I just pulled one my 1588Bs out of its can the other day. Then today I found this in the description of an evilbay auction. I wanted to throw it out there for discussion.
The seller's evilbay name is nclayton. I've actually bought some transformers from him. I'm going to send him an email requesting more info on this:
"The 1592B is a modular mixer designed to take up to five inputs from a variety of types of sources using dedicated plug-ins for the particular source to be used. Usually they were equipt with 1588 series microphone preamps to be a 5 into 1 mic mxer. They're very good sounding units -- they use Peerless transformers on every input and the output; they use all class A circuitry with all discrete transistors, no opamps; and they have a nice two band tone control with gentle broad boosts and cuts, which can actually be surprisingly usable, subtle and nice for general brightening or warming (it can also be switched out of the circuit completely). I'm a big fan of the 1592 sound, and have probably owned ten of them in all over
the years (both A's and B's). With the 1588B input module their topology is suprisingly similar to a Neve 1073: transformer in -> three cascaded transistor single ended preamp with feedback controlled gain (through the pad control)-> out to volume control and EQ -> three stage output amplifier directly coupled with the output transistors pulling their DC supply
current through the output transformer. Their biggest drawback is that when used for recording in a modern studio, they're basically just a single channel preamp since there's only one output. Every now and then I
find myself using a second channel either to mix two mics directly to tape or as a second input not to be used at the same time as the first (if that makes sense), but on the whole it seems like a waste that you can't use more of the inputs at once. I've bought and sold a few of these on ebay, and whenever I have, the question always comes up "can these be modified to have direct outputs" to which the answer is a
qualified "no".
The problem is that each 1588 module provides only about 33 dB of gain, then they're mixed together and the output amp adds the additional gain needed to bring the mic signal up to line (+30dBm max). The mixer as a whole system has TONS of gain, 87dB worth of gain, but as you can see, direct outputs taken from the 1588's would be pretty low level, good for very loud sources like drums or cranked up guitars, or maybe medium level sources if your recorder uses -10 inputs, but unusable for quiet sources, low output mics, etc.
So....Here's how I'm modifying all my 1592's now. The first three preamps (the ones with controls left of the VU meter) are stock, mixing into the master volume and output amplifer as usual. The last two (right of the VU meter) are rewired for direct output.
IN ADDITION, the two 1588B preamps in the direct output slots have been internally modified to increase their maximum gain. These 1588B's are modified for about 52dB, much more than the stock 33dB -- still on the low side for a mic pre, but much much more usable. 1588C's are not amenable to this type of gain mod, so I only use modified B's in these slots. Most 1588B's are getting old and a bit noisy, so while I have the cans opened up I also replace one critical carbon comp resistor with a metal film, and one electrolytic capacitor.
The extra gain is a result of decreasing the 1588B's internal negative feedback, kind of what the pad control does but in reverse. The 1588B circuit, open loop without any feedback, is capable of much more gain than they actually use. They were intentionally brought low by Altec so that when the mixer was used with 1588's for mic and 15095 transformers for line inputs, the mic inputs would have 45dB more gain than the lines (since the 15095 stepped down gain by about 12dB). The low gain of the 1588 modules was strictly a matter of not overwhelming the whole system with much more gain than was necessary. In fact, they're capable of much more gain even with more than sufficient linearizing feedback still applied. So, *theoretically* this modification increases distortion, but in practice the biggest difference in sound between these and normal inputs is that these don't go through the output amp, and the output amp does add some coloration. To make a long story short, the direct preamps don't sound exactly like the mixed preamps, but they do sound cool, and most importantly, with this 1592 you're getting three simultaneously usable mic channels instead of just one, and two of which can be used for stereo.
The outputs of the 1588B's are, of course, unbalanced. On this mixer I've added a pair of transformers to balance the outputs. I chose to use Peerless 4834 transformers for this job for a few reasons. First of all, they're about the same age as the mixer, and being original Altec gear, hey're a good "cosmetic" match I guess you could say (the 1588B's use the
Peerless 4838 input transformer, for example). Second, they're a one to one ratio transformer, so they don't loose any gain and have tremendous bandwidth and accuracy. Thirdly, they're NOT 600:600, they're actually rated for somewhat higher impedance. This is important since the way this circuit is laid out, the transformers come directly after the fader pots with
no buffer in between. In this circuit 600 ohm transformers could noticeably loose some bass due to the source impedance of the pots, but the 4834's have high enough primary inductance to keep a nice flat and rich bass response all the way to the bottom no matter where the fader pots are set. This is all worth mentioning, because these transformers DO have a downside which is that for best performance the direct outputs shouldn't be plugged into anything with lower than about 2K impedance, and preferrably higher. I doubt if this will be a problem, since most recorders/soundcards are 10K or higher."
:wink: 
