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Pro Audio Asylum Pro studio recording equipment and music production/industry. |
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In Reply to: Best TL-072 op-amp replacement posted by donnie7 on April 8, 2002 at 21:52:26:
No simple answer to this one, I'm afraid. -However I've worked on a bunch of Ameks over time, and I might have a few ideas...In different applications, the various op-amp options have different advantages & disadvantages. -For ordinary garden-variety audio use, an NE5532 will replace a TL072. The NE5532 is a dual version of the NE5534, but without the 5534's external compensation ability. I think that the gain of the individual device might be an issue, since this has an effect on unity gain stability. -Also, the 5532 is a bipolar input device, the 072 is a JFET input device. A number of devices use 07x series amplifiers in filter circuits (EQ stages, hi/lo pass, x-overs, and so on) and 5534s elsewhere.
In Some manufacturers' designs (SSL for example) there are fusible resistors in the power feeds to any circuit which has a bunch of op-amps in it. If you switch to different op-amps that -for example- consume twice as much current, you'll want to adjust the value of these resistors accordingly. (replace with half-resistance resistors if it drinks twice the juice!)
An 07x (e.g.071, 072, 074) has slightly better figures than an 08x, (081, 082, etc.) and the 05x series is a low current version, (for battery powered applications) -You might like to try the Burr-Brown OPA series. -My advice is to get a few devices from each type, and make your own choice. -Do part of only one channel with each device, and make very certain that you carry out your testing under a variety of conditions. Here's a few examples of some things you might like to try out:
1] Extremes of gain setting for input amplifiers. (e.g. =20dB and -20dB for line... or +/-10dB if that's all the range your particular console allows.)
2] Extremes of cut/boost in the EQ stages, and prehaps rather more importantly, extremes of bandwidth ('Q') control settings, if your console EQ has them. -This is actually the true 'gain' of the EQ filter stage, whereas the cut/boost control is merely the amount of like-or-unlike polarity signal is fed to the filter circuit to be re-summed with the original signal.
3] Output drivers might react quite well with a new chip on the bench, but suddenly become wildly unstable when presented with a reactive load. -This actually happened with the early Amek BC2 (broadcast) colsoles. -The output drivers are a cross-coupled (I call them "push-me, pull-you") pair of op-amps, the idea being that they *sort of* simulate a transformer, in that you can tie either leg to ground and the other leg will compensate by swinging twice as much... Anyhow, under certain circumstances, driving long lines (we're back to cable choices here!) they tended to oscillate just over 1MHz. -Not a problem, you might think, but when three or four drivers start oscillating around the same frequency, the difference frequencies started weaving in & out of the audio band... -really not very funny at all! If you're using the board for sound reinforcement and driving long lines, you might want to scope the outputs *with the ones you're going to use connected to the console* before you decide on a winner!
Finally, never re-chip a whole console at once unless you're absolutely certain of the sonic advantages. -You'd be denying yourself the opportunity to be certain that it really is the best it can be. -Do one channel at a time, one section at a time, and perform some honest, unbiased listening tests. -Best of all is to have someone else replace the chips for you and *not* tell you which is which... Example: "Well, donnie, Channels 21, 22 and 23 have different chips in the line inputs, one has 5532's, one had Burr-Browns, one has 072's. You tell me which sounds better without knowing which is which!"
Next time, do the same for the EQ filters, then the group summing amplifiers then the output drivers. Work on ONLY one section at a time! -You might miss a great EQ filter substitution option if the chip in the line input position is ruining the signal at the front end for whatever reason.
Also, check noise levels and also make sure you don't neglect low-level performance. -One op amp might perhaps have a very low offset, but if it achieves this at the expense of higher crossover distortion, you won't hear the harshness until the signal gets quiet. -remember, distortion doesn't just get worse as the signal gets louder... crossover distortion is the other way round.
Soldering the chips in is great if you're deft with the iron, but by all means use Chip sockets if you need to, -it makes replacement much easier, but AVOID cheap, 'spring-contact' sockets. Get ones which have machined contacts that make very positive, firm pressure contacts with the IC pins. A Gas-tight connection is a corrosion-proof connection, and you won't be building future problems into the board!
And one last thing... One studio owner a good number of years ago decided to re-chip his entire harrisson console. -He asked a tech friend of mine his thoughts, but there really wasn't any point asking, -he'd already made up his mind.
-"Who's going to do it for you" my friend asked...
"I'll do it myself, -I mean, it's not rocket science, is it?" replied the studio owner.
-Well, my friend dropped by a few weeks later on a social call, and by chance the owner was slotting the last module into the console.
"Hey, you're in luck! -I'm just about to turn it on and hear the difference" he announced.
The owner went out into the machine room to turn on the power supplies, and within five seconds thin, grey smoke began to pour out of the gaps between the modules, through the fader slots... -everywhere!!!
My friend ran into the machine room and cut the power as quickly as he could. The owner had by now made it into the smokey (and rather smelly) control room, and was standing with a 'deer in headlights' look... "What the...?" seemed to be all that he could manage.Even my tech friend was baffled... -how could the result be so absolutely catastrophic?
After a couple of minutes, asking various questions trying to understand what could have happened, he finally asked the owner:
"-You're sure that you put them all in the right way round?"The owner looked at him to make certain that this wasn't a joke... there was a long, awkward pause after which the now rather red-faced owner asked "You mean they have to go in a particular way round?"
-Phew! another marathon post... -Hope it's been helpful!
Technically yours,
Keith A.
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Follow Ups
- op-amp replacement observations: - SSL Tech 06:48:14 04/09/02 (1)
- (typo) - SSL Tech 06:58:53 04/09/02 (0)
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