Altec 1628a Mods - and 1674 and 1678 .....

[RodC]

I'm sure someone has done this before, but I couldn't find any info on the web. I thought I would put a doc together with my mods/ideas cause ppl always ask me when they see this thing, how do you use that Mono mixer? Well it aint a mixer anymore then I explain the following.

I really wanted to pick up another 1628a before I started shining another light on it, but heck, its Christmas.

If you have anything to add please let me know, When I get this doc done Im going to post it on my website.

A paper for your review:

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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 1588C 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 some 1588B modules for more gain. However, after working with the 1588Cs 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 1588C 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 1588C 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 ?? 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 1588C 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 ?? 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 1588C 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 1588C 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.


Any feedback welcome.

[A.David.MacKinnon]

Thanks for this.
I've been working on mods for the altec 1606b mixer/power amp and have been having a hard time finding info on it. This helps alot.

I tapped out of the volume pot on each channel but also left each channel connected to the main mix. It doesn't seem to have effected the sound of the direct outs and it gives me the option of mixing 2 or more channels.

As for removing the LEDs and replacing them with a gain pot, i'd suggest using that hole to mount a phase reverse switch instead.

[RodC]

A phase reverse would be cool, but my Delta 44 allows me to do that for each channel, and I can always flip it in the software.

I ment to leave a link to the altec 1692C, here it is:


http://alteclansingunofficial.nlenet.ne ... index.html

[bayoucables]

That's great info on the 1628's. I have a couple of those. I have some other Altec mixers that I have modded for multi channel. The 1588's didn't have quite enough output, so I modded them as per the mods posted by tablebeast. The best thing I have found about the mixers is that with a 15095a input transformer they make a great DI. So good in fact that I sold my Avalon U5. I have a lot of info on Altecs, if anyone wants to exchange info, please email me.

[A.David.MacKinnon]

How are you using it as a DI?
Are you going right into the 15095a with an 1/4" - XLR adapter? Do you get enough level to go right to tape or do you have to run it into another pre?

[bayoucables]

Yes, I am going direct with a 1/4" - XLRM cable. The input of the 15095a is rated at 15K impedence. The output is from the normal output of the mixer, which has a lot of gain. I use a 1592B usually, which has a main volume knob and both 150 and 600 ohm output. I have used a 1599a mixer as well which doesn't have a main volume adjustment. The 1599a only has a 600 ohm output. It has plenty of gain for the input of my M-Audio interface. You can't use the 15095a with a mixer that has been modded for direct out from the preamp as the 15095a has no preamp circuit in it, it's just a transformer. A very good transformer!

[leigh]

I finished cleaning up and modding an Altec 1592b last week. It came with 4 1588b cans and one 15095a line transformer. I had started with the idea of following tablebeast's mods, but wound up doing something a little different.

First, I got complete schematics from this place, without which I would have been lost. The 1592b is divided into 3 sections: what I'll call the "main" section (all the point to point wiring), the power amp board, and the tone board. The schematics use different part numbering schemes for each section, so for example there's an "R5" in each of the sections. So to identify a part, you have to say "R5-main" or "R5-power."

Second, I re-did the power supply. To simplify wiring and free up some terminal lugs, I removed all the wiring for battery power, since I was never going to use that. Then I replaced C2 and C3-main (the big power caps), leaving the big transformer cans hanging off the back. Having those extra terminal lugs freed up from battery wiring gave me some anchor points to wire in new power caps. I also removed C4 and C5-main (the "death caps" from the power line to chassis). Opinions vary on the importance of both these steps.... make sure to search for second opinions over at The Lab.

Third, I opened up 2 of the 1588b cans and did the high-gain mod tablebeast describes. I also rewired the pad resistors for those two channels (channels 3 & 4).

Fourth, I was going to remove channels 3 & 4 from the mixer section and just make them direct outs. However, after reading junkshop's approach, I thought, why not just leave them hooked into the mixer AND mult them for direct outs. Well, there's kind of a reason not to leave them connected if you've modded the cans for hi-gain, but I'll get to that in a second.

I made direct outs for channels 3 & 4 from the "master mult" RCA jacks that were already on the back. These were put there to be line level aux-ins that inject signal right before the master mixer level pot. On mine the two RCA jacks now grab the direct outs from channels 3 & 4 directly from pin 1 on each of the cans. This way, the level of those direct outs is NOT affected by the channel's mixer pot, and the signal stays a little cleaner by not running over to the front panel, through the pot, and back. Note that, since I didn't add a transformer, these direct outs are unbalanced... good enough for me, for studio use with short runs.

Now, back to what I alluded to before, about it being a bad idea to leave hi-gain cans hooked up to the mixer. When turned up past about 7 on the mixer knob, these channels started "motorboating", or creating a low-frequency oscillation from internal feedback. (Again, search the Lab or elsewhere for a full explanation of this phenomenon.) In short, the rest of the Altec circuit was not designed to have the cans running in such high gain, and an internal feedback loop develops. I found a solution was to increase the filtering on the power supply feeding the cans, namely C16-power. It comes stock as 1000uF; I upped it to about 3000uF to get the motorboating to stop.

