User "Beck" previously suggested to replace the cheap carbon resistors in the audio path with metal film. But I think I'll leave them be.
I had a quick look at the M-300 series, and can't seem to find any for sale at the moment near Belgium and more importantly, it seems like even the 8 channel version would be way too big for my small apartment - studio.
+ I don't need the extra functionality as I work mostly in the box. The mixer would be mostly a creative tool & cheap preamp. I plan to convert the tape rca's to outputs because the tape input doesn't pass through the gain knob (the gain is fixed). That's why I can repurpose the FLB/tape switches (if that's what you mean with sacrificing the "monitoring").
Thanks for explaining all that. That all makes sense for your situation. Yes the M-300 series has a relatively bigger footprint that the M-200.
On the “cheap carbon resistors”, you will find carbon film resistors are the most commonly used in these types of applications, and you’ll find them in garden variety stuff on up to boutique stuff. Metal film can offer advantages, but it depends on the device, the application and the goals. The most common advantage is the potential lower thermal noise coefficient…metal film resistors may produce less noise, but, again, any potential gains would depend on the device, the application, etc. and we are only talking about resistors in the audio path, so input and output resistors, and feedback loop resistors. In the case of the former you probably couldn’t even measure any difference unless you had a LOT of open channels and multiple I/O stages. “These are not the droids you are looking for.” On something like the M-200 it’s my opinion it’s a waste of time and money to replace carbon film I/O resistors with metal film. If you have a lot of opamp stages with gain you *might* be able to hear a difference if you replaced the feedback loop resistors with metal film? But in that case maybe only if running a lot of open channels and at high gain. If you are wanting to lower the noise floor a much better approach is recapping the power supply, analyzing local filtering in the device and recapping there as well or adding local filters, and, depending on the opamps, upgrading relatively higher noise spec opamps with lower noise parts. But not all things are created equal and depending on what you upgrade to you may need to employ other modifications to avoid the new parts oscillating, and as I mentioned before you need to be aware not everything is a drop in part. The M-200, IIRC, is all JFET opamp stages and the 5532 is bi-FET. Sometimes, depending on the circuit, you can replace the JFET opamp with bi-FET and it works, but you have to test if it works by measuring the DC offset at the output of the original opamp, putting in the 5532 and measuring again. And if the DC offset increases significantly, you can’t use it in that stage. A good example I recall of this was experimenting with different opamps in my prototype Tascam console, the “M-__”. The EQ section alone for each channel has six TL072 opamps. I’m not an ‘072 hater. I was just curious. You’ll find the 072 in tons of garden variety stuff, but you’d be surprised at how many coveted vintage boutique devices used the 072 as well. And people hate on it. It’s not about the opamp itself, it’s about the circuit design as a whole…all the staff ahead of, around and downstream from the amp stage. Opamp “upgrades” are not a silver bullet and using the same opamp in your device as a coveted device will not turn it into that device. No free lunches. And a lot of times an opamp “upgrade” won’t end up sounding “right” in your gear, and this can be true even with something like the “humble” 4558 (which, BTW, seems to be coveted by the vintage guitar effects pedal crowd…see? Your mileage may vary, objects in the mirror are not as they appear, etc). So you have to have an open mind and reasonable expectations and be willing to A/B the results and then decide…that’s my advice. Anyway, I was doing listening tests with different material and A/Bing stock 072 channels against modified channels. Which is a great way to tell you how often opamp “upgrades” are not worth the time and expense. Anyway, one of the six opamps was just an LED driver, so leave that alone, another, the input buffer, had some particular sensitivities around the circuit and it was best to leave that alone (but a new TI branded part worked better than the original JRC branded part). For the remaining four I plopped 5532s in place of the stock 072s and did my offset measurements. The 5532 worked in two of the four stages. I ended up keeping the two 5532s that worked in place, and also liked OPA2134s in the other two stages. But remember this is in a console with a couple dozen amp stages in each channel just because of the complexity of the topology…completely different circumstances with the M-200. So there are more potential gains to be had with noise and distortion when you have lots of cumulative amp stages, and those can manifest in a number of ways, and sometimes sound “better”, and sometimes not, and some of it can be measured, and some of it is subjective. In my example I chose the changes purely based on subjectivity. I think the noise factor may have gotten worse a little actually, but I liked the sound better.