Opamp Labs 325EQ - Inside the Can

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rackmonkey

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The benefit of Covid19 shutting everything down is a dropoff in build/repair work, so I can get to projects that I've put off.

Next up in this CJ-inspired series of dismemberments is the Opamp Labs 325 Equalization Amplifier. A couple of people asked about this one when I did the 425 deconstruction many moons ago, here: https://groupdiy.com/index.php?topic=69637.0. I’d been curious about this unit for a long time too.

(In addition to the 325 eq amp, I also tore up a non-working 360BM mic preamp. Next post will be on that. Also planning on looking at the Model 37 limiter module. OAL also made a 325L Limiter module, but I’ve never come across one. So the 37 will have to do.)

Here’s some older documentation on the 325. This is from a 1974 binder of OAL stuff.
ftJQx1uh.jpg


One thing that’s become clear having dissected a few of these is that starting with the torch is a mistake. When you heat the can and slide the potted board out, it tends to crumble and tear into pieces, unlike a transformer. Best way to get into these things with the guts intact is to cut the mumetal shield off completely, then use the heat gun and a very small tipped common screwdriver to pry off the potting epoxy bit by bit.

When you get the can off, you end up with this:
eDw0mlvl.jpg


Whatever thermoset they used, it’s excellent for the task. Hard as a rock and nearly impervious to heat.

Once through the tedious task of removing as much potting junk as possible, it’s clear that this thing is a 425 on a longer PCB with a few differences in the passive components. Nothing more.

dj7SHJll.png


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The differences between this board and the one out of the 425:

2 10k resistors on the inputs (see diagram below)

1 15uF tantalum capacitor to ground at the input (see diagram)

1 15uF tantalum coupling cap on the output (the 425 has no coupling cap on the output)

Shielded cables for the inverting and non-inverting inputs to their attachment points on the board

The pink crap was hard to get off to the point where not all component values could be identified by printed value. I was able to pull a couple of key resistors that the stripes had melted/got scraped off of and measure them though.

Now, the irony of all this work is that I came across the text snippet below from a Model 201 console channel kit AFTER I shortened my life breathing in the alien technology super-thermoset fumes doing this teardown! Don’t go to Vegas with me. The bright side is that it gives a better explanation of the circuit than I would have:

What I was able to trace out on the input:
A1GpJMJl.jpg


And from the belatedly discovered f*$%in’ manual:
pGDD84ul.png


The explanation above should tell you what you need to know to use this thing, but next post I'll outline the rest of what I found researching it. There are at least a couple of designs worth playing with that you could start from.

 

rackmonkey

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EQ Schematics

Below are some of the EQ circuits that I’m aware of that Opamp Labs used to sell kits for, or put into the consoles they built (which were fairly rare, I think, since their main focus was providing the parts/kits to build you own console). From what I can tell, they also provided a number of suggested EQ schematics that you could build from individual parts they sold that they didn’t package into kits. There may have been others than the ones I’ve dug up. If anyone has other schematics, post them please.

It would be pretty straightforward to copy any of these circuits if you have inductors of suitable values. And any of these designs could easily form the basis of an eq that you customize for your purposes using different inductors. Cinemag could easily make you some with the values needed if you wanted to copy these or create your own. There are probably others who would as well. Don Audio might have some pre-made ones that are fairly close to the values needed, but I haven’t checked. (EDIT: Don Audio sells custom inductors with up to 7 taps/windings per core. See post a couple of responses down for info.)

Model 109 (3 knob):

F5QNvoWl.png


EDIT: added full size image attachment as these schematic values are impossible to read.

Model 109. This is a rare one, but I’ve heard good things about it from personal sources I trust. I haven’t found or built one myself. Probably the best of the designs from OAL I’ve seen for our purposes. It uses 6 of the multi-tapped model 87-x/8709-x inductors described below. These are really nice ferrite core inductors, and you could get them dirt cheap up until last summer. Irene Losmandy (the founder’s widow) may still have some hiding somewhere from what she told me, but she may have to dig for them.  More on the inductors in the next post.

Models 103/104:

A 2 knob shelving EQ design that I’ve usually seen included on the many Model 201 channel strip kits that still show up online. I think these were probably designed with speech input/broadcast duty in mind. The 8083-x series inductors are about half the size of the 87-x/8709-x series  in the 109. But it’s a space-saving design that may be worth experimenting with. There are pictures of a set of these I bought in the next post, where you can see just how little space they take up.  And more info on the 8083-x inductors there as well.

