B&K ST-202+ with DC servo

[jyhfeei]

I have 30 year old amp that I'm replacing some electrolytic and poly capacitors. On this version of the ST-202, there is a DC servo circuit using an FET input op amp. Mine came with a TL072, but others come with an AD712. Is there any benefit to upgrading this op amp? I have AD712, OP249 and OP823 op amps that I can use or any other recommendation (OPA2134 or ?).

 

[Mooly]

TL072 is a perfect choice for a DC servo and there would be no benefit in using anything else imo. Servos are not in the audio path and their purpose is to set the DC conditions within the amp. What is fitted is the correct part for the job.

 

[jyhfeei]

Thanks. I thought that may be the case.

One final question. There are small mica caps (30pf, 200pf) that look to filter high frequencies. I hear they can be microphonic. Should these be replaced with PP or polystyrene?

 

[Mooly]

It really depends what they do in the circuit but such small values suggest they are used for defining stability somewhere. Generally small poly or films would be better than mica.

 

[jyhfeei]

Thanks for the feedback. The attached schematic is pretty close to my 1992 amp. The mica caps I'm referring to are C7 and C11. I plan on replacing them with (150pF or 220pF) and 33pF Wima FKP caps.

 

[Mooly]

Those are critical parts although 220pF and 33pf should be fine.

The ones I would use are these type. Put the striped outer foil connection toward the low impedance part of the circuit:

https://cpc.farnell.com/lcr-components/fsc-160v-33pf-1pf/capacitor-160v-33pf/dp/CA05849?st=capacitor

https://cpc.farnell.com/lcr-components/fsc-160v-220pf-2-5/capacitor-160v-220pf/dp/CA05856