Modifying an Altec 1588B Mic Pre for Higher Gain

[tablebeast]

People are always saying that the Altec 1588B modules are noisy, thats BS. If you recap them there are no noise issues, its a very clean pre amp under normal gain settings, but gets wonderfully hairy when you drive it. I find it to be an incredible pre on rock drums, I can color each to taste. Anyway, the mod to give the 1588B module enough gain to stand alone and have a direct output pulled from all these altec mixers (1592a and B, 1599A and B, modular 1581A, etc) is fairly simple, plus while you have the shell off the module you can go ahead and swap out the electrolytic caps. Here are the pics describing the mod.

http://circuit-bent.com/1592bmod1.jpg
http://circuit-bent.com/1592bmod2.jpg

Jesse

[tablebeast]

For the direct output mod the first picture shows you whats going on there. I have a 1592B that takes channels 4 and 5 out of the mixer section, each of them going to individual XLR outputs. The important thing to note is that you want to change the 1 MEG output pot to a 1 K pot. You can tap the output of the pot directly and add a buffer stage or an output transformer to taste.

[leigh]

Nice work on this 1592b. I've read your posts here on TOMB about it and also read the Ebay item page. I just got a 1592b, and I'm planning on doing similar work to it.

For modding the 1588s, do you have any advice for opening those cans gently?

Also, if you didn't care about balanced output for the direct outs, do the 1588s have enough juice to drive at least a short line? I'd guess so, since you were just adding a one-to-one ratio xformer.

Mine came with 4 1588B cans and one line level (just a transformer) can. The previous owner had modded it with phantom power, by connecting the V+ of the input cans to the mic +/- lines (what becomes XLR pins 2 and 3) through 3.3k resistors. I don't know if that's such a hot idea, since, for one, that V+ is only at about 12 volts, and some mics want the full 48 volts.

Thanks for sharing all your work on these units!

Leigh

[leigh]

Also, am I correct in reading your first schematic there that you'd remove the two 1K resistors for the direct outputs? (for channel 5, R15 and R20)

Leigh

[RodC]

I have a few of these preamp modules I use direct out sorta like you describe. Everthing sounds pretty good through them except Kick drum. It always seems to distort these units.

Have you had any success using them with Kicks?

[nclayton]

Nice work on this 1592b. I've read your posts here on TOMB about it and also read the Ebay item page. I just got a 1592b, and I'm planning on doing similar work to it.

For modding the 1588s, do you have any advice for opening those cans gently?

Also, if you didn't care about balanced output for the direct outs, do the 1588s have enough juice to drive at least a short line? I'd guess so, since you were just adding a one-to-one ratio xformer.

Mine came with 4 1588B cans and one line level (just a transformer) can. The previous owner had modded it with phantom power, by connecting the V+ of the input cans to the mic +/- lines (what becomes XLR pins 2 and 3) through 3.3k resistors. I don't know if that's such a hot idea, since, for one, that V+ is only at about 12 volts, and some mics want the full 48 volts.

Thanks for sharing all your work on these units!

Leigh

Hi. I know this thread is kinda old, but I just ran in to it, and since this mod is public info now, I might as well make a couple of comments on it. I sold tablebeast that modded 1592 originally, and for good or ill am responsible for making the mods and the schematics posted here. The 1588B mod does have a couple of caveats. First, they won't drive true low impedance loads all that comfortably. The cans say that output impedance is 600 ohms, and stock, they can drive 600 ohm loads, but only just. Then if you do the mod, you're increasing gain by reducing negative feedback, which increases output impedance a bit. Furthermore, although the output impedance technically is 600 ohms (or now a bit higher), the emitter follower output isn't biased to quite a high enough bias current to drive a full signal into a real 600 ohm load. Also, you further increase output impedance with the volume pot. If you use a 1K volume pot the output impednace is now variable, from nominally 600 ohms to a maximum of about 850 ohms (250 ohms maximum added by the pot), and that's without takinginto consideration whatever's added by the reduced feedback. The upshot is, if you do this and decide to balance the ouptut using transformers, I wouldn't recomend just any 1:1 impedance ratio output transformer for this mod, 600:600 would be too low impedance. Something like a 2K primary would be about as low as you'd want to go. The transformers I used in Tablebeast's 1592 for output were actually 10K:10K, high enough inductance that the low end didn't suffer at all, but with the tradeoff that the outputs should only be sent to high impedance inputs of somekind. Of course most gear these days has at least 10K input impedance so this mod is pretty useable, and definitely makes 1592's more interesting.

The first schematic posted here shows how to rewire the volume pot. It is best to completely replace those 1M pots so output stays as low impedance as possible (with the stock 1M pots wired as stock output impeance can be as high as about 1,000,000 ohms turned down, which is a bunch of noise you don't want), and to discard the fixed 1K wiper resistor that isn't needed in the new wiring scheme. Also note that the pad resistor values are changed so that the pad can still be used with for APPROXIMATELY -10 and -20dB cuts with the altered 1588's. If you use the stock values I think you get about -25 and -30dB cuts (just a ballpark guess to illustrate the basic problem).

The resistors mentioned to give 12V phantom may not be unoriginal. All 1592B's had 12V phantom power always applied.

Opening the cans is a huge pain in the ass. If you're going to bother to do it, be sure to not just change values for high gain, but also replace the electrolytics and at least that one 27K carbon comp resistor with MF while you're in there.

Anyway, I don't imagine there will probably be lots of folks trying this mod, but I just wanted to give these small warnings just in case other people find this thread in the futue.

Ned

[mjau]

FWIW...I just got a 1592b (unmodded), and it's a cool beast when pushed hard.