0FysMQMl.jpg


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Models 105P/106P

Uses the larger model 87-x/8709-x inductors, rather than the little 8083-x series. Another two knob design, and another that would be good to experiment with/modify to suit where space is tight. Similar in design to the 103/104 above. Like those, it uses 1 PCB attached to the bottom deck of a dual, concentric switch, 2 switches/PCBs per set. Would be easy to recreate or customize from using perfboard.

UrUGaNbh.png


Next post I'll share what I know about the inductors.


 

Attachments

  • Opamp Labs Model 109 3 Knob EQ - 325 lg.png
    Opamp Labs Model 109 3 Knob EQ - 325 lg.png
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rackmonkey

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OAL Inductors

As far as I know, OAL sold 2 different inductor sets for several different EQ kits, but also sold the inductors individually.

You can still find full OAL EQ kits here and there on Reverb or eBay now and then. Most of them I’ve seen are Model 103/104 kits described above that were made for Model 201 channel strips (see schematic above). The more desirable Model 109 kit is really rare, on the other hand. Unless you find kits, and given the difficulty/impossibility of obtaining the original inductors by themselves, you’re probably better off winding your own/getting some made if you want to try to recreate any of these circuits. But I did the research, so I’ll share what I know.

87/8709 series:

Some documentation refers to “87” series inductors, while other docs refer to “8709” series inductors. From the documentation I've found, these were probably the same parts with different numbers.  From the schematics, the frequency bands associated with each tap of the 8709 series inductors are close enough to the 87 series that this could be the case (see Model 109 EQ above and the chart of values for the 87 series below), but if it’s true then the nominal frequencies are off in some of the bands by a fair amount.

These are the inductors found in the Model 109 and the Model 105P/106P set, above. They have large, 1 inch diameter cores. The series consisted of 3 multi-tapped, Siemens ferrite inductors. 

3noHyVuh.png


The model 87-x inductors were still available to buy from OAL up until last summer. I bought 10 sets of 3 then, at $10 per inductor. I was told that that was the last of the ones that they don’t have to dig for. (Sorry, not interested in selling)  :). I was going to deconstruct a set so that I could share the specific ferrite cores used, wire gauge and turns/tap so anyone who wanted to could get some cores and wind their own. But the cores are these old Siemens ferrites from the Pleistocene, and I can’t even find the data for them in the 1983 Siemens ferrites catalog I’ve got. I also can’t find any of the numbers in any of the old TDK/Epcos catalogs, which seem to use the same nomenclature Siemens did. We could probably calculate AL for each core, but I was hoping it would be a simple matter of looking them up. Maybe someone who knows more about ferrites than I do could suggest equivalents. If CJ wants to tear them down and tell us how to build them, I’m happy to send him a set.

XCScVOVl.jpg



8083-x series:

These were smaller inductors (half the size of the 87 series) and are the ones used in most of the vintage consoles and kits I’ve seen online. These are the ones used in the Model 103/104 described above. I have one set of these that’s part of a completed EQ kit from a Model 201 channel strip (Models 103 and 104 EQ kits). I bought them already built, but I haven’t tried the kit out yet. The smaller size of these makes me wonder how quickly they’d saturate. But they do conveniently fit onto a small board, making for a space saving design, as in the Model 103/104. I’ll eventually get around to experimenting with the 103/104 kit that I have, and when I do I’ll post what I find out.

PMycmfBl.jpg


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That's it.

 

rackmonkey

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Forgot to post the details on the Model 87-x inductors.

Note the simple two-band bass/treble control design at the bottom of the page (Model 111). I've never seen a kit for this one. Note also that this design uses the smaller 8083-2 inductor from the two knob EQ kit mentioned above (Model 103/104 kit). Just kind of smashed together on a page with the details of the larger 87-x inductors.

RxrrBiEh.png
 

rackmonkey

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mjrippe said:
Wow, incredible wealth of info.  Where were you 20 years ago when I had a pile of these parts?  ;)

The fact that I was sitting on the same pile of parts is why I started doing this  ;)

These things are still available new, and they’re so cheap,  it amazes me more DIYers don’t experiment with them. I don’t fancy being the pied piper for these things, but for anyone that would build a Melcor or Quad Eight clone channel, I’d say try out the 425-based mic pre in the schematic I posted in the 425 tear down post. Cheaper, less effort, and it’s right in that wheelhouse performance-wise.

