Got my AVCC 2.1 Shunt Supply from TPA
THE SRA2.1 (“SHUNT REGULATOR AVCC 2.1”?)
Superb workmanship and finish.
Here is the “SRA1” version that was bundled with the Buffalo II DAC (the one I have)
Here is the “SRA2” version that was available prior to the introduction of the AVCC 2.1. This “SRA2” version has been available for quite a while, at least since the the BII switched from an 80 MHz clock to a 100 MHz clock. Notice the thicker traces and different circuit topology (gone are the n-channel transistors QN1 and QN2)
Here is on a BIII
Here is the “SRA2” on a BII-100 [link]
Boy, my AVCC is two generations old!, and I didn’t even know it. Maybe because it has been hiding on the bottom of the DAC board🙂
Lets start with the “ideal opamp” characteristics:
- Infinite voltage gain
- Infinite input impedance
- Zero output impedance
- Infinite bandwidth
- Zero input offset voltage (i.e., exactly zero out if zero in).
|Parameter||LMP7732 (Old)||OPA2209 (New)
||Data sheet has graphs but they use widely different units, so it is hard to compare.
|Input offset (max)
||0.006 mV||0.035 mV
||Seems the old one has an edge for these first 5 parameters
|Input Voltage Noise Density
||Here the new opamp has an edge. This seems a critical component for good performance [link]
||Here the new opamp has a slight margin for stability
The comparison above does not give a clear indication as to why the new opamp is superior to the old one. I am sure the enhanced performance is in the circuit design and choice of component values. Without doing a circuit simulation, and testing/measuring, is is not possible to say which one is better. We trust, however, that TPA has made the right selection for opamp.
Indeed, as Russ has commented,
Bottom line is that the old op-amp was excellent on paper (and probably for other applications), but was not so good for AVCC in practice. It was also strangely finicky, meaning it could work perfectly usually – but occasionally be upset just by changing rail voltage or applying the right kind of external stimuli or load.
The output voltage of the AVCC starts at 3.6V. After the LEDs warm up, the output voltage settles at around 3.56V. This is typical of regulators that use LEDs as references. Input voltage is 6.2 V. The load resistors for the test are 72 ohm. The output current is therefore 3.56/72= 50 mA.
At first I had use a 33 ohm resistor. This load was pulling 108 mA and the voltage dropped down to about 2.5V. The regulator cannot source more than 100 mA (as specified).
Compared with the V1 (shown below), the LEDs are evenly lit.
HURRY AND UPGRADE
Original Buffalo IIs (the ones which came with the SRA1 AVCC supply) should benefit the most from the new AVCC 2.1 because both the circuit configuration, layout and opamp has been updated/upgraded. NICE!
If upgrading from the later 2.0 version, the layout, opamp has been updated/upgraded and the circuit has been tweaked with the compensation capacitors. Nice upgraded but not as nice as upgrading from the original BII🙂