Re: Revox A700 repro IC
Generosity doesn't come into it - the machine was designed around the available or projected technology and was a quantum leap in value for money and complexity at the time. The use of ICs was central to keeping the cost and complexity within bounds. 931s came in three grades, according to noise. I take it you are using 931-3s in the rep amp? If not, swap one in from a less critical position.
As to modern ICs being better,the 5532 is still the class of the field for low noise audio work, so the problems of bias current remain. With the benefit of hindsight, the 931 wasn't as low noise in this application as it was intended to be. Bipolar discretes would have been a better solution at the head, as Studer themselves recognised, but the difference is that between a perfectly adequate configuration and one that is near-perfect. If nobody tried to innovate, we'd all still be running around in woad, as my old boss used to say...
If you must fiddle, note that the magnetisation problem with 5532s occurred on switch-off. The preamp worked perfectly well the first time it was used. The +and -12V rails decayed at different speeds, as the +5V was derived from the +12V, if memory serves, so it collapsed more quickly, causing a current pulse through the head. Some jiggery-pokery with diodes, downstream decoupling and defined decay through drain resistors may fix this, or it may not.
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