Vince, thanks so much for all the help to Jeff on this.
Jeff, I've been (very much on the periphery) following the thread just to see how things turned out. I highly suspected the relays...they were working fine back here but they do seem to go bad out pretty quickly when the time comes and simultaneously at that, and the symptoms you were experienceing were
identical to those of my 58.
Good job and what a relief.
And, yeah, Vince is certainly right about older 'lytic caps in the signal path and the (IIRC) 4558 opamps on the 48 amp cards are late 70's vintage, but, a word of advice, don't go there. At least not now. Get things operating in the studio. Do some music. The 48 sounds great in stock trim so opamp upgrades are for a rainy day IMHO. The opportunity will still be there later. Don't get distracted.
The 4558 opamp was used prolifically...a predecessor to the very popular 072, but the 4558 is slower with higher self-noise and distortion specs. Typically what opamp upgrades might give you is a "tighter" sound (because the chip is "faster"...has a higher slew rate which is basically a measure of the dynamic latency between input and output of the chip and the tighter the ouput signal matches the input the more accurate the sound will be), and lower noise and distortion..."cleaner". BUT, can you hear the difference? I love Vince's answer to the question about why he did it and I betcha he's right about the result (
what chip did you put in, Vince?) that it sounds "fuller". And he is comfortable doing those swaps and he was "in there" already. I bet it could be backed up by measurement too, but it all depends on the system, program material, how the deck is setup...there are so many variables and let's not forget that any time you take an iron to PC board you are entering risk. So I've learned that, for me, it is really important to assess the need or reasoning behind an opamp upgrade. I specifically chose NOT to replace the 4558 chips in my Tascam RS-20B...Dr. ZEE turned me around on that one. The RS-20B is a nice sound spring 'verb unit...smooth and smeary. Well, how much of that smooth smeariness is attributed to the 4558's? I have no idea but if it sounds good why go messing with it. OTOH I will experiment with some different chips on my CRL SEP400 multiband compressors. Why? These will be used for mastering and I want them to be as accurate transparent as possible, and they are loaded with 4558's at present. Other factors to consider is that ALL the opamps were socketed at the factory so opamp upgrades are easy and relatively risk free to try out. I DID upgrade specific opamps on the two channels of my Tascam M-__ mixer when I refurbished them and the result left me longing for the sound of the stock channels...I have done little more on this yet (as far as A/B'ing or trying different combinations of chips), but maybe the slower noisier stock chips just sounded more natural. I'm having an issue with monitoring off the board right now as I'm doing some more work using it more mixing, but the soundstage sounds kind of choked. It might be the room or maybe I should shift my focus from upgrading the mono strip input and ouput chips to looking at the control room and stereo out opamps and most importantly the summing amps...for another day, year or decade. You likely get the point.
It seems to me that a lot of the ideal behind opamp upgrades is to get a more clean accurate sound, and, again, a lot of what we like about analog is there are smooth distortion artifacts...its not "clean" and "accurate" and that's a part of what sounds pleasing...the circuitry adds something to the source material that is pleasing to us as enthusiasts and so it seems counterpoint to try and erradicate that, though many open reel machines are put on a pedestal because of how "accurate" they are. This is application and engineer dependent IMO. Many models of Studer machines seem to be regarded this way. Loved by many for their sophisticated and precise transports and accurate reproduction, while others call them "sterile" sounding. The ears of the beholder...But for somebody (for instance) who does analog to digital transfers, they are going to want an analog machine that effects the source tape signal as little as possible, vs. somebody like me who wants "mojo" added to drum tracks.
Other things to consider with opamp upgrades:
- Often times newer opamps draw more current. Not a big deal if you are doing one here and one there on a smaller system, but can become a real consideration if you are going through and doing a whole mixer...might be too much for the PSU.
- Newer "faster" chips have the propensity to go into oscillation and it is a common practice to put a film "bypass" cap between the + and - input pins on the chip to prevent this from happening. How deep does the rabbit hole go...
Stay the course, sir.
