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Op amps in small-signal audio design - Part 1: Op amp history, properties

5/25/2011 04:51 PM EDT
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Bert22306
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re: Op amps in small-signal audio design - Part 1: Op amp history, properties
Bert22306   5/25/2011 10:50:50 PM
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Very interesting article. Many years ago, I designed a simple audio preamp for use in my home stereo setup. I used it for many years, and finally gave it up to get something with remote control. But it served me flawlessly and sounded great. Based on the TL082 dual JFET input opamp, similar to the TL072 mentioned in the article. Two very important lessons I learned. The first one was, beware of capacitance in the load! The thing started hissing loudly after being powered up for a few seconds, and would not work. Turns out, the capacitance of the shielded output cable was enough to put the opamp into oscillation (it shifts the phase of the feedback signal). All it took was a few hundred ohms at the output pins, in series with each output cable, to stabilize the circuit. I used 470 ohms per output. Probably 220 ohms would have been enough. The second was, read up on thermal noise! The combined resistance to the + and - input pins creates thermal noise. For example, the feedback resistor and ground resistor connected to the - inpout pin. Keep that parallel combination low. Thermal noise manifests itself as a constant background hiss. The idea is, if possible, design the circuit so the thermal noise caused by the various input resistors is less than the noise inherent in the opamp itself.

agk
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re: Op amps in small-signal audio design - Part 1: Op amp history, properties
agk   5/26/2011 8:26:27 AM
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By reading this artiicle i call back year 1975 my design of a low noise pre amplifier for my casette player's head with NAB equalization. I tried many practically found producing a hiss noise in the gaps of silence or the fading music. I tried fnally LM381 dual low noise amplifier by NS in the single ended input configuration which gave better results. Also i designed a 2 transistor preamp with equalization given me still better results.

David Ashton
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re: Op amps in small-signal audio design - Part 1: Op amp history, properties
David Ashton   5/26/2011 12:22:27 PM
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Excellent and very interesting article. But could you clarify if the 5532 is a true dual 5534 - the characteristics in both the tables given (Slew rate / Noise) are different?? Thanks.

bcarso
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re: Op amps in small-signal audio design - Part 1: Op amp history, properties
bcarso   5/26/2011 4:09:32 PM
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Bert, the ability of the TL series to source/sink current is limited, hence the strategy to lower the impedance of the feedback network to reduce noise is similarly limited (however, bad news/good news, the intrinsic voltage noise of the amp is high enough that you can indeed make the feedback contribution fairly small by comparison). David, Douglas mentions the distinctions between the 5534 single and the 5532 dual in his book in a later chapter. They are indeed different, albeit fairly close. The 5534 is slightly superior and also allows input offset trimming. As testimony to the longevity of the 5534, I just noticed a phono preamp in the latest Stereophile (June 2011) that uses one per channel as the sole active signal chain components! And they still want 1300US :)

Rick_Hille
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re: Op amps in small-signal audio design - Part 1: Op amp history, properties
Rick_Hille   5/26/2011 5:30:18 PM
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I recall using LM1458's and LM358's extensively in telecom circuits (tone generators, filters, voice coupling, etc.). Not Hi-Fi, but low enough noise and distortion to meet telecom requirements of the time. We encountered significant performance variations between different manufacturer's devices of the same part number, as well as some date code dependencies. One trick that still sticks in my mind was having to add a "forced class A" modification (output pulldown or pullup resistor) in small signal stages to tame output crossover distortion from certain makers.

bcarso
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re: Op amps in small-signal audio design - Part 1: Op amp history, properties
bcarso   5/26/2011 11:58:43 PM
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I have a tube of "GL358s" which are actually not unity-gain stable! A very good technician kept reporting that a very-low-frequency twin-t notch filter (with feedback to greatly increase the Q) was oscillating, and I assured him that he must have wired something up wrong! Nope! The plain 358 (or as a quad, the 324) almost always has severe crossover distortion, except at very low output currents, and some companies actually treated that strategy of forced class A as a trade secret! For years I never saw a voltage noise spec, but recently noticed that it is now listed as 40nV/rtHz.

