muting transistors / Rotel RCD-975

[focal7C]

Since the analog output stage seems a target for mods - would replacing the caps in the output stage for better quality ones provide a similar improvement?

If so, which ones to replace in the first instance and what do I replace them with?

Assume same V/uF values should be kept?

[Mooly]

Only the 100uf cap could be changed really, and tbh, any perceived gains would more than likely just be wishful thinking. I wouldn't short it out as the relay pulls the output to ground and that could upset the feedback under start up conditions.

Changing the opamp is a definite area to look at, and I'm going to go with one of my favourites and say OPA604. Solder direct to the PCB, don't use a socket and also add a small cap of 0.047 to 0.1uf direct across the supply pins soldering to the IC itself.

If you haven't a scope that's as far as I would go... if you have I would look at the output of the opamp direct and see what level of HF is present and perhaps even try removing the output stage transistors as the OPA604 has a pretty decent current delivery. You could try that without a scope of course... but it's good to see what is actually happening.

[focal7C]

Seems like a good approach by changing opamps and listening what the difference is.

What the Reason for cap across supply pins? Does a socket impair signal?

Have also looked at Audio-gd Discrete OPAs as an option. ºÍ§Ó*µ响

[Mooly]

Personally I don't use or like sockets... they add a little stray capacitance and also raise the impedance of the supply as seen by the opamp... but they can be useful for quick evaluating of devices.

A small cap direct on the supply pins gives a low impedance at HF which is essential for stability with many very wide bandwidth opamps. Some will argue that the cap should really e in series with a small (1 ohm or so) resistor. On linear supplies where the rails are essentially clean anyway I have never found an issue and the cap on its own is fine.

Can't really comment on the discrete ones other than to say that a "large" unshielded PCB will be prone to stray pickup and that although certain aspects of the design may seem appealing I can't believe it can compete with modern purpose designed devices that have had huge resources thrown at them.

This was from an earlier thread...
The best sounding audio integrated opamps

[focal7C]

Like your suggestion of OPA604APG4 + caps. Any caps you can suggest in particular? How would you characterise the sound in your setup?

Looked at the earlier thread you referred to and others. Everybody has their opinion. With popular choices:

LT1028ACN8 (neutral with good bass)
LME49860 (good compatibility) couldn't find exact model number?
AD797BRZ (x4 +2:1 adapter)
AD8620ARZ (with a similar CAP addition as yours - I think)

The Audio-gd discrete opamp circuits seem to be best with +-15V (does my supply do this?). Earth model the safer choice with "3-D sound". Then there is the IC vs discrete argument...

Depends on the TDA1305T and other circuit matching as well.

My Cyrus 8vs2 and Focal DIY speakers are very "detailed" but on the lean side (could do with 2ohm resistors on tweeters to balance sensitivity for starters). Maybe the LT1028 may be a good match with its realistic bass? Although next upgrade is probably a pre-amp, active filter and power amps.

[Mooly]

Any caps like this are fine. The leads can be spaced easily to fit exactly across the supply pins direct on the opamp.
--|MCRR25104X7RK0050|CAPACITOR, 100NF 50V | CPC

Your circuit piccy doesn't show the supply clearly enough so I would just measure and be sure from ground to pin 4 and pin 7.

All I can say as for what to use is try a few as it's so personal a choice. The OPA604 is (to me) very musical with no edginess or stridency at all. I used an OPA604 and AD845 in my micromega... which is what the snip in post 9 is

There's nothing like a bit of marketing waffle, have a scroll down to "sound quality" near the end,
http://www.datasheetcatalog.org/data...n/mXyzsxss.pdf