Is the EXAR XR-084 quad bi-fet opamp any good? - Page 2 - diyAudio
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Old 19th March 2012, 05:14 PM   #11
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I don't thoroughly understand some parts of the schematic.

Is the R20 pot some kind balance between the push and pull sides?

Last edited by cyclecamper; 19th March 2012 at 05:17 PM.
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Old 19th March 2012, 06:37 PM   #12
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R20 (500 ohms trim pot) is used to bias the power mosfets. By turning this pot, you can bias the amp into Class AB.

There's no necessity to upgrade the fans. Sensors on the heatsinks automatically adjust the fan speed.
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Old 21st March 2012, 12:32 AM   #13
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http://mysite.verizon.net/gregorypin...-schematic.pdf

.4 v across 180 ohms is only .002 amps.

There's a zener and a diode on each side CR12/13/14/15; are they also making it class "B" making the drivers only work on half the waveform, or are they just setting bias for the drivers?

Last edited by cyclecamper; 21st March 2012 at 12:35 AM.
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Old 21st March 2012, 04:13 AM   #14
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Yes, 0.002 amps (2mA) is very low but that is not the main issue.

The power mosfets need about 1.5V to switch on. So, +1.5V for the top mosfets and -1.5V for the bottom mosfets, that makes 3V across the 180 ohms. At 0.4V, the mosfets are off when the amp is idling, hence in Class B.

The zeners and diodes CR12/13/14/15 are not for biasing. They are there to prevent the mosfet gates from exceeding the maximum limit, I believe +/- 20V. They clamp the gate voltages at +/- 12V.
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Old 21st March 2012, 05:57 AM   #15
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Quote:
Originally Posted by abraxalito View Post
TI has the OPA1644 these days too....
and its superb!
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Old 21st March 2012, 06:01 PM   #16
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Micheal Chua, thanks for your help. I wonder what the range of that adustment pot is (in bias volts)? One way to find out...

I have 9 of these amps now, 18 channels...and despite inefficient speakers (sealed isobaric and planar dynamic) I often run them at low average power but I use lots of EQ, dynamic range expanders, and devices that make "fake" sub-bass, all of which make large occasional demands. The sound of that "first watt" is certainly important! But then I need a lot of overhead! Sometimes I also use them for live-performance music, which also has extreme dynamic range (at least the way I do small PA).

So it's actually got both sides practically off completely for some small values very near the zero-crossing? I guess that's just another way of saying "class B"...I thought in all real implementations both sides would be on a little bit.

OK the current across the resistor isn't important (DOH!) If I increased bias to 3v across the resistor, approx. how many watts do the outputs and related resistors have to dissipate due to bucking each other? Then does the linearity suffer at some higher signal level where the push and pull stop bucking each other and it goes back to being class-B (more efficient and also running in a better-performing part of its curve)? Or does the feedback loop make up for those ills?

I also notice that the paralleled MOSFETs are connected directly to the rails and to the "S" output without any 'current balancing' resistors, so I assume the FETs must be matched reasonably well?

I'm still an idiot looking at transistor schematics, instead of the few tube circuits I understand.

Some day maybe I should build a small hut outside the house, and install the amps in a rack with a large furnace-type air handler to depressurize one side of the entire rack, and a big air filter on the air input side. And just run the balanced signal lines and speaker wires thru the wall. I used to have them in the garage in the old house, with dedicated power lines and remote relays to turn them on and off.
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Old 22nd March 2012, 12:15 AM   #17
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Originally Posted by cyclecamper View Post
OK the current across the resistor isn't important (DOH!)
No, it is important. Just that the voltage across the 180 ohm takes precedence. If the gate voltage is not high enough to switch on the mosfets, the current of the drivers Q10, Q11 doesn't even come into the picture.

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I also notice that the paralleled MOSFETs are connected directly to the rails and to the "S" output without any 'current balancing' resistors, so I assume the FETs must be matched reasonably well?
Yes, if no Source Resistors are used, the mosfets must be Matched.

Quote:
I wonder what the range of that adustment pot is (in bias volts)? One way to find out...
Before you start adjusting the bias, I suggest you measure the voltage across R14 (0.27 ohms/5W) first. The voltage drop, if any, across R14 will tell you whether the mosfets are biased up and if so, by how much.
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Old 22nd March 2012, 12:59 AM   #18
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Measure voltage across R14 with no signal?

Interesting that that point CSA goes back to the power supply...near the "V0 trim" and "VTH Trim" pots. Is this protection circuitry or remote-sensing for the regulated supply?

Looking at the device spec sheets, what's the critical "turn-on voltage" spec called? Is that "Gate-Source Cutoff Voltage"
min -0.15 max -1.45 V for 2SJ56
min 0.15 max 1.45 V for 2SK176
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Old 22nd March 2012, 01:01 AM   #19
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Should I adjust pot for 0 V DC at the speaker terminals after replacing big power supply caps?
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Old 22nd March 2012, 01:04 AM   #20
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What does "VTH TRIM" likely stand for?

Sure are plenty of heat sensors on each of the 4 heatsinks and transformer.
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