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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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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 :)

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.

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.

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.

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