vref) relative to the negative rail (Ground). This followed by the need to gradually rise the Vref during power up, to prevent a surge of power flowing out the input and outputs. I also didn't want the feedback loops and terminators of each channel, effecting the other channel's Vref. Hence the independent Vref buffers for each channel.|
|The ground plane is not just a means to return current to the power source, it also shields the sensitive traces on the underside of the PCB among other things in this amplifier.|
Full MB schem.
Larger Schem T.B.D.
|Home > Ideas > HPA17r16, Intro.|
HPA17r16 Operating Environment.
HPA17r16 RFI filters.
The Main Amplifier Board.
HPA17r16 Vmon & Sources.
HPA17r16 Tests & Afterthoughts.
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