Altec 1588B Discrete Preamp Build

[dgibney]

Hey there!

I have decided to design and build my own mic preamp using the Altec 1588b as my foundation, and I'm in need of some design help.

After doing some research, I came upon this thread and learned that I need to build an output stage for the 1588b. This is where my confusions starts. The two options I'm looking at are using a DIY 1290 output stage or a 2520 output stage.

My inclination is to use the 2520 as the basis for my output stage as I have a few of these laying around that I could use, and I love the sound of them. What would this look like? How would I wire a 2520 to the 1588b? What are my other design considerations for doing this? I assume I would also need some sort of transformer?

Any help will be greatly appreciated.

P.S Here are some addtional links to info I have found:
1588b as mic preamp
1588b High Gain Mod (I don't want to do this modification, this simply has some schematics and info on the 1588b in case anyone needs that.

 

[dmp]

I guess to start with, what specs do you want the mic preamp to have?
The 1588b is a transformer & amp that is switchable from 34dB to 20dB gain. Want more gain than that? Then you need to add another gain stage. The 1588b wants 18 VDC based on those threads you linked.
Look at the Altec 1599C mixer schematic to see how the 1588b can be followed by a second gain stage. If you are following the 1588B with a nice discrete opamp like a 2520, then the 2520 would typically want +16v/-16v. It's not going to work on just 18V. So you could make the simple 3 transistor circuit in the Altec 1599C schematic with just 18V to give you up to 60dB gain. But do you want an unbalanced out or a balanced out? If you want it to be balanced, then you could use a output transformer. You might be OK putting a 600:600 line out transformer after the Altec 1599C gain stage, but you'd want to try it and make sure it is happy. Not sure it could drive the load, depending on what you connect it to. If you were going to build a 2520 line driver, You could put a line output transformer (2503) after the 2520, see the 312 schematic. The output from the 1588b would be connected to the (+) of the 2520, probably with a cap between them. You could probably build a +16v / -16 psu and run the 1588b on the +16 rail. You'd have to decide how much gain to set the 1588b at and how much gain to select in the 2520.
Or, finally, you could just also use a balanced line driver chip, like DRV135 after the 1588b (if 34dB is enough gain) or the 1599C gain stage (giving you up to 60dB).
If you also want phantom, then you need 48VDC.

So, decide what you want from the preamp and then decide what direction to go.

Edit: removed 'op'

 

[k brown]

I don't see why it would need an 'output stage' - has less than 10 ohm out Z and sufficient current to drive cables; unless you need it drive some ancient Low Z inputs (under 600 ohms). Only needs an output transformer for balance out, or even simpler, an impedance-balance out, by adding a 47 ohm build-out resistor to XLR pin 2, and another 47 ohm from pin 3 to another electro (same as already on the output) to ground. I've built several of these (and 1578s) powered by 2 9v batts, and they drive long cables into 10k inputs no problem. You'll want to be sure not to have phantom on, due to the polarity of the out cap(s).

 

[dgibney]

I want a versatile mic preamp that can handle a variety of sources. My hope was that it would have a balanced output. I guess it would be best to start by building the entire circuit for a 1588b gain stage: power, switchable gain, and output attenuation. Once I have this built, then I could build a second gain stage with balanced output.

As far as building this simple design, I see a few different options depending on the schematic I see. Some mixers implemented "0, -10db, -20db" gain switches, while others just had "Low, Hi" gain switches. My mixer uses the "Low, Hi" gain switches so I was hoping to implement that. Is the schematic I attached correct for that?

Additionally, it appears that people have used different values for the output attention pot. I have seen 750 ohm, 1m, and 1K pots being used. Does this matter? Anything else I should consider?