And there ends my tale.

Bayoucables, I'm looking forward to trying your idea of running a line-transformer channel as a bass DI!

Cheers,
Leigh

[A.David.MacKinnon]

I've been toying with the idea of adding switchs to allow me to take any of the 1588'a out of the main mix - This would allow you to route any number of channels to the main mix and still be able to use some as direct outs. This would give you the greatest flexability.

[leigh]

I've been toying with the idea of adding switchs to allow me to take any of the 1588'a out of the main mix - This would allow you to route any number of channels to the main mix and still be able to use some as direct outs. This would give you the greatest flexability.

If you wire it like I did, you don't need switches - you just turn the mixer pot all the way down in order to take a channel out of the main mix. The direct outs are active all the time. The only "disadvantage" I see to this scheme is not having level control over the direct outs (although the pad switches still work for them). I say "disadvantage" b/c I actually like it better this way. One less component for the direct out to pass through, one less control to set incorrectly in the heat of a recording session.

Leigh

[leigh]

Yes, I am going direct with a 1/4" - XLRM cable. The input of the 15095a is rated at 15K impedence. The output is from the normal output of the mixer, which has a lot of gain. I use a 1592B usually, which has a main volume knob and both 150 and 600 ohm output. I have used a 1599a mixer as well which doesn't have a main volume adjustment. The 1599a only has a 600 ohm output. It has plenty of gain for the input of my M-Audio interface. You can't use the 15095a with a mixer that has been modded for direct out from the preamp as the 15095a has no preamp circuit in it, it's just a transformer. A very good transformer!

I tried this today on bass, and I agree, it sounds very good! Pretty thick in the low mids, so it'd be good for slower or fingerstyle stuff. I also found that running a passive DI (I'm using a box I built that's pretty much just a Jensen transformer) into one of the mic channels on a 1592b works well too for bass, and gives more string sound so articulation comes through. Not as thick in the low mids, but the bass fundamental freqs are there in full. Better for faster or flatpicked bass lines, IMHO. As a common point of comparison, both these variations had much thicker low end than the DI on my RNP.

One caveat about the cable for this: the ground on the 1/4" side needs to connect to both pin 1 and pin 3 of the XLR side. I tried it first with just a regular TRS to XLR cable, and the sound was very quiet, with low freq attenuation. It might work fine if your bass has an output jack that's shorting the ring to sleeve, but on mine that did not work.

Also, a previous owner had modded my 1592b with pseudo phantom power on all channels (by running the 16V line that powers the 1588b cans through a pair of 3k3 resistors to XLR input pins 2 and 3 on each channel). The 15095 line transformer did NOT like this, however, and produced terrible, low level mushy sound until I went in and clipped off the connecting resistors on that channel.

Leigh

[A.David.MacKinnon]

Have you guys tried routing the output of the 15095a into one of the 1588's and then direct to tape? Just curious.

[@?,*???&?]

I am looking into doing something similar with my two Altec 1678s. I'd like to run them as two 8 channel units. These have fixed gain switches which make them work well for voice, but little else.

Thanks for the tablebeast posting. I'll look for some info there.

[RodC]

Jeff Robinson, I have seen the 1678s for sale before, I have never heard any of that series. Those are post green face units, do you have any examples of how they sound?

You can download the schematics for those here:
http://alteclansingunofficial.nlenet.ne ... index.html

[RodC]

Jeff, on the 1678 take a look at the schematic I pointed out. its marked page 9. You should be able to change the S102 switch to and the R103 (S202 and R203 for the other channel) to a pot, this would give you a variable gain. I would start with a 1K.

Looks like an easy unit to get what you want, it already has a line out for each channel. Im sorta regreting I passed a few of these up.

[leigh]

Have you guys tried routing the output of the 15095a into one of the 1588's and then direct to tape? Just curious.

Nope, but interesting idea. Impedance matching might be a bit rough: the 15095a can be wired for 150 or 600 ohm output, while the 1588b is labeled with a "150/250" ohm input (I think they mean a range there, because it's not switchable). In a 1592b mixer, the 15095a is wired for 600 ohm output (pins 3 & 4 tied together), so you'd have 600 ohms into roughly 200, and bigger into smaller impedance is NOT the way you want to go. Older audio gear tried to match impedance for maximum power transfer; these days most gear have low output impedances and high input impedances, aka "bridging" inputs.

With some rewiring, the 15095a can be set for 150 ohm output (break the tie between pins 3-4, and then tie pins 1-4 and 3-6), which would even out the impedances. If I'm correct in my thinking, a DPDT toggle would be enough to switch the 15095 output impedance. Might not be a bad mod to play with.

Anyway, I'm curious now, so I'll probably try jumpering this arrangement together to try it out!

Leigh