old crown IC-150a preamp

[otaku]

Hi, I bought an old crown IC-150a preamp recently, and have some question regarding possible mod. I scanned the schematics, but to make it show clearly, I kept the image fairly large and uploaded to the following link.
http://sung.orcon.net.nz/crown-IC150a_preamp.gif

I noticed that this unit has been serviced before replacing just one channel's original opamp LF356H with TL071. I decided to replace both op-amps with OPA134 as they looked somewhat obsolete.

This mod made it much quieter and unpleasant hiss is almost completely eliminated. The voltage is +/-18V, which are ok to OPA134.

It sounds ok, but there seems a bit of clipping happening. I'm no expert in both audiophile and electronics, but don't you call it clipping when there is noticeable distortion at some high frequency?

Please note that there is a gain switch so that I could select between -10 and 0dB. I chose -10dB.

Some info on the web recommends to use OPA627 instead of OPA134 and remove output electro caps(perhaps C134/C234 25uF/15v NP) since OPA627's very low offset(0.4uV/C offset drift) eliminates the need for it..OPA134 seems to have slightly higher offset(2uV/C) so I just left it as it is.

I'll be pleased to hear any comments and suggestions.

Thanks

[MrTransistorm]

It sounds like there might be some high frequency oscillation. It could modulate the audio signal making it sound like distortion, particularly with higher frequencies.

Try putting a small capacitor each across R133 and R233. It shouldn't take more than 50pF.

Also, the schematic doesn't show power supply connections. There should (ideally) be a small cap (around 100nF) from each op amp supply pin to ground. They should be as close to the ICs as possible.

[otaku]

Originally posted by MrTransistorm

Also, the schematic doesn't show power supply connections. There should (ideally) be a small cap (around 100nF) from each op amp supply pin to ground. They should be as close to the ICs as possible.

Yes, it is very true. I closely examined the pcb and noticed that the power supplies to the op-amps are not bypassed!! I'm amazed to see 70's engineers's different way of using op-amps!! Perhaps they didn't know about bypassing, or op-amps those days were too slow to oscilliate?

Unfortunately, it doesn't seem an easy job to fit some ceramic caps around opamps..Ground rail is a bit far...Perhaps with some wiring from ground rail and solder the cap directrly to IC sockets and to the end of the wire? (yes, they used IC sockets, which made it easy to replace ICs...)

Your suggestion definitely brightened the way. Cheers!!