rpell2
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re: Op amps in small-signal audio design - Part 1: Op amp history, properties
rpell2   5/28/2011 3:19:58 PM
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I first saw this technique ("forced Class A") used in Walt Jung's "POOGE" modifications for the Philips DAC960 DAC unit in the early 90s. It has since become de rigueur in the DIY/audiophile community to "bias op amps into Class A", where it seems any op amp in a circuit is an excuse for an accompanying pull-up/down resistor or CCS - even op amps driving high-input-impedance buffers: http://tangentsoft.net/audio/opamp-bias.html

Nic Cohen
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re: Op amps in small-signal audio design - Part 1: Op amp history, properties
Nic Cohen   7/27/2012 8:20:41 AM
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Does that work that well in comparison? Regards Nic www.kdweb.co.uk

Robotics Developer
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re: Op amps in small-signal audio design - Part 1: Op amp history, properties
Robotics Developer   5/26/2011 6:29:34 PM
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Really nice article with great details and information! Makes me want to read the book (BSEE - digital guy, but I like analog audiophile applications). Thanks!!! I always look forward to Doug Self's stuff..

anon9303122
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re: Op amps in small-signal audio design - Part 1: Op amp history, properties
anon9303122   5/26/2011 7:03:42 PM
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Gasp. Reminds me of the pre-amp I made using a pair of 741's for my new Technics turntable to go into my ancient receiver that didn't have the phono inputs, only line inputs. Ok, this was 1981 vintage, freshman EE cobbling, but it worked just fine. I saw it in the basement piles of old "junque" a few weeks ago.

Bert22306
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re: Op amps in small-signal audio design - Part 1: Op amp history, properties
Bert22306   5/26/2011 7:40:03 PM
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Bcarso, right you are. The TL082 noise spec is 16 nV/SQR(Hz), and my resistor combinations at the + and - input terminals created 5.59 nV/SQR(Hz). But in my first attempt, where I used higher value resistors, the resistor noise was clearly audible when the circuit was idle. I lowered the value in two stages, and noticed the deminishing returns of lowering resistor values. If I'd used very low noise opamps, with noise spec around 3 nV/SQR(Hz), it may not have been as easy to lower the resistors enough to make a negligible contribution. Because of interfacing issues with other components. Just thought I'd mention those two items, load capacitance and theral noise, because I didn't think the article mentioned them explicitly.

jewilson
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re: Op amps in small-signal audio design - Part 1: Op amp history, properties
jewilson   6/1/2011 5:26:22 PM
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I have found the performance levels of the TL072/82 and the 5534 to be sub standards when compared to the current generation of high performance OP AMPs like the AD797 and AD8597/AD8599. In addition, the uA739 was superior in performance to 741 and other early opamps designs, excluding the 5534. The AD797 is a very low noise, low distortion operational amplifier ideal for use as a preamplifier. The low noise of 0.9 nV(root)Hz and low total harmonic distortion of -120 dB at audio bandwidths give the AD797 the wide dynamic range necessary for preamps in microphones and mixing consoles. Furthermore, the AD797's excellent slew rate of 20 V/Ás and 110 MHz gain bandwidth make it highly suitable for low frequency ultrasound applications. The AD8597/AD8599 are very low noise, low distortion operational amplifiers ideal for use as preamplifiers. The low noise of 1.1 nV/vHz and low harmonic distortion of -120 dB (or better) at audio bandwidths give the AD8597/AD8599 the wide dynamic range necessary for preamplifiers in audio, medical, and instru-mentation applications. The excellent slew rate of 14 V/Ás and 10 MHz gain bandwidth make them highly suitable for medical applications. The low distortion and fast settling time make them ideal for buffering of high resolution data converters.

Laser Man
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re: Op amps in small-signal audio design - Part 1: Op amp history, properties
Laser Man   6/2/2011 2:02:26 PM
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I will never buy another book authored by Douglas Self. I have one of his books on audio circuits - they are simply reprints of his articles from a magazine and they contain no design details at all. Poor material for engineers.

sinsinsin49
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re: Op amps in small-signal audio design - Part 1: Op amp history, properties
sinsinsin49   6/4/2011 4:00:07 PM
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I agree with the opinion that "performance levels of the TL072/82 and the 5534 to be sub standards when compared to the current generation of high performance OP AMPs. My top three devices for serious analog audio work are LME49990, OPA1611 and OPA211. LME49990 is the only one capable of true 24-bit analog performance when powered with +/- 15 V supply (S(N = 159 dB,THD = 146 dB). As the THD of LT1028 is 96 dB, it is "only" good for 16-bit systems. NE5534A is even less linear, THD = 93 dB.

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