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LM318 distortion
LM318 distortion
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Old 2nd February 2020, 08:52 PM   #1
dotneck335 is offline dotneck335  United States
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Default LM318 distortion

I'm refurbishing an old audio circuit that uses LM318 opamps; datasheet does NOT show any THD specs. Has anyone measured these opamps?
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Old 2nd February 2020, 09:02 PM   #2
rsavas is offline rsavas  Canada
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iirc Walt Jung has, in some of his old paper back IC opamp books.
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Old 2nd February 2020, 09:15 PM   #3
Joseph K is offline Joseph K  Hungary
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Lm318 was one of the best, low distortion, high open loop gain, high slew rate, quality devices of it's time.
Like : it's been used in distortion analyzers like HP339a etc..
I have never measured it stand-alone, but in the Myref composite amps of Mauro Penasa, applied as input stage / control amp. The obtained results were put on here, at around 0,00015% up to close to clipping.

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Old 3rd February 2020, 03:41 AM   #4
keilau is offline keilau  United States
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Quote:
Originally Posted by dotneck335 View Post
I'm refurbishing an old audio circuit that uses LM318 opamps; datasheet does NOT show any THD specs. Has anyone measured these opamps?
The LM318 is a fast Opamp for instrumentation with a small signal bandwidth of 15MHz. It is NOT meant for audio use. Noise or THD are not critical in general application Opamp. National Semiconductor designed the NE5534/5532 series for audio which was very successful, but has since been superseded by better Opamp.

You can find information on measuring and reducing noise for LM318 in this old National Semiconductor application note that TI re-published.
http://www.ti.com/lit/an/snla206a/snla206a.pdf

The major issue against using LM318 in audio because it has poor damping, or in other word,it rings. The transient response of LM318 is poor, but perfectly acceptable in wide band, steady state, instrument type measurement application. If you want something faster and quieter than NE5534 (5V/uS), the LME49710 (20V/uS) is a good choice, but somewhat expensive.

Last edited by keilau; 3rd February 2020 at 03:46 AM.
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Old 3rd February 2020, 04:31 AM   #5
abraxalito is offline abraxalito  United Kingdom
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Quote:
Originally Posted by keilau View Post
National Semiconductor designed the NE5534/5532 series for audio which was very successful, but has since been superseded by better Opamp.
Just a point of fact - it was Philips who developed the TDA1034 which later was renamed NE5534 (a Signetics ID).
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Old 3rd February 2020, 04:56 AM   #6
dreamth is offline dreamth  Romania
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http://www-f9.ijs.si/~margan/Audio/Ironing_RIAA.pdf
page 37-38 and your riaa pre too i wouldn't go fet all the way , bipolars are better for CCS and cascode transistor, but you're a smart guy
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Old 3rd February 2020, 05:03 AM   #7
dreamth is offline dreamth  Romania
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There's something i always though trying but never did: to burn the 12k resistors by applying a high voltage over them .That would allow me a better input solution, but i don't know what will really happen when doing it.
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Old 3rd February 2020, 08:02 AM   #8
Elvee is offline Elvee  Belgium
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Bob Pease tested its linearity (heavily related to distortion), and it doesn't come out very well: it is one of the worst he tested, ranking between the 709 and 741:

LM318 distortion-lm318-png

In addition, as already mentionned, its noise and dynamic behavior are problematic in many instances
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Old 3rd February 2020, 08:37 AM   #9
Joseph K is offline Joseph K  Hungary
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While I do not wish to argue much, because basically You are right - It turns out from the text that the measurement was done at the 2k load zone. So the output stage is heavily involved. And as he points out, at lighter loads the values can get much better.
Just trying to understand why I did get much better distortion results than from this test could be projected. (the load in the Myref topology is 47Kohm). And it did get applied in precision low frequency oscillators like Krohn Hite, distortion meter like that Hp339A etc.
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Old 3rd February 2020, 01:00 PM   #10
dotneck335 is offline dotneck335  United States
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Quote:
Originally Posted by Elvee View Post
Bob Pease tested its linearity (heavily related to distortion), and it doesn't come out very well: it is one of the worst he tested, ranking between the 709 and 741........In addition, as already mentioned, its noise and dynamic behavior are problematic in many instances
Wow, that's interesting---318 not very good at all! I see that the LM4562 fared quite well----was there any mention of the NE5534's performance?
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