Altec 1674 (line mixer) input transfomers for API?
« on: December 10, 2020, 06:22:01 PM »
I have 4 of these little square can, shielded mic input transformers from an old busted Altec 1674. I'm building a buddy an API preamp for Xmas. I was wondering if they look okay for an API 312 input?

The only info I can find about them is briefly mentioned here: https://groupdiy.com/index.php?topic=44710.20 where BruceO says:  "Altec 1674 1678 1684 automatic mic mixers have some marked ALTEC/671-1323/651-8957 in them, square Mu Metal can, center tapped, 1:4 and 1:8 ratios. Tested pretty good, well shielded. These mixers also have a line output transformer in them (in the case of the 167x it is the same size as the api output trafo's I have not tried it though).

The primaries are in series coming out of the transformer. But I unpotted one, and inside I was able to seperate the center tap and I believe I can now wire the primary in parallel like API.

I'm having a hard time getting dcr readings. Maybe my dmm

But I have been able to measure 24 ohms between the top and bottom windings (with series wiring,) on the primary side. On the secondary (also series wiring) I was able to measure 1.3K between the top winding and the center tap. The bottom winding was connected to the case and didn't seem to get any continuity with other windings.

Does this calculate to a 1:8 transformer if I wire the primaries in parallel and leave the secondaries in series?

I attached a screen shot of the Altec schematic input, the best I could find. I'm able to read a 200K termination resistor.
« Last Edit: December 10, 2020, 06:27:33 PM by JW »


Re: Altec 1674 (line mixer) input transfomers for API?
« Reply #1 on: December 15, 2020, 05:23:15 PM »
I've harvested these little Peerless 4859s out of 167x mixers in the past. They are good sounding little inputs. But their max level is a bit lower than the 2622. It's been years, but my notes show max ratio of 1:10.

But I pulled out a couple that I still have, and I mine don't have the primaries in series coming out of the can. I have split windings on both primary and secondary.

You should have two groups of 3 pins on each side. If you lay the transformer down pins up with primary on your left, the bottom left two pins (the two closest to you) are one primary winding (DCR= ~13 ohms). Move to the group of 3 pins above that, and the bottom two of the group is the other primary (DCR= ~13 ohms). The top left pin is the shield (sometimes soldered on the inside of the can, sometimes on the outside of the can).

The secondary is laid out the same way on the opposite side - bottom right two pins are one secondary winding (DCR= ~750 ohms). In the group of 3 above that, the bottom two pins are the other secondary (DCR= ~750 ohms).

This gives you two primaries and two secondaries to work with, and thus you have several configurations possible.

As for your question about your measured DCR "calculating" to 1:8, it doesn't work that way. "1:8" is an impedance ratio (or a winding ratio, not sure which you think), not a DCR ratio. DCR ratios may or may not agree with impedance ratios, as the DCR is only the resistive part of the impedance and doesn't account for the inductive reactance. We often estimate impedance ratings in audio transformers by multiplying the DCR times 10. So, from the measurements detailed above, each primary winding would be estimated to be about 130 ohms (likely a nominal 150 ohms as that's a conventional winding nominal impedance for mic/line transformers) and each secondary would be estimated at about 7500 ohms nominal. But these are just estimates and can be off by quite a bit.

Even so, the 10x estimation approach is still useful for making educated guesses at characterizing an unknown transformer.

If it's a 1:8 or 1:10 winding ratio you're looking for, impedance ratio is proportional to the square of winding ratio.

IIRC, the 2622 was 1:10.
« Last Edit: December 15, 2020, 05:49:39 PM by rackmonkey »
Whether you think you can or you think you can't, you're probably right.


Re: Altec 1674 (line mixer) input transfomers for API?
« Reply #2 on: December 17, 2020, 11:23:24 AM »
Well, the EA2622 is 1:7 so I guess if we're around there. . ..

Thanks for all that valuable info Rackmonkey. My primary readings are the same as yours. I now have the primary wired in parallel, and I'm getting about 9 ohms across the paralleled windings. The secondary is a little different though. If you imagine mine, upside down, and with the primary on the left (3 pins per side) The transformer can shield connection is actually on the secondary, bottom right.

Just need to get this 312 up and running and I think I'm at 'try it and see' stage. Thanks again!


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