My third Composer is functional with the exception of the DES direction sensing chip. I'm thinking of just using it mainly for the Axial Tilt circuit. With my current recording set up I go through the Composer using the tilt correction then to the computer's sound card input. The problem that I have is that when I play the recording during editing via the same audio editor I get some low level feedback as the sound cards inputs are still active. It is a pain to have to disconnect the input cables or mute the inputs via the sound cards control panel.
When I first got my original Composer I wrote Steve Kennedy suggesting among other things that the unit should have a tape monitor. That would have added cost to the already expensive product. Most vintage decoders do have a tape monitor (most useful if you have a three head machine). Even if the tape output was switchable would be a useful feature, but I digress.
Why use the Axial tilt anyway? For years I didn't bother as it is such a pain to adjust, most test records have test tones that are far too short, and the pots are awkwardly placed on the back of the unit. I started using it a couple of years ago.
With my Sony cartridge I was able to improve separation from just over 20 dB to almost 30 using the tilt circuit! Using the new Audio Technica cartridge the improvement was less, a few dB maybe. When listening to stereo that improvement is not even noticeable. It did seem to have the biggest effect when tilt corrected recordings were decoded with other (lesser) decoders. The tilt correction circuit is similar to Sonics pre-synth/phase balance circuit, just that it operates over a much smaller range, adjusted to cancel out the crosstalk in the opposite channel. It would be a very easy DIY project.
The S&IC comes stock with TL074 quad op-amps. To tweek better sound out of them Audionics ran them in Class A. That was done by means of a 2.2K resistor from each op-amps output to the negative or in some cases the positive supply rail. Over the years I swapped op-amps to the AD713. Close enough to the TL074 that the mod is likely still a good fit. I tried the OPA4123PA but they were unstable, I don't know if that was because of their wide bandwidth or because I bought them from a Chinese seller and so may be fakes! More recently I bought some LME49740 from a U.S. seller depleting his supply. Sadly the LME49740 has been discontinued. There are lots for sale from China at attractive prices but they are likely fakes. I just ordered some more LME49740 from a U.K. seller. I used one to make my active adjustable Dyna type decoder, they have an incredible CMRR (typical 120 dB), blending the stereo signals out of phase with each other using one of these almost totally eliminates the vocal. That attribute has got to be a plus in a matrix decoder! They seem to work/sound fine simply dropped into the Composer. I'm in a bit of a Dilemma, is class A operation beneficial with this op-amp? It has such low distortion already, it is possible that the added resistors are now degrading performance?
Audionics glued heatsinks to the chips running class A as they do run warm. The Composer with the old National chip set has no heatsinks on the chips. I guess that it was an afterthought.
As a test I removed the resistors from the crippled (third) Composer and installing my last LME49740 the rest are AD713 and one TL074 . It still sounds fine to me with mostly the old op-amps still installed and no class A.
I intend to upgrade all units by substituting the LME49740. My dilemma is do I leave in the resistors and hope for the best. Do I remove them from all units still hoping for the best. Do I replace the resistor with a JFET, JFET Cascode, Current Regulating Diode or change it's value, 2.2K always seemed rather high. What current is optimum, is this class A thing totally unnecessary. I'm leaning toward the latter.
Here is a very interesting link about Class A biasing of op-amps.