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Old 2nd December 2012, 05:52 AM   #11
agdr is offline agdr  United States
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LMH6643 amplifier
Originally Posted by jcx View Post
I don't know if a few cuts and jumpers would fix the "3-channel" silliness of the mini3 pcb
Possibly! Photos attached

Last year after the O2 came out I got a burr under my saddle to see if I could improve the mini^3 virtual ground by replacing the OPA6901D chip with a 200mA OPA551UA (surface mount). Turns out with only a couple of cuts and jumper the OPA551 just fits right in the existing mini^3 PCB, as that photo shows.

In addition I converted the two point virtual ground (signal and output grounds separate in the mini^3) to a single point virtual ground, which turned out to be fairly easy too. In the photo I replaced the two electros pinning the VG with two 1uF mlcc caps. And on the back added that diode I wrote about above, on the negative battery lead, to make it a balanced circuit with respect to the new ground reference point (the VG).

Another benefit that came for free was that although the AD8397 output chip doesn't have current limiting, hence another reason AMB added those 6.2R output resistors to keep the TRS shorting from frying the thing (the LMH6643 does have current limitng, BTW, so 0.5R is OK), the OPA551 does have current limiting. And since the VG puts it in series with the AD8397, in theory at least the whole thing is now current limited against TRS shorts! I haven't tried to test that theory (yet) with multiple output shorts, but the OPA551 limits at around 220mA and the AD8397 is good for 310mA peak, so the potential is there at least.

Since the circuit was now current limited against output shorts, I also modified that PCB to have 1R output resistors instead of the standard 6.2R, to improve damping, and re-wired the feedback loop on both channels with a cut and jumper to take off before the 1R rather than after, so that the output resistor is no longer in the feedback loop.

All of this greatly cleaned up the simulation FFT (modified vs. original circuit), which was interesting. But when it came to real-world listening, of the modified version vs. a mini^3 I have here built the standard way, zero difference to my ears. Both sounded great. So although the mods probably reduced crosstalk and distortion, whatever amount it was is apparently below my threshold of hearing. And my philosophy is always along the lines of "if I can't hear it, then it doesn't matter".

AMB knows about my experiments.

I'm off for the holidays. Have fun!
Attached Images
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File Type: jpg IMG_1202.JPG (141.8 KB, 120 views)
File Type: jpg IMG_1194.JPG (196.6 KB, 120 views)

Last edited by agdr; 2nd December 2012 at 06:13 AM.
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Old 3rd December 2012, 10:13 AM   #12
croccodillo is offline croccodillo  Italy
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Location: Bergamo
Hello all, and thank you for your answer.

I was out for the weekend.
Last days I focused on the amplifier supply stage, and decided few tings:

1. I really need to charge it through USB: I have USB charger all around my house, my car, my office; even more, I have PCs all around me that can be used as battery chargers...
2. I want to use Li-ion batteries, they are the future, last forever (if correctly used), and you can find really cheap batteries from mobile phone world.
3. My instinct says me to do not use a VG, so I decided to go with REAL ground.

So, these are the solutions I have thinked about:

1. Real dual batteries, with dual battery charger.
You need two isolated 5V supply, two charger ICs, and a circuit to cuut-off everything if one of the two batteries disconnects itself from the circuit (I assume I would use self protected batteries).
This solution is quite complicated.
I know I can charge the batteries in series with a single charger, as for AMB X1 battery stage, but in that case you cannot balance your battery cells, which is really dangerous and will shorten their life.
For two batteries, you really need two chargers.
Click the image to open in full size.

2. Single battery, voltage doubler.
Quite simple, you only need a beefer battery (not a problem, nowadays).
Click the image to open in full size.

I would go with the second circuit, cheaper, simpler, smaller.

Third possibility is to develop a transformer coupled, dual output DC/DC converter from scratch.
Not too hard to make it wok, really hard to make it work fine with an audio amplifier.

What do you think?
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Last edited by croccodillo; 3rd December 2012 at 10:21 AM.
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