Altec 1628a Mixer Mods Convert to 8 channel preamp Rod

Altec 1628a Mixer Mods
Convert to 8 channel preamp
Rod_Childers@hotmail.com
12/27/05

My goal for this project was to utilize the 8 1588B preamp modules in my Altec 1628a mixer without any of the other circuitry of the mixer. My initial goal was to do the 8 output conversion then look into modifying the 1588B modules for more gain. However, after working with the 1588Bs alone I found that their 33db of gain was enough for my needs. (I actually put a –10db pad on 2 of the channels)

A lot of this was put together using other Altec schematics, I have not been able to find one for a 1628a, if you have one please send me a copy. Thanks!

With my modification I totally isolated the 1588B modules from all the other circuitry. You could try leaving it all in place and still use the mixer functions.

The following assumes the 1628a mixer is equipped with 8 1588B preamp modules.

Creating 8 output channels:

1. Remove the ribbon cable that connects all the modules. (This will be reused, don’t throw it away)

2. Remove all the preamp circuit boards and the power supply / master circuit board.

3. Remove all the wiring to connectors on the back of the unit that will not be used. This step is optional, I did it to reduce clutter/noise. I removed all the wires coming from the large connector on the power supply module leading to the back of the unit. (Link In, Link Out, Output terminal strip, Auto Man switch, Master Ext switch, input priority jack) I left the Auto Man switch in place to use for my –10 db pad for channel 7 and 8. The input priority jack became my channel 8 output.

4. Drill holes for 8 ¼” output jacks, I arranged them like the input jacks. Remove the Master Ext switch and enlarge the hole to accept the jack for the channel 7 output. Use the input priority jack for channel 8 output. You should be able to squeeze them all in below the model number. Don’t mount the jacks until you are ready to solder wires to them.

5. Now would be a good time to check all the solder joints, I re-soldered all the connections on the bottom of the preamps and the Mix pots. My boards were a bit short of solder and were showing some cracks.

6. If you trace the output of each 1588B you will notice that it leads to the front of the preamp module and ends up on the center lug of each of the front panel pots. (Mix/Volume pots) This is where you will pick up the output of each preamp. If you want to isolate each module, use a blade to cut a small section out of the circuit board print that connects to the center lug of the Mix pot. Solder a shielded wire (or twisted pair) to the lugs of the mix pot. The center will be the positve output. You will notice that one of the other lugs connect to ground, use this for your shield/negative. Run this wire to the rear of the unit to be soldered to your output jacks. (You may choose to put an isolation transformer here, my unit is right under my DAW breakout box so noise was not much of an issue.)

7. If you wish to add a pad to some of the preamps read the section below before mounting the preamps.

8. Mount the preamp boards one at a time, solder and mount the ¼” jacks to the rear of the unit.

9. Mount the power supply board and replace the ribbon cable that connects all the boards.

Adding a –10db/-20db Pad:
If you look at the diagram on the side of a 1588B you will notice that pin 2 is the gain control. If you connect this directly to ground that is full gain (0 db)

If you look at the schematic for an Altec 1592b mixer you will see how to utilize this gain control. (There seems to be lots of info in this mixer, but I have not been able to find a schematic for a 1628a) You will notice that if you place an 11K resistor between pin 2 and ground you will have a –10db pad, if you place 111K (11K + 100K) between pin 2 and ground you will have a –20db pad.

If you look at the preamp circuit board for the 1628a you will notice that there is a small green resistor looking component connected between pin 2 of the 1588B and ground. (It even has an R indicator on the board) This is not a resistor, its just a jumper, jumpers made like resistors were easy to load into assembly machines. To add the pad you must remove this jumper and replace it with a resistor of your choice, or you can use a switch to connect the pin 2 to ground or go through the resistor. I utilized the Auto Man switch to switch resistors/ground for channels 7 and 8.

Notes.
The switchs on each channel marked IN OUT appear to be a low cut filter. If the switch is in the IN position the output of the 1588B is forced through a 1uf capacitor, thus blocking the lows. If the switch is in the OUT position the capacitor is bypassed.

Other Modifications I have considered:

Output transformers, I haven’t needed them yet.

All the LEDs on the front look cool, but I have considered taking them all out and replacing them with a Pot to use as a gain control. This would be cool, just wire the pot in place of the resistor mentioned in the Adding a –10db/-20db Pad section above.