Design Article
Op amps in small-signal audio design - Part 1: Op amp history, properties
Douglas Self
5/25/2011 12:51 PM EDT
Bipolar-input op-amps not only have larger noise currents than their JFET equivalents; they also have much larger bias currents. These are the base currents taken by the input transistors. This current is much larger than the input offset current, which is the difference between the bias current for the two inputs.
For example, the 5532A has a typical bias current of 200 nA, compared with a much smaller input offset current of 10 nA. The LM4562 has a lower bias current of 10 nA typical, 72 nA maximum. In the case of the 5532/4 the bias current flows into the input pins as the input transistors are NPN.
Bias currents are a considerable nuisance; when they flow through variable resistors they make them noisy when moved. They will also cause significant DC offsets when they flow through high-value resistors.
It is often recommended that the effect of bias currents can be canceled out by making the resistance seen by each op-amp input equal. Figure 4.1(a) shows a shunt-feedback stage with a 22 kΩ feedback resistor. When 200 nA flows through this it will generate a DC offset of 4.4 mV, which is rather more than we would expect from the input offset voltage error.
If an extra resistance Rcompen, of the same value as the feedback resistor, is inserted into the non-inverting input circuit then the offset will be canceled. This strategy works well and is done almost automatically by many designers. However, there is a snag. The resistance Rcompen generates extra Johnson noise, and to prevent this it is necessary to shunt the resistance with a capacitor, as in Figure 4.1(b).
Figure 4.1: Compensating for bias-current errors in a shunt-feedback stage. The compensating resistor must be bypassed by a capacitor C2 to prevent it adding Johnson noise to the stage
This extra component costs money and takes up PCB space, so it is questionable if this technique is actually very useful for audio work. It is usually more economical to allow offsets to accumulate in a chain of op-amps, and then remove the DC voltage with a single output blocking capacitor. This assumes that there are no stages with a large DC gain, and that the offsets are not large enough to significantly reduce the available voltage swing. Care must also be taken if controls are involved, because even a small DC voltage across a potentiometer will cause it to become crackly, especially as it wears.
FET input op-amps have very low bias current at room temperature; however, it doubles for every 10°C rise. This is pretty unlikely to cause trouble in most audio applications, but a combination of high internal temperatures and high-value pots could lead to some unexpected crackling noises.
Op-Amp Properties: Cost
While it may not appear on the data sheet, the price of an op-amp is obviously a major factor in deciding whether or not to use it. Table 4.3 was derived from the averaged prices for 1+ and 25+ quantities across a number of UK distributors. At the time of writing (September 2009) the cheapest popular op-amps are the TL072 and the 5532, and these happened to come out at exactly the same price, so their price is taken as unity and used as the basis for the price ratios given.
Table 4.3 was compiled using prices for DIL packaging and the cheapest variant of each type. Price is per package and not per op-amp section. It is obviously only a rough guide. Purchasing in large quantities or in different countries may change the rankings somewhat (even going from 1+ to 25+ causes some changes) but the basic look of things will not alter too much. One thing is obvious – the 5532 is one of the great op-amp bargains of all time.
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Bert22306
5/25/2011 6:50 PM EDT
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.
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agk
5/26/2011 4:26 AM EDT
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.
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David Ashton
5/26/2011 8:22 AM EDT
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.
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bcarso
5/26/2011 12:09 PM EDT
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 :)
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Rick_Hille
5/26/2011 1:30 PM EDT
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.
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bcarso
5/26/2011 7:58 PM EDT
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.
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rpell2
5/28/2011 11:19 AM EDT
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
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Nic Cohen
7/27/2012 4:20 AM EDT
Does that work that well in comparison?
Regards
Nic
www.kdweb.co.uk
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Robotics Developer
5/26/2011 2:29 PM EDT
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..
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Work to Ride comma Ride to Work
5/26/2011 3:03 PM EDT
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.
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Bert22306
5/26/2011 3:40 PM EDT
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.
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jewilson
6/1/2011 1:26 PM EDT
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/µs 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/√Hz 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/μs 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.
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Laser Man
6/2/2011 10:02 AM EDT
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.
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sinsinsin49
6/4/2011 12:00 PM EDT
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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