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OPA2132: DC Servo OpAmp Replacement in Technics Amplifier

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Replies: 8

Views: 1258

Part Number: OPA2132

Hello,

I need to replace the operational opamps in my amplifier due to transistor failure, and --54v went through -17v rails.

DC Servo circuit is below its a new class at Technics SU-V9 from the 1980s. The old part was AN6552 which is gone and modern replacement is RC4558   (voltage is 15+/-). But maybe the OPA2134 or OPA2132 or a better suited Opamp for the circuit would be best

Thank you

Best Regards

Sam

  • Hi Sam,

    it can be problematic to exchange a slow OPAmp like the AN6552 by a much faster one. I wouldn't do that. On the other hand, the AN6552 has not very much impact on the signal integrity, as it's only purpose is to create a DC voltage. Even the noise is no problem because the output signal enters the signal path only via a voltage divider. So, I would take a pretty normal RC4558 for that job. If I had to use a better OPAmp, for whatever reason, I would give the NE5532 a try.

    Kai
  • Hi Sam,

    Kai's suggestion of using an RC4588 as a legacy option is a good one, but I'd suggest OPA1678 as a performance upgrade instead of the NE5532.

    In this case, the large feedback capacitance in the DC servo should help keep it stable even with a faster op amp. I simulated the DC servo circuit by itself using an OPA1678 for IC301, and didn't see any obvious signs of instability in the simulation. Intuitively, that feedback capacitance should cause this to behave as a unity-gain buffer (IC301 + C321) fed from a low-pass filter (R375 + C319) at high frequencies. Since there's no significant load capacitance (R379 isolates IC301 from anything downstream), I'd expect OPA1678 to remain unity-gain stable in this application.

    While not necessary in this circuit, I'd recommend OPA1652, OPA1642, or OPA1688 as FET-input op amps instead of the OPA2134 for audio signal path applications.

    OPA2134's input JFETs are not dielectrically isolated, and therefore exhibit a changing input capacitance with variations in common mode voltage. This can contribute additional distortion unless source impedances are matched. The parts I listed above do not exhibit that changing input capacitance, and offer lower THD+N and as-good or lower noise that the OPA2134 as well.

    Regards,

    Alex Davis

    Precision Amplifier Applications - TI Tucson

  • In reply to Alexander Davis:

    Hello Kai;

    Thank you for the help and advice. The RC4588 would be great, using that with the load comparator opamp.

    Hello Alexander

    Thank you for the help and advice. I would prefer to upgrade so the OPA1678 as you suggested sounds great, and that it can be used in the circuit without going unstable.

    Great, as i do have though maybe past their shelf life Jan 2017 in a sealed bag from Mouser OPA1642 , three of them so enough for the tone opamp and phonostage. I will get the adaptors them .

    Thank you again Alexander and Kai

  • In reply to Sam Macdonald:

    Hi Sam,

    have a look at the layout of your Technics and check whether decoupling caps are sitting close to the AN6552. If not, add 0.1-µF ceramic bypass capacitors between each supply pin and ground, placed as close to the device as possible. Eventually, the signal ground routing arround the OPAmp needs an improvement by adding some ground wires.

    If the voltage divider R379/R381 is far away from the output of OPAmp insert a 47...100R placed close to the device.

    Kai
  • In reply to kai klaas69:

    Hi Kai,

    Phono opamp is close with 47uf to ground for pin 4 and 8.  DC Servo - bipolar caps with diodes for pins 1,2 and 5 and 7. R379 is relatively close as is R381. So they should be fine or need resistors or bypass caps ?

    Also are my batch (3 OPA1642) in a sealed  bag from Mouser 1Jan 2017 still okay if shelf life on bags says 12months?

    Both you and Alex have been great help , thank you again

    Regards

    Sam

  • In reply to Sam Macdonald:

    Hi Sam,

    if the OPAmp becomes unstable, I would connect some decoupling capacitance from pin 4 and pin8 to the GND island at the bottom of the picture, where the feedback components are mounted to. Do also short circuit these two GND island by a piece of wire so that a local GND is formed where the feedback components and the decoupling caps are connected to. You can even solder a piece of copper foil to the GND island and make it end close to pin 4 and pin 8 so that the decoupling caps from this pins to GND need to have only very short terminal wires. This gives you sort of a solid ground plane and can considerably decrease the unwanted inductances of GND connections of decoupling caps. More cannot be done without doing a total revision of layout...

    By the way, have you thought about exchanging the electrolytic capacitors by fresh ones? The PeakTech 3705 allows to measure the capacitance directly in the circuit. You can check all the electrolytics and exchange those ones which have lost their capacitance.

    The pots and switches can be cleaned and sealed by "Kontakt 2000".

    The shelf life on the bags has to do with the MSL (moisture sensitivity level) and means that the OPAmp should be baken and dried by a defined procedure before using them in an automatic soldering machine.

    Kai
  • In reply to kai klaas69:

    Sam

    We haven't heard back from you so we assume this resolved your issues. If you need additional help just post another reply below.

    Thanks
    Dennis
  • In reply to Dennis Goeke:

    Hello Alex and Kai, and Denis
    Sorry for the late reply. I got caught up with other issues with the amplifer -

    I had a look at element14 i can get what Alex and Kai sugguested, and again thank you again

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