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Old 19th August 2004, 09:30 PM   #1
p53 is offline p53  Germany
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Default noisy opamps

Hi there,

could somebody please help me with following stuff?

Ive got a so called SPL Vitalizer (one of the first models from the 90s). Its a nice thing but quite a little bit noisy. The opamps used there are all on sockets, so I could easily exchange them. My problem is that Im a bloody beginner and I dont know which opamps I could try instead. The opamps are:

TL071CN, TL072CN, TL074CN and LM13700N

Thank you in advance,

Rudolph
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Old 19th August 2004, 10:12 PM   #2
rgsotr1 is offline rgsotr1  Germany
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The replacement for the TL072 is simple, use LM833, its low noise and low distortion at similair speed. May be someone else has a good tip for the others.
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Old 20th August 2004, 01:51 AM   #3
sam9 is offline sam9  United States
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TL071, TL072, and TL074 are single, dual and quad versions of the same opamp. They are pretty low-noise to begin with but there are ones with better specs. A common upgrade for audio is the OPA134, OPA2134, OPA4134 single dual quad family. Also the OPA132, OPA2132, OPS4132 family. (I'm not sure on the relative merits.) These are not all that expensive. I would guess that using all from the same family to attempt an experimental upgrade of the TL07x family would be the most conservative approach.

The LM13700N is a "tranconductance" op amp. The applications on the data sheet suggest it's use for current controlled amplifies, current controlled filters and other control circuits. Although, it looks like it is fairly noisy, relatively high distortion device, I'll bet that in your Vitalizer it is not in the signal path but rather supplies DC voltages to control the functions that the unit performs. With luck the unit was designed such that it does not contribute to the Vitalizer's noise and distortion figures. On top of that, there may very well not be a psart that could be substituted without altering the functionality of the Vitalizer. I suggest you leave this item alone unless you or someone else who realy (I mean REALLY!) understands how the thing works can determine whether there is any potential improvement in this area.

From the description, of this product, it looks like it is big bundle of filter elements most likely realized by many, many capacitors. These could well be a noise source equal or greater than that of the opamps. You could try replacing them with the same values but higher quality, but the improvement might be marginal at best. I triesd something like this with something called a Phase Linear 1000, but it was hopeless -- the signal simply passes through so many RC filters, gain and attenuation stages that the unit was just inherently noisy regardless.
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Old 20th August 2004, 09:41 AM   #4
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I'd concur with that. TL07x series are reasonable devices and I doubt much improvement to noise can be had just by chip changes. Most likely the layout, topology and/or component value choices is at fault. Probably not a lot you can do about it.
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Old 20th August 2004, 12:55 PM   #5
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Well the noise has to come from somewhere??

So where does it come from....

The 2 main noise sources are the thermal noise of the resistors... which I doubt will be higher than the 18nV/sqrtHZ.. the other source are the active devices... you can get 3nV/sqrtHZ instead of 18nV/sqrtHZ if you use opa227 instead of TL072 .....

Than again 18nV/sqrtHZ is -92dB ref 100mV (20KHz BW).. noise shouldn't be a problem... do you use a propper input level?
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Old 20th August 2004, 01:01 PM   #6
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ooops 20KOhm = 18nV/sqrtHZ at room temp? than resistor noise could be your problem and switching to a low noise BJT input opamp will not improve the matter.... sorry

can anybody check my calculations...
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Old 20th August 2004, 01:11 PM   #7
djQUAN is offline djQUAN  Philippines
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maybe the unit uses carbon composition or carbon film resistors which are a possible source. the solution is replace them all with metal film resistors which I don't think beginners should mess with.
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Old 20th August 2004, 01:36 PM   #8
alecwek is offline alecwek  Denmark
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hey i checked up what these Vitalizers are they sound very intresting what do they sound like do they make a difference in the mix or is it hype, what do you use it for when u use it....
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Old 20th August 2004, 02:14 PM   #9
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if the opamps are socketed, get rid of the sockets and solder the replacement opamps right into place -- the exception would be if the sockets had gold wipers or machined pins.

make sure that the power supply pins are bypassed with 100N or 10N capacitors, placing the bypass caps as close as possible to the power supply pins as possible.

consider tackling the power supply -- if it uses standard 3-pin regulators you might want to explore the lower noise discrete component regulators (search this forum under Sulzer, Super-Regulator, ALW etc.)

jack
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Old 20th August 2004, 03:09 PM   #10
p53 is offline p53  Germany
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Wow, you guys are fast! Great board!

Thank You all.

Maybe a few words more about the Vitalizer. The parts used there are all of very high quality. As I can see, there are no electrolytes in the signal pathway and the PSU is also of very high quality. The resistors, well - I donknow. But SPL is known for well engeneered stuff.

The unit that I have is from 1995 and has a S/N ratio of 78 db. SPL produces the same Vitalizer Jack today, but thay claim it was reengeneered and has a S/N ratio of 86 db. I dont beleave they did very much changes because in the 90s there were companies which tuned this Vitalisers to make the S/N better.

I like this thing, because it does wonders to older recordings and even to better recordings when listening at low lewels (in the evening). I plugged it between my preamp and active studio monitors and I do hear some faint noise when it is silent. Well maybe I should try to attenuate the output of the Vitalizer and crank up the input because I never see the peak-LEDs blinking on it.
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