[RodC]

Thanks nclayton, I have been doing some work with the 1588C modules. Do you have a schematic of one of these? Do you know what output impedance they are best at?

[nclayton]

FWIW...I just got a 1592b (unmodded), and it's a cool beast when pushed hard.

Yeah, they're a nice little bargain. The unmodded channels are definitely cooler than the modded ones since they go through the summing/output amp for the extra gain and drive needed by a real mic preamp. I just like to mod the last two channels with direct outs since you never end up plugging five things into a 1592 in the studio with the intention of mixing them to tape.

[leigh]

The upshot is, if you do this and decide to balance the ouptut using transformers, I wouldn't recomend just any 1:1 impedance ratio output transformer for this mod, 600:600 would be too low impedance. Something like a 2K primary would be about as low as you'd want to go. The transformers I used in Tablebeast's 1592 for output were actually 10K:10K, high enough inductance that the low end didn't suffer at all, but with the tradeoff that the outputs should only be sent to high impedance inputs of somekind. Of course most gear these days has at least 10K input impedance so this mod is pretty useable, and definitely makes 1592's more interesting.

Hi Ned,

Thanks for chiming in here. So you did the mods to Tablebeast's unit... that explains why he wasn't answering my more detailed followup questions!

I have two channels on my 1592 wired for direct output, but going straight to the RCA jacks on the back, unbalanced. Plus the 1 Meg pots are still wired to those channels, so that would be raising the output impedance if I understand you correctly. In all, it does sound a bit thin in the bass, maybe it's having problems driving even the 10K inputs on my DAW's converters?

So I'm thinking of adding transformer balancing to those channels, and I could swap the 1 Meg pot for a 1K as well.

You start off your discussion of transformers with an assumption of a 1:1 ratio. But what would you think about using a 10K to 600 ohm transformer on the output?

What maker of transformer did you use in Tablebeast's unit? They looked nice and compact in the photos.

Thanks,
Leigh

[nclayton]

I'd probably say THE MOST IMPORTANT thing to do when making direct outs on a 1592 is to change those pots. The original pots are fine for going into the internal high impedance mix bus, but they're not good for output, and they're wired "wrong" so that as you turn down you put a bigger and bigger series resistance on the ouput which is not good. Definitely repalce them with 1K's, rewire so that the 1588 output goes to the high lug (it's on the wiper stock) and the wiper goes to the output. Also, there's a 1K resistor that sets a lower limit to impedance the 1588 output sees. If the output's going to the fixed high lug of a 1K pot, this resistor is no longer needed, so go ahead and remove it.

As far as transformers go, the 10K:600 is a good thought, but the problem's that it will cut the output voltage down 4 times, and since the 1588's already run off a fairly small rail voltage you won't have any headroom. In Tablebeast's and one other than I did I used a pair of vintage Peerless transformers that were actually 10K:10K. This is a little weird since it means you should only drive high impedance inputs, but it's good for frequency response, since 10K is so much higher than the possible source impedance of the 1K pot. I personally like the compromise since you hardly ever have to drive a 600 ohm input anymore (and anyway, the little 1588 cans are honestly a little bit wimpy to be driving 600 ohm loads directly).

Still, I'd rather use a 600:600 than a 10K:600. The extra source impedance provided by the pot (250 ohms max) shouldn't cause too much trouble as long as you find a 600 ohm transformer that REALLY provides a 600 ohm load right down to 20 Hz and hopefully below. I think the Jensen JT-11-YMPC looks like a pretty good choice. It's only spec'ed to +8, but you don't need more than that to handle what the 1588 puts out, it has low distortion, the price is great, and most importantly it looks like it has the highest input impedance of any Jensen "600:600" transformer, and probably won't sound noticeably thin with the 1588 + 1K pot source.

If you don't need a balanced output, though, it might be best to just replace your pots and leave it at that.

Ned

[leigh]

Awesome, Ned, thanks for that info. I'll be trying out the 1K pot today, and recording before and after clips to know what it's doing.

Cheers,
Leigh

[ilcaccillo]

Hi,
thank you all for this thread, specially Ned for the detailed descriptions and advises.

I also have one of this mixers, and like everyone else I would like to have some direct outs.

I would like to ask if anyone has the 2 pics describing the High Gain mod that "tablebeast" posted in the first post?


Also,
What do you think Ned about adding a gain stage after the Pot and before the transformer? (for a direct out)

I was thinking in using an 2520 style opamp to provide more output and then drive a transformer to balanced the output. Would this be a good idea?

thanks

[leigh]

I also have one of this mixers, and like everyone else I would like to have some direct outs.

I would like to ask if anyone has the 2 pics describing the High Gain mod that "tablebeast" posted in the first post?

I will look for those pics - I may still have them around.

Also,
What do you think Ned about adding a gain stage after the Pot and before the transformer? (for a direct out)

I was thinking in using an 2520 style opamp to provide more output and then drive a transformer to balanced the output. Would this be a good idea?

I'm not Ned, but IMHO, an API 2520 (discrete) opamp would be overkill in this application.

I wound up just bypassing any kind of output pot, and wiring the preamp cans' outputs directly to the RCA jacks on the back. No transformer, no balancing. Works fine in my place, where cable runs are short, and I don't suffer from undue RF problems.

cheers,
Leigh

[leigh]

I would like to ask if anyone has the 2 pics describing the High Gain mod that "tablebeast" posted in the first post?

OK, found 'em and posted them here:

http://picasaweb.google.com/leighmarble/TechFiles#

I'm assuming tablebeast's files were just down due to being old links... if you want them removed, TB, just let me know.