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Author: Paul Hunting - TNT UK
Published: January, 2023
This piece suggests an approach that will hopefully go someway to make op amp rolling more likely to succeed in terms of op amp stability and, in so doing, to realise the op amps performance capabilities.
Along with a performance upgrade, op amp stability is a goal of op amp rolling. The host CD player, or DAC, is designed for the factory fitted op amp, in particular with regard to the surrounding op amp resistor bias, feedback and gain circuitry. Stability problems: like oscillations, phase, overshoot and ringing are likely to emerge if there is a mismatch between the replacing op amp specifications and the surrounding circuitry, see a previous article here on TNT-Audio.
The important criteria in selecting an op amp is by listening to alternatives. All that performance metrics and stability specifications do is to help you narrow down the op amps to listen too. Performance metrics are noise, common mode and supply mode rejection and slew rate and has previously been covered here.
Table 1 tabulates stability specifications of eleven op amps. The NJM2114 and the NJM4560 are here the target op amps to be replaced and are fitted into a number of CD players. The seven op amps MUSES8820, LM833N, OPA1662, NJM8068, OPA1692, AD797, OP275, OPA1602 and the OP27G are potential replacing op amps.
You don't actually need to know what each specification does to roll an op amp. But it's just as well to have a brief idea of what's going on. The input offset voltage is the voltage applied between the op amp input terminals to obtain zero volts at the output. The Input bias current is the average of the currents into the two input terminals at a specified output level while the Input offset current is the current difference between the terminals. Errors occur from tiny differences in applied voltages and current at the op amps inputs. Later stages in the op amp may attempt to correct these noise errors, but a proportion of the errors will get amplified by the gain stage and will appear in the output.
The players surrounding bias circuitry to the op amp is there to set correct bias levels and minimise errors and is designed for the target op amp that is the op amp fitted as standard in the player. Rolling to a substitute op amp is a risk. If the replacing op amp is very different in its offset and bias values compared to the original op amp then this may create problems and might increase op amp noise and reduce op amp performance.
The suggested principle of op amp rolling is to upgrade to op amps with offset and bias, current and stability values close to the typical values of the target op amp. The CD player has been engineered with the fitted as standard target op amp in mind. By fitting a replacement with values close to the target op amp we are going someway in ensuring compatibility of the new op amp with the host CD player and its circuitry.
The input offset current of the AD797 (Table 1) looks a little high compared to either of the two NJM target amps, but the AD797 values are in the range of values of the NJM4560 and NJM2114. The OP27G op amp looks low with regard to the input offset voltage and is out of range compared to the target op amp, NJM4560.
Fitting the OP27G in the Philips CD753, this is factory fitted with the NJM4560, produced noise on both outputs. The OP27G is not compatible (Table 1) with regard to the input offset voltage (0.03 to 0.1 mV) and is below the value range of the target NJM4560 (0.5 to 6 mV). That is the CD753 is likely to supply too much offset voltage for this op amp as the CD753 has been specified for the NJM4560 and this is thought to account for the output noise. But other factors might come into play.
It is best if the replacing op amp has a typical bias current figure around or equal to the typical bias figure of the target op amp. In terms of input bias the AD797 (250 to 900 nA), OPA1662 (600 to 1,200 nA) , LM833N (500 to 100 nA) or the NJM8068 (250 to 1,000 nA) maybe replacements for the NJM2114 (typical value 500 nA) while the OP275 (100 to 250 nA), OPA1602 (20 to 200 nA) or MUSES8820 (100 to 500 nA) could replace the NJM4560 (40 to 500 nA) by the input bias specification.
Input bias current figures are usually matched by offset voltage and current figures. The possible replacing op amps in Table 1 were pre-selected and I used input bias current as part of this pre-selection screening process, and I did not consider op amps where the potential replacing op amp had a typical input bias value exceeding the target op amp maximum or a lot lower than the typical value. The OP27G slipped through, as on first view it looked compatible.
Voltage is specified as a maximum and you just need to check that the voltage of the new op amp is about the same or specified above the original op amp figure. But voltage to the op amp can also be checked in the CD player service manual.
Common mode input voltage range describes the voltages necessary for normal op amp behaviour without problems. If the op amps maximum range is exceeded, the op amps transistors may become saturated and output non linear. This will be so even at lower voltages than the specified datasheet maximum value. Voltages to the op amp may be low and common mode input voltage range is lower for lower voltages. But if the voltage ranges between op amps don't match at 12 V, then they will not match at 5 V.
Looking at Table 1, the AD797 (11 to 12 V) and OP275 (10.5 V) have a lowish tabulated common mode input voltage range figures compared to either of the tabulated figures of the target op amps (12 to 13 V NJM2114 and 12 to 14 V NJM4560). If the host CD player drives voltages above the AD797 or OP275 maximum value, and this is likely to be the case for the CD753, then this may result in saturation. Listening to the OP275, in the Philips CD753, there is, at times, mild saturation. It was not so bad and the saturation came across as a bloating to some music. It's not unpleasant but it is discernible. With the AD797 in the Marantz CD53 there was no bloating heard, however the upper AD797 voltage range value (12 V) matches the lower range value of the NJM2114 (12 V). But other factors might come into play.
