Opamp Labs - Inside the can

[ruffrecords]

Anybody have any insight into what they might have stuck into the 325 can that would make the frequencies different compared to the 425 in a similar circuit? 325 circuits show .8H for 100Hz and 425 circuits show 2H.

In most circuits, the Q (sharpness) depends on the inductor and the overall circuit resistance. So different inductor values for a frequency indicate ether they were aiming or a different Q or the circuit resistance was different.

Cheers

Ian

 

[ruffrecords]

Interesting pics of the innards of one of their consoles:

https://reverb.com/item/1556129-opamp-labs-r2008-16mmo-recording-console-quad-eight-langevin

Love the 100% wooden frame.

Cheers

Ian

 

[rackmonkey]

I like they way they look too.  What's really interesting as I'm pulling this schematic together is just how close to  the Quad Eight stuff this thing is.  The 425 is like a CA127 with a second differential pair on the input. Which kind of makes it like an AM10, does it not?

Still need to pull that second 425 apart, but from the traces I can see so far on the first one (plus what I now know about the 4009 innards), it's looking that way. 

 

[PRR]

> It's just a ... feedback. What exactly is beyond my eyeballing skills.

You found the clues to solve a 45 year old mystery for me. Here's the broad solution:

 

[rackmonkey]

> It's just a ... feedback. What exactly is beyond my eyeballing skills.

You found the clues to solve a 45 year old mystery for me. Here's the broad solution:

You nailed it, brother. Spot on. I’ll post the whole thing at once in a few days.

 

[ruffrecords]

> It's just a ... feedback. What exactly is beyond my eyeballing skills.

You found the clues to solve a 45 year old mystery for me. Here's the broad solution:

Which seems uncannily similar the the Soundcraft Series 2 front end:

https://groupdiy.com/index.php?topic=44786.msg889340#msg889340

Cheers

Ian

 

[buildafriend]

> It's just a ... feedback. What exactly is beyond my eyeballing skills.

You found the clues to solve a 45 year old mystery for me. Here's the broad solution:

so a differential amp followed by a transimpedance stage followed by an emitter follower?

 

[Lee_M]

One of my friends cracked one of these open a little while back.

The unidentified PNP output transistor is almost certainly a 2N4036 or 2N4037 and the white things are possibly diodes, if my memory is correct.

 

[Lee_M]

Anybody have any insight into what they might have stuck into the 325 can that would make the frequencies different compared to the 425 in a similar circuit? 325 circuits show .8H for 100Hz and 425 circuits show 2H.

I think the main difference with the 325 is the two Ri resistors (both 10kΩ on the "Tone control using model 425 amplifier" schematic) are mounted inside the can, although that seems somewhat undeserving of it's own module imho.

If you've got a 325 module to hand, you could try measuring the resistance from pin 7 to pin 8 and then also from pin 1 to pin 5.

Comparing the pin 1 to pin 5 measurement to the same points on a 425 will probably tell you something.

As for what's causing the difference in frequency, the series resistor (either before or after the inductor) is responsible for that.

Also, there's nothing that I can see on those two EQ schematics to indicate that they were supposed to give the same frequencies, it's simply the same circuit with different LCR values.

 

[rackmonkey]

One of my friends cracked one of these open a little while back.

The unidentified PNP output transistor is almost certainly a 2N4036 or 2N4037 and the white things are possibly diodes, if my memory is correct.

Yes, the white things are indeed diodes. Thanks for the reminder to finish tracing it out.

 

[opacheco]

Yes, the white things are indeed diodes. Thanks for the reminder to finish tracing it out.

Dear rackmonkey,

did you finish the schematic for the 425 ? how about the 325 and 4009?

Thanks anticipated
opacheco

 

[rackmonkey]

Hey Opacheco.  You must have psychic powers!  I just pulled the 425 drawing back out to finish it a few days ago.  I got a good deal on a big box of 425s, 325s, 350BMs and 34As a couple of weeks ago off of eBay (about 35 plug in modules in all) and I thought I’d finish and post the 425, and then crack open a 325 and do the same thing. I also just built an octal test board that I plan to test several different circuits with and post the measurements, including a single 425 pre, a dual 425 pre with a second 425 configured to switch between low and high gain, and a couple different 325-based eqs. I plan to test the 425 pres both with and without transformers.

