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741 op-amp revisited |
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Graham Slee
Admin Group Retired Joined: 11 Jan 2008 Location: South Yorkshire Status: Offline Points: 16298 |
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Posted: 09 Feb 2012 at 6:15pm |
741 op-amp revisited
Introduced in 1968, the Fairchild Semiconductors LM741 op-amp was derived from Bob Widlar's LM301. It was used in numerous audio applications. You can read all about it here: http://en.wikipedia.org/wiki/Operational_amplifier so I'm not going to go into its history. In the 70's it was just about the only op-amp that would easily lend itself to audio designs. Today many pour scorn over it because of its limitations, but are those limitations really bad for audio? That's what I'm going to look into. I just purchased a tube of 40 and I'm going to see what the humble old 741 can do today. I used them over 30 years ago and my memory suggests they sounded quite good, but can they compete against the faster op-amp designs of today? We'll see. |
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Fatmangolf
Moderator Group Joined: 23 Dec 2009 Location: Middlesbrough Status: Offline Points: 8943 |
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What sort of applications did you have in mind Graham?
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Jon
Open mind and ears whilst owning GSP Genera, Accession M, Accession MC, Elevator EXP, Solo ULDE, Proprius amps, Cusat50 cables, Lautus digital cable, Spatia cables and links, and a Majestic DAC. |
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Graham Slee
Admin Group Retired Joined: 11 Jan 2008 Location: South Yorkshire Status: Offline Points: 16298 |
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They're too noisy to be considered for a phono preamp - or are they? We'll just have to see. I could see if they do anything for the sound of the Solo UL. I'm interested in some key areas of audio performance but need to run some tests first to see if the 741 is close enough to be able to deliver.
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Graham Slee
Admin Group Retired Joined: 11 Jan 2008 Location: South Yorkshire Status: Offline Points: 16298 |
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So here we have the first part of this investigation: the 741 sanity model which has it driving 10kOhms and 100pf with the addition of a 47pf compensation capacitor between output (pin 6) and inverting input (pin 2) which it didn't actually need, but I wanted to see what it would do over-compensated.
So, all looks well? I then put it in a headphone amp test circuit to drive a complimentary darlington pair which were already proven to work with a number of op-amps. This represented around 100pf and around 10kOhms load... It took off! In proper language it oscillated. Therefore the sanity check model is insane, and therefore I assume the uA741 sub-circuit model was either written by a lunatic or a liar. Placing a 470 Ohm resistor in series with the output (between the 741 output and complimentary darlington input) solved that particular problem, and the first THD measurement was obtained: 0.0068% - quite good considering it included the transistor output stage. So off for a quick listen, am I impressed? Well, it's not at all bad... yet. It still needs some "burn-in" - obviously quite a bit, and that's where I'm leaving off until Sunday or Monday. |
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Fatmangolf
Moderator Group Joined: 23 Dec 2009 Location: Middlesbrough Status: Offline Points: 8943 |
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Thanks for sharing your 741 experience. I think we will be following you on another learning journey!
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Jon
Open mind and ears whilst owning GSP Genera, Accession M, Accession MC, Elevator EXP, Solo ULDE, Proprius amps, Cusat50 cables, Lautus digital cable, Spatia cables and links, and a Majestic DAC. |
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iamalexis
Senior Member Joined: 07 Feb 2009 Location: London Status: Offline Points: 119 |
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interesting...i like the fact you are always searching for that extra something, seeing what you might find. looking forward to what you might find.
i assume the fairchild company is the same that made the highly sought after 670 stereo valve compressor found in many top recording studios?
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Graham Slee
Admin Group Retired Joined: 11 Jan 2008 Location: South Yorkshire Status: Offline Points: 16298 |
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I've no idea on the 2nd question. The 741 is a rather old op-amp design produced before many audio professionals/enthusiasts started to delve deeper into solid-state and its effects upon music reproduction - that didn't really kick in until the mid 70's. In my case I think recently I was suffering a nostalgia attack. When I put my first 741 op-amp circuits together in the mid 70's, and because I'd made them for me, and because there wasn't anything much better, obviously they must have sounded good. I've had to pinch myself and ask myself that if the 741 sounded so good, then why was I doing things with BC109C transistors... yes nostalgia plays strange tricks with the mind. I didn't waste too much time - just enough to establish what it could do (or not do). A gentleman from the "fatherland" was looking around for a replacement HPA for a discontinued one using a dual "741", the Raytheon RC4558 dual op-amp, and considering it was held in such high esteem and sold well over the years at around £200 (the HPA that is), I wondered what I was missing. My 741 findings: Slew rate is as advertised: 0.3V/uS, which you can work out from 0.3Ft, the transition frequency (or unity gain bandwidth) being 1MHz. The RC4558 boasts 2.5MHz which I thought quite incredible given it shares the same internal circuit. Maybe I wasn't looking quite so hard and the integrator cap had been reduced in value? If it had, then the phase margin must have been reduced with it. As I commented, unless precautions are taken the 741 oscillates like a good-un. But I suppose when directly driving a 32 Ohm load, as the said HPA was designed for, then perhaps that was sufficient load to slug it down? The slew rate manifested itself as a slow rising (and falling) squarewave - at least I found it symmetrical. The only fault being that it still exhibited ringing on the negative going "top". So what should I do with the remaining 38 pieces? To be honest, I'm not worried. I once bought 1,000 pieces of the LM318 which is far superior performance... they say. Those are still awaiting an application. The 741 is 0.3V/uS and the LM318 50V/uS. The 741 ought to be able to drive (charge and discharge) a reasonable capacitive load, but IMO with the basic test I gave it... it couldn't. The LM318 at 50V/us should be the bees knees but slew rate is only OK if it can produce the current to match it... the LM318 simply cannot. In fact most op-amps and discrete equivalents simply cannot either. Having tried most op-amps including those raved about, I cannot see what all the fuss is about. There are a couple of 80's designs that can do it all, if the circuit provides the right compensation. The rest sound great at first but once they've trashed the passives surrounding them the truth comes out in a teeth itching sound. The 741 by virtue of its slow slew rate leads to a rising THD (distortion) before reaching 10kHz but the one thing it won't do is wear out its support circuitry. From that stance I would rate the 741 as a vast improvement over some of todays "popular" op-amps. So what do we use? Sorry, that's our living. However, one I can tell you about is the really nice sounding op-amp in the Genera... |
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