Is the EXAR XR-084 quad bi-fet opamp any good? - diyAudio
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Old 9th March 2012, 05:46 AM   #1
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Default Is the EXAR XR-084 quad bi-fet opamp any good?

My power amps all have balanced input stages that use EXAR brand XR-084 quad bi-fet opamps. Is this generally regarded as a good-sounding device or are there better-sounding upgrade substitutions? I think these are the only op-amps in these power amps.
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Old 9th March 2012, 05:47 AM   #2
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specs:
http://81.70.242.211/eab1/manual/Com...uct%20Guide%20[49].pdf
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Old 9th March 2012, 05:50 AM   #3
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It's a direct replacement for a TI TL084
same 14-pin pin-out as XR-3403 or LM324
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Old 10th March 2012, 06:51 PM   #4
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No opinions?
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Old 10th March 2012, 11:11 PM   #5
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A '084 is a lower-spec version of a '074, with the difference remaining somewhat mysterious.

Generally the choice among good quad opamps is quite limited. In terms of FET input, there's the OPA4132 (and its lesser spec cousin, OPA4134) plus the TLE2074. As far as bipolars go, there's LME49740, LT1359 and if you require significant output currents, MC33179.

I'd want to study the circuit a little further before determining what to use as a replacement... impedances at the opamp input pins, available decoupling, stuff like that. (Service docs would help.) Tossing an opamp into the circuit at random may result in instability or at least subpar noise and distortion performance. (DIYers like to do just that, and surprise!, they get random results.)

I'd expect a bal/SE converter among other things, an application in which even a '074 tends to be OK as it mainly demands good transfer linearity, though its voltage noise would still dominate the resistors'. The '2074 would make a good inexpensive upgrade then, a '4132 a better and more expensive one. Impedances tend to be highish, which means the '49740 with its fairly high current noise may actually turn out to be noisier than its FET input colleagues. Now the question is what they are doing with the second pair of opamps.

In any case one of the FET input models shouldn't be any worse than what's in there now at least. Note that they are about a factor of 3 faster though, so requirements in decoupling are likely to be stricter than for the old '084 and one should inspect the PCB layout in this regard.
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Old 11th March 2012, 12:57 AM   #6
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Quote:
Originally Posted by sgrossklass View Post
Generally the choice among good quad opamps is quite limited. In terms of FET input, there's the OPA4132 (and its lesser spec cousin, OPA4134) plus the TLE2074.
TI has the OPA1644 these days too. And don't forget the venerable old LT1058.
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Old 15th March 2012, 05:51 PM   #7
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I may just leave the stock opamps if they're generally regarded as OK; I'm actually impressed they used bi-fets considering the age of these amps. One early reviewer complained that the input section was noisier than the rest; I have no clue as I've never injected signal after. I always use the balanced inputs.

Thanks for the input. I have 9 of these monster amps now, and I like them much better than their old class-H amps. Early SMPS. I got my money's worth of 216 FET power output transistors in TO-3 packages alone!

I hosted the schematics:
http://mysite.verizon.net/gregorypinkowski/PR1800-schematic.pdf
The big power supply electrolytic caps seem OK on them all (the previous owner replaced them in one of the amps). I should probably replace the non-polar electrolytic coupling cap in each channel with something better (but cheap if I need 18 of them). Any other good ideas? Is it practical to consider changing the bias (and turning up the fans)? Is the output stage class B or class A/B now? What do you guys think of the amplifier sections (not power supply) in general? It sounds good to me, and has for 25 years; but there's always room for improvement. The seller confirmed very very low quantities of DC at the speaker terminals. This is good to know. But is it more a test of the power supply or the amp?

Last edited by cyclecamper; 15th March 2012 at 06:18 PM.
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Old 15th March 2012, 06:04 PM   #8
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Don't even think of changing the bias. The Exar IC's should be fine also.

The only changes would be to replace the coupling and feedback DC blocking caps. You might try putting film caps across the main filters but you will need at least 50 uf to get a significant change.
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Old 15th March 2012, 07:01 PM   #9
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According to the schematic, I wonder why the bias is only 0.4V across R26. The output mosfets are in Class B.
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Old 19th March 2012, 05:05 PM   #10
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I was going to say that it was a "professional" PA amp looking for high output and efficiency, but the PM1600 is the same exact schematic for the output section, it just deletes the balanced inputs, and was their top-of-the-line home hi-fi amp.

Of course if I increase the bias and try to put it farther into class-a I've got to upgrade the fans also, but they've never gone on 'high' in normal use.

Last edited by cyclecamper; 19th March 2012 at 05:13 PM.
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