Got my AVCC 2.1 Shunt Supply from TPA
THE SRA2.1 (“SHUNT REGULATOR AVCC 2.1”?)
(Update 12/2/12): Russ White commented on the design rationale behind the SRA 2.1. Check the comments section.
Superb workmanship and finish.
PREVIOUS VERSIONS
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 🙂
NEW OPAMP
According to TPA, the new opamp (OPA2209) is a better choice than the original opamp (LMP 7732)
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) |
COMMENTS |
| Voltage Gain |
130 db |
132 db |
|
| Input Impedance |
38 Kohm |
200 Kohm |
|
| Output Impedance |
“Low” | “Low” |
Data sheet has graphs but they use widely different units, so it is hard to compare. |
| Bandwidth (GBW) |
22 MHz |
18 MHz |
|
| Input offset (max) |
0.006 mV | 0.035 mV |
Seems the old one has an edge for these first 5 parameters |
| CMRR (typical) |
130 dB |
120 dB |
|
| Input Voltage Noise Density |
2.9 nV/sqrHz |
2.2 nV/sqrHz |
|
| Slew Rate |
2.4 V/usec |
6.4 V/usec |
Here the new opamp has an edge. This seems a critical component for good performance [link] |
| Phase Margin |
60 degrees |
80 degrees |
Here the new opamp has a slight margin for stability |
| Price |
~$3.53 |
~$4.29 |
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.
OPERATION
Output voltage
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.
Maximum Current
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 🙂
H i F i D U I N O 
I think you have the labels in the table above switched. as it is your comments beside dont make any sense =)
Hi qusp,
I think the labels are right. To tell you the truth, I can’t tell why one is better than the other when used in the shunt regulator. Can you provide some insight? Thanks…
Edit… The labels (in the table were right). the sentence above was wrong…
Hi glt,
Let me try shed some light on thing for you – at least provide the back story.
The op-amp is important – but only part of the picture. It cannot be looked at in isolation.
The two op-amps are internally implemented in two different ways. I wish I had realized how much so when I first designed the SRA2. 🙂 Fortunately for the community we have excellent dedicated users – one in particular did some excellent analysis (with equipment I would love to have access to!) that led me to where I could sensibly redesign the module. This is essentially the same redesign as we did earlier for Trident – only tailored for the AVCC role.
The previous op-amp is internally implemented and compensated much differently than the new one. In the SRA2 the old op-amp had a negative effect on slew rate especially at higher frequencies. It also had a detrimental effect on stability in some cases because of the reduced phase margin. 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.
Reasons for the revision that relate directly to the new op-amp:
1) it is more suited for the load we create by virtue of it’s AC performance.
2) It has a much broader supply voltage range making it far more robust.
3) It performs better when the output is sitting closer to a rail.
4) Voltage *and* current noise were nice and low while maintaining the above virtues.
5) When myself and few beta testers where checking it out – it just seemed a standout from many of the others we tried. This is the “How did it sound” factor I guess. 🙂
The SRA2.1 (Yes that stands for Shunt Regulator for AVCC 2.1) regulator circuit itself is pretty close to that of the SRA2 – with the addition of degeneration at the shunt elements and some external compensation at the error amp to make it really bullet proof in it’s designed application. The new version has an improved layout – you just get better at this over time. The new layout takes advantage of certain parasitic properties as a form of compensation/local feedback.
As I said in the DIYAudio thread – this is one one the times where I have to take my hat off to our excellent users. I would give the ones involved directly with AVCC/Trident public credit, but at least a couple of them I know wish to remain anonymous because of their industry involvement. They did not divulge any industry secrets to me, but better – they told me where to look for answers and offered to critically test prototypes etc. They used gear to test I can only dream of owning at the moment to measure things like phase margin and such. They also are good at critical listening – which is of course key. They taught me how to fish.
I hope that helps you understand the motivations for the updated version.
I personally was startled how much better the new version turned out. I wouldn’t have thought it probable, now I know better. It was a humbling situation to be certain. I guess we can all use a bit of that. It reminded me that I am an amateur – or rather a “hobbyist” . I do this because I love to – so I will probably always consider myself a hobbyist. It is users like ours that help keep me going. You included.
Sorry for any typos. I need an editor. 🙂
Cheers!
Russ
Wow!
Thank so much for taking the time and explain the rationale behind the SRA2.1. Just as some would experience greater satisfaction from a good looking piece of gear, I get more satisfaction by knowing the engineering behind a design :-).. so thanks again.
Oh and I think you have one small mistake above – it is the OPA-209(and 2209 for AVCC) that is the new opamp. The other is the old op-amp.
And – of course- you are quite welcome. 🙂
Also offset voltage is not a critical parameter in this application at all.
The most important factors are low voltage noise (we have a low source impedance) – followed by AC performance and robustness (immunity to things like output and phase reversals. The other opamp we checked into was the OPA211 which also worked quite well, but measured no better in the real world, and did not sound any better (at least not to us) either, but it cost quite a lot more. We later found that this was because that opamp is designed for even lower source impedance. It turned out the the OPA209 was actually very ideal in a number of ways.
We also tried the newer super low noise FET types like the OPA827 etc but found that while excellent for current noise they actually had more voltage noise. The OPA827 is a superb opamp, but not actually ideal for this application.
Hi Russ, thanks again for the information. When you say AC performance, what upper limit frequencies are we talking about?
Well here you get into limitations of copper traces – inductance etc. 🙂
At some point you must look at capacitive loads and the threshold at which the dominance of bypass caps takes over. I think we have found the sweet spot. Just listen for yourself. 🙂
If you look at the GBW of the OPA209 it is pretty much ideal for this application. keep in mind though the rest of the circuit effects the over all gain.
Also 20deg of phase margin is no small thing. But it actually works out more than that in the final analysis.
Cheers!
Russ
Allow me to test my understanding on the “AC” characteristics. The opamp is only used as an “amp” inasmuch as it is ensuring the output follows the input which is the reference voltage. So it is not really handling high frequencies, unless it noise is sneaking in through the components, where in such cases we want high PSRR and the gain going to zero if the noise comes through the input. I suppose this can be determined by simulations. I suppose also that the “optimal GBW” is to allow fast response to changing load, but not so fast as to respond to external noise. In any case, I think my original question was supposed to mean “what was the highest frequency noise disturbance simulated/measured” -if that makes any sense.
I’ll have to study this some more to get a good feeling… Thanks again..
You can think of the opamp as an inverting “error amp” which basically senses any difference between the output and the reference (due to load usually – but other factors as well) and applies an opposite correction to the shunt element. So you want an error amp that is fast enough for the load – but still stable and with good phase margin for capacitive loads. In our case the AC segment of the load is mostly in the audio band, with a very small component at ~1.5625Mhz (stock). The high frequency ripples are actually mostly handled by the bypass caps, but the shunt reg plays a part in smoothing that out as well. 🙂 So basically that is about the highest frequency we care about. As a complete circuit (which is more than just the error amp) the AVCC 2.1 does a nice job at both ends of the spectrum.
Very good info. Lucky me I ran across your blog by chance (stumbleupon).
I’ve book-marked it for later!
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