The Burr-Brown sound? - diyAudio
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Old 21st April 2010, 08:16 PM   #1
Moset is offline Moset  Sweden
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Default The Burr-Brown sound?

I've just finished piecing together a preamp for my LM3886-based chipamp. It uses relays to switch inputs and the volume is controlled by an LM1972 followed by an OPA2134 at a gain of about 7db (I wasn't able to run it at unity gain reliably - kept spiking awfully on turn on/off).

It's been a while since I used this amp, but after adding the pre the sound is remarkably different, at least from what I can remember. It's also different from my other, LM1875-based amp with a passive pre. The sound's much.. softer. Highs, like snare drums and cymbals, which I usually find pretty harsh, are very smooth. Perhaps a bit smoother than they ought to? On the other hand, the sound overall is really, very pleasant. Strings and pads sound awesome, for example.

Now, the OPA2134 datasheet mentions impedance matching to minimize distortion in non-inverting applications (which the LM1972 requires, iirc). The LM1972 has a variable output impedance between 25k and 35k. I was aiming to keep the feedback network at about 30k but during testing I ended up at about 18k (33k and 39k paralleled). Could this mismatch increase the distortion enough to cause the sound I've described above, or is it simply "the Burr-Brown sound"?

Also, if anyone has any other comments on the importance of impedance matching of the OPA2134, I'd be interested in hearing about that as well.
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Old 24th April 2010, 06:01 AM   #2
magick is offline magick  Serbia
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Impedance-mismatch crossed my mind while I was still on the second passage...

I don't have much experience with particular devices but I can assure you that impedance (mis)matching can significantly influence frequency responce and alter the "sound".
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Old 24th April 2010, 07:35 AM   #3
Mooly is offline Mooly  United Kingdom
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Think you have something more than just the "Burr Brown" sound going on here.

The only real way to tell, is do some tests with a 'scope and look how the amp behaves.

Does the sound change if you change the opamp for something else. Try something like a TL072, an NE5532 and an old 4558 (really) and see if the same listening impressions are there.
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Old 24th April 2010, 10:12 AM   #4
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The OPA134 does not like to be loaded hard. See the data sheet and also the work of Samuel Groner. He made an in debth comparison of a lot of popular op amps.
Do you realy think that Burr-Brown tries to make a "home sound" ?
I do not think so. I asume that they try to make their Opamps as good as they can in a technical sense.
P.S. D.Sef measured the OPA134 too and he found raised treble distortion when loaded down to 600 ohm. Also if you have a mismatch in impedance of the feedback resistor and the source impedance you end up with common mode distortion in this OP in the case of a non inverting series feedback arangement.
The OPA827 and ADA4627 are in improvement if you want to use Fet input.
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Old 24th April 2010, 10:14 AM   #5
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Sorry, it was Douglas Self. Samuel Groner has a homepake where you can download his
survey.
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Old 25th April 2010, 09:57 PM   #6
Moset is offline Moset  Sweden
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Great, more fiddling. One of the vias on the board is getting a bit tired from me de-/soldering parts to it, so I wasn't too keen on more testing if nothing seemed out of order. Will now, and grab a hold of some higher precision resistances as well, while I'm at it.

The OPA should not be loaded down by much. The chipamp.com amplifier boards I'm using have an input impedance of about 23k (if I'm reading the schematics correctly), well above the 600 the OPA has problems with.

I'll try to get a hold of an oscilloscope and see if I can measure the output with the current (~18k) and more appropriate (~30k) impedance in the feedback network. Hopefully the latter will prove sufficient, given that the LM1972 has a variable output resistance. Would hate it if I should have to compensate this in the feedback network. (Though it would be interesting )

What would be useful tests? Square waves at a few different frequencies? Sines?
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Old 25th April 2010, 11:22 PM   #7
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Try to repair the via with a touthpick or bore out the hole with a 0.6mm syringe.
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Old 26th April 2010, 06:32 AM   #8
Mooly is offline Mooly  United Kingdom
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Quote:
Originally Posted by Moset View Post

What would be useful tests? Square waves at a few different frequencies? Sines?
Look round about post #1017
The best sounding audio integrated opamps

You have to measure and test all the time. Squarewave testing is extremely useful and shows up many problems.
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Old 26th April 2010, 10:39 PM   #9
Moset is offline Moset  Sweden
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I've no equipment to do any signal testing, apart from a simple multimeter. I did some simple loopback tests with my soundcard but the results were utter garbage. I'm looking at picking up a Tektronix 2215 second-hand (looks like around $100). Should be more than sufficient for my needs, I presume?
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Old 27th April 2010, 06:06 AM   #10
Mooly is offline Mooly  United Kingdom
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The 'scope should be fine. You need a good signal/function generator too.
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