I gutted a dead Altec 1592 and plugged 425’s into the octal sockets. Powered them at +/-15V using an encapsulated Acopian supply and an Mnats phantom board. Stuck old Jensen JE-6110-k’s I had on hand in reverse on the inputs (for 150/600:10k) and Edcor 1:1s on the outputs. 5 channels of 70s DOA sound for dirt cheap and I did it in a few hours over a weekend. 
 

rackmonkey

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If you wanted to build any of these circuits, Don Audio sells custom inductors for  34,58 €:

https://www.don-audio.com/Custom-Inductor-RM8-Core

Not bad. You can do up to 7 taps/windings, so you could easily shave the 6 inductors required to build a 109 EQ down to fewer. Depending on whether they can do separate windings vs taps on a single core, you may be able to get it down to 2 or 3 inductors per EQ.

The values you'd need are:

2 x 1.6H
2 x 800mH
1 x 400mH
1 x 100mH
1 x 50mH
2 x 25mH
1 x 12mH

I emailed Irene Losmandy from Opamp Labs last week to see if she has any inductors left. She still hasn't answered. I hope she's okay. She's in her late 80s. She always takes a while to respond, but not this long.
 

rackmonkey

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Thanks for scanning, Mike! Hard to find those old full catalogs.

While we’re here, it’s worth mentioning that the mysterious Model 109 EQ kit has finally surfaced - at least in part.

Same guy that sells the 201 channel strips, 103/4 and 105/6 EQ kits (and those anodized aluminum western electric style console knobs) listed a few 109 kits on Reverb recently. They appear to have sold now:

1 x 1968 Opamp Labs 109EQ Kit 3 band Siemens inductor fully stepped mastering EQ Quad Eight Langevin | Folie Audio | Reverb

No electronic components were included other than the inductors. I suspect that, like the 103/4 and 105/6 kits, there were once separate bags with the resistors & caps included with the kit. He also listed it as a “mastering” EQ. These things aren’t stepped for mastering though - they’re clearly tracking EQs, just so there’s no confusion there.

I wasn’t going to drop the $700 per kit he was asking, but he had an extra couple of PCBs that he sold to me for research purposes.That cleared up a question I had.

I had speculated in the original post that the type 87-x inductors are likely the same as the 8709-x inductors included with the kit - just different part numbers. That’s true electrically, but on a mechanical level they’re different. The 87-x inductors that I got from Irene aren’t the same as the 8709-x inductors included with the kit in one way: at least one of the leads from the inductor taps is attached to the wrong mounting pin to fit into the PCB. There’s a pair of pins available for each tap, and one of the taps would need to be soldered to the other pin in the pair to make it fit. It would be easy enough to solder a jumper wire from one pin to the other and cut or bend the unneeded pin, but it does show that there’s a difference between the 87-x and 8709-x inductors. Why, I have no idea.

An oddity w/regard to the kits is that the schematic shows that you need 6 inductors for a 109 channel (3x 8709-1, 1x 8709-2 and 2x 8709-3). His kits only show 5 inductors in the pictures, and the listing only mentions 2x of the 8709-1 inductors are included. The PCB has slots for the 6 specified in the schematic. So it would appear that the kits were missing one 8709-1, in addition to the other passive components mentioned above.

Documenting in case anyone ever runs across one of these things again.
 
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beezer4

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Does anyone on this chain have a 360BM module they'd like to sell or trade for a couple fully working 425 modules? I'm building a pair of Opamp Labs channel strips and need one more 360BM. I also have an 325EQ module, T-10K / T-20 / T-25 / T-30 transformers I'd be willing to give up.

I've built a few stand-alone preamps and 500 series preamps with the 360BM / 425 with Jensen JE-123-S output transformers. Really enjoy the sound of them. I'm a big Quad Eight / Electrodyne fan and these fit in nicely.
 

kludge

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You will notice that the 325 is pretty much the same as the 425 with the addition of a resistor inside to get signal from Pin 7 to the opamp input (and the external reactance network) and the DC blocking cap. There's no reason you can't make an effectively identical equalizer circuit with a 425.
--scott
 

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