OPA2132: DC Servo OpAmp Replacement in Technics Amplifier

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.

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

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

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

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