Following the rolling principle, the operating current draw of the replacing op amp should be as close as possible to the typical current draw of the fitted as standard target op amp. To handle changing voltages op amps need current as specified in its datasheet and if this is not delivered by the CD player, then this may compromise op amp performance. Though manufacturers may well over specify current. Available current depends on how the CD player has been engineered and built.
As a general rule it is safer, when fitting a replacement, to stick as close to the target op amps typical current figure. By this specification, the OP275 (4 mA), OPA1602 (2.6 mA) and the OPA1692 (1.3 mA) are replacements for the NJM4560 (4.3 mA), while the higher current draw AD797 (8.2 mA), LM833N (5 mA), MUSES8820 (8 mA) or NJM8068 (5 mA) may be replacements for the NJM2114.
I found it difficult to isolate current inconsistencies to sound issues. In some cases it appeared that the replacing op amp/CD player combinations were sensitive to current anomalies, in other op amp/CD player cases it seemed immune. This may be because some CD players are overspecified when it comes to op amp current supply (count the number of regulators), but equally op amps may just not react the same when there is insufficient current.
Gain is the loudness of the output signal and says something about music fullness. Loudness per se is controlled by the volume.
Take the Unity Gain figure, the frequency when gain is equal to one, as an approximate measure of gain and use this to compare gain between op amps. IC Op amp datasheets generally tabulate the Unity Gain Frequency. Further this gain figure can also be read from the gain over frequency chart as the frequency where gain is equal to zero. In Table 1 gain values vary from 3 mHz to 30 mHz. If the gain value of the replacing op amp is higher than the target op amp figure then loudness, or fullness, of the signal may come across as weak and may sound less full. The replacement op amp should have a gain value with the same or lower gain as the target op amps.
Looking at Table 1, OP275 (10 mHz), MUSES8820 (5.8 mHz) and the OPA1692 (3 mHz) seem like a replacement of the NJM4560 (10 mHz) in terms of gain.
I tried the OPA1602 (30 mHz) in the CD 753, originally fitted with the NJM4560 (10 mHz), and I thought the OPA1602 had gain instability in this CD player. The sound of the OPA1602, in this context, came across as weak as if there was an issue with the gain, it was limp and lacked presence. The OP275 (10 mHz) had a matching gain and was compatible with the CD753 gain circuitry, and the sound with this op amp fitted came across as full and has presence. But other factors might also come into play.
Some CD players include an integrator with a gain control circuit. This means the CD player should self-adjust to maintain the same gain whatever the gain of the op amp.
Try to pick an op amp designed for audio and not one designed for scanners or video. By picking an audio designed op amp at least the op amp designer has thought about op amp stability in an audio circuit.
Most audio op amps are voltage feedback. If this is the case with the original op amp, replace it with a voltage feedback op amp and not a current feedback op amp.
Op amps either use bipolar transistors or FET transistors. Best practice is replacing bipolar with bipolar and FET with FET op amps.
If an op amp is Unity Gain Stable this means that the op amp is unlikely to oscillate or cause major issues. Again best practice is to replace unity gain stable op amps with a unity gain stable op amp.
There are other issues with op amp rolling. CD player designers may use cascading op amps to add filtering or will apply circuitry to force the op amp to operate in Class A (Arcam CD82) and this may complicate the compatibility and stability of op amp rolling.
Manufactures often work within the same technology limitation and in developing op amp series, tweaking improvement in updated circuits. You might pick up a stable replacement by selecting op amps from the same approximate period or series as the target op amp.
The best op amp selection criteria is listening. Stability specifications, and performance metrics, help to narrow down the likely op amp choices to listen too. To advance stability compare the typical stability specifications of the target op amp with those of the replacing op amp. Take into account how the host CD player was built and check, if possible, the circuitry and how this might affect a likely successful rolling. There is leeway with specifications. Select a range of op amps to listen to, some closely matched with regard to stability specifications and others just ballpark matched.
What are op amp stability issues? As well as the obvious ones like oscillations, phase, overshoot ringing there are less known ones like internal capacitor and transistor saturation. Op amp issues can include mild added harmonics, subtle forms of soft-clipping compression and minor losses of performance. Some distortions can sound very nice, soft, warm and give a mild overfull sound. And these issues may only become apparent after a long period of listening and then only on some types of music. If it sounds wrong there is likely to be a reason for this.
The next time you decide to roll an IC op amp, it might be worth comparing your original op amp data sheet specifications with the intended new op amp specifications. And consider rolling to a replacement op amp with better performance specifications and one that has near similar current, bias, offset, voltage and gain values as the target op amp. This will go someway to ensure that the replacing op amp is indeed an upgrade and that it has a chance of being compatible in the surrounding CD player circuitry.
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© 2023 Paul Hunting - firstname.lastname@example.org - www.tnt-audio.com
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