Unfortunately, I can’t find the 87xx inductors on OAL’s site anymore. I hope they haven’t stopped selling them.  If I have to, I may just use some UTC Vic variable inductors to test with. But I’d really like to get hold of a few of the ones originally sold with the 325s.

Good timing on the resurrection! anyway, stay tuned...

 

[opacheco]

Hey Opacheco.  You must have psychic powers!  I just pulled the 425 drawing back out to finish it a few days ago.  I got a good deal on a big box of 425s, 325s, 350Ps and 34As a couple of weeks ago off of eBay (about 35 plug in modules in all) and I thought I’d finish and post the 425, and then crack open a 325 and do the same thing. I also just built an octal test board that I plan to test several different circuits with and post the measurements, including a single 425 pre, a dual 425 pre with a second 425 configured to switch between low and high gain, and a couple different 325-based eqs. I plan to test the 425 pres both with and without transformers.

Unfortunately, I can’t find the 87xx inductors on OAL’s site anymore. I hope they haven’t stopped selling them.  If I have to, I may just use some UTC Vic variable inductors to test with. But I’d really like to get hold of a few of the ones originally sold with the 325s.

Good timing on the resurrection! anyway, stay tuned...

Oh man!!...jejejeje...Thanks a lot for take time to do that schematic work by hand!....Well I will wait for these info.

Thanks a lot again
Opacheco.

 

[Winetree]

I just found 2 - 360Bm and 2 - 425 in drawer in the tech room.
How could these be used to build a preamp?

 

[rackmonkey]

Several ways. You could get 2 channels, 3 channels or 4 channels, depending on what you want to do with it and your preferences. Just think of them as opamps. Specs for each are on the Opamp Labs site.

The 360BM is a self-contained mic pre with a little Triad input transformer stuffed in with it (probably a 425+transformer). Those things overload pretty easily. In my mind, you're better off just using the 425 and sticking your own input transformer in front of it (or going transformerless). I haven't calculated the optimum transformer secondary impedance for it, but I've experimented with 1:2, 1:5 and 1:10 and all sounded fine. I didn't measure noise though (will do that when I do the circuit tests I'm planning).

You can just use a single 360BM or 425 if you're only going into a >10k load. If you want to drive 600 ohms, use a second 425 as a voltage follower/unity gain buffer.

The attached schematic shows a pair of 360BMs going into a single 425 used as a summing amp. You could use this as a starting point if you want to use the 360s and modify to taste.

See Reply #6 for a schematic for a mic pre using the 425. That's the circuit I did a quick test with when I first posted this thread. It sounded like what it is - an early 70s discrete pre with useful character.

 

[rackmonkey]

360BM specs/app note.

 

[rackmonkey]

Finally got around to finishing this.  I still don't know which of the Sprague TD series dual matched transistors serves as the front end differential pair in this, but it's less important than understanding the design overall so I won't dig deeper into it. I've rechecked this several times and am fairly confident everything not inside the 4009 op amp is accurate. However, the schematic of the 4009 I got from the guy who worked for the manufacturer of it has some uncertainties about it, as you'll see when I post it in the next post.

Of course, if you see any obvious errors, let me know.

Edit: corrected schematic posted.

 

[rackmonkey]

4009 schematic. I have question marks in the schematic for the 425 posted above based on the "corrected"/drawn in values in this schematic. Two values for some parts, parts drawn in, etc.

This is a larger image than the original the guy sent me. What I thought then was an emitter follower after the diff pair is actually a pnp common emitter stage. The arrow is clear in this one.

 

[Ethan]

If you've got more to mutilate try submerging it in pure acetone. You can get it at most hardware stores--don't use wifey's diluted nail polish remover.  It took me about 3 weeks to "undo" a block of epoxy, but I wonder if it would've been faster if I put it in a tank with some kind of mechanical agitator.

 

[rackmonkey]

Good to know, Ethan. Although in this case, I wonder if a long soak in acetone would have taken off the resistor color codes, which is the main thing I was struggling to get at with that hard, crumbly pink stuff. Two 425 autopsies later, I finally got at all of them.