Opamp Labs - Inside the can

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rackmonkey

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There have been several threads over the years where people have asked what's inside these old Opamp Labs cans. I had a couple of noisy 425s and decided to donate them to science. Thought I'd share what i came up with.

The 425 is shown in schematics on the OL web site as an output stage/line driver, often with mic pres like the 360x series in front of them. It's also documented as a standalone mic preamp, a summing amp, and a makeup gain stage in limiter circuits. It's a pretty versatile little module.

p1a3MZal.jpg


The pink epoxy potting is nasty and refuses to melt. Instead, it just gets darker and crumbles when heated enough. Hard to get all of it off the board without melting the plastic components. Had a heck of a time getting this one apart, and I tore the edge of the circuit board off trying to pull it out. Due to the damage and the residue left on the underside, I've not been able to completely trace it out. I plan to do that with the second unit now that I know what I'm dealing with - hoping I can get that one out more cleanly.



Also, some of the active components aren't marked. But it's easy enough to see what's going on. A hybrid circuit like many of the late 60s/early 70s, with what looks like an 8 pin proprietary opamp (I believe it's the Opamp Labs 4009; see picture of that unit below), with a matched pair of BJTs on the input (the black, 6 pin cylinder) and a complimentary pair on the output. The 4009 is the shorter round metal can. You can see on the underside in the picture above that it's got 8 pins.

1OlOJOKl.jpg


OL 4009 proprietary opamp (from  web source - the one in this module is not labelled):

g2cdScPl.jpg


Differential input stage looks like one of the old Sprague TD series matched duals. I've had a few of the TD-101's around for years (pic below). The owner/guru at the local (old school, fantastic) electronics shop here told me he thinks Sprague just binned maybe BC-54x - BC-55x transistors and matched hFE & Vbe, then encased them in resin. Nothing like the super matched stuff in the LM394 or MAT parts, but very useful nonetheless.

ntZd91Al.jpg


TD-101s from my stash:

uPqoAJIl.jpg


One of the output transistors is marked. It's a 2N2102, used on so many discrete output stages of the era. The PNP of the pair is not marked, but it's probably a 2n4036, as that combination was very common back then. The rest is passive stuff. The white parts look like they might be diodes or inductors, maybe?

Jy79Lzxl.jpg


I seem to recall Scott Dorsey posting on another forum (maybe the old rec.proaudio board?) that the 360xx mic pres were just a 425 with a Beyer peanut transformer on the input. That seems credible enough. IIRC, he had (has?) a full OL console in his studio, which he bought from Norman Lear productions (TV shows from the 60s/70s).

It's crazy that you can still buy these things brand new directly from OL. Last time I looked they were still around $40 each, which is cheaper than most of the used stuff on evilbay.

Enjoy.

BT




 
Dmichel123 said:
Anybody ever busted open a 325? The link for using a 425 in an EQ circuit seems to be dead...

I don't think this is the one you're looking for, but i'm pretty sure I have it. This is basically for a full channel strip. Incorporates the 425 on the output, after the 325 eq (which is immediately followed by another 425 as a summing amp, then the second 425 output stage).

I'm traveling right now and don't have access to all my stuff, but I'll find it eventually and upload if someone else doesn't beat me to it.
 

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Old datasheet I dug up from the Wayback machine; with 425 as a "high quality mic preamp". If it's true what Scott D said about the 360 preamps being one of these with a Beyer transformer stuck on the front end, using these with better transformers on input and output might just make a nice quick and dirty discrete pre. Schematic shows transformerless, but input/output impedance specs are shown.

I will say that I have played with these to see about making an old Altec 1592 mixer into a 5 independent channel unit, with the mixer output pulled and 5 of these 425s subbed in and a 600:600 transformer out on each. Never got around to building the whole thing, but tests with those little Altec cans followed by a 425 sounded pretty good in a crayon sort of way. Also played with the Ampex plug in pres for the 440 units, and they sounded better than the Altecs. +/-18 volts is just about right for these things.
 

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more schematics that may be of interest. These were graphics that NY Dave had uploaded ages ago that now have dead links. All hail the wayback machine!
 

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Was looking for information on the 4009 op amp and came across this interesting nugget that’s part of an eBay listing for one:

“The Op-Amp Labs 4009 is a terrific piece of early 1970s technology. Bel Losmandy was one of the early fathers of integrated circuit technologies. While there were some usable IC chips back then few sounded as good as his 4009 op amp.  He coupled these fine little 8 Gold plated pin hybrids with additional output buffer transistors, input transformers, etc. into industry consoles for which he received an Academy award in 1971 for advancing the sound industry from tubes to solid state.
""ACADEMY AWARD" TO
OPAMP LABS'
B. J. LOSMANDY
At "Oscar" ceremonies on April 15,
1971 the Society of Motion Picture
Arts and Sciences honored OPAMP
LABS President Bela Losmandy with
its Class Ill technical award.
Mr. Losmandy's award recognized his
contribution of concepts and designs
for improved motion picture recording
equipment, through the application of
micro- miniature solid state amplifier
modules.
' B. J. Losrnandy""

They used a bipolar 24 volt supply unlike most of the other consoles. Most of the others utilized a bipolar 15 V or single polarity 24 V. So his consoles could swing 48 V on its rails which probably few people know. 

These devices were fabricated at CTS Microelectronics where I worked at the time and may or may not work to max ratings.  I have tested these to function only in a simple flasher circuit at about 15VDC on both the + and - rails or about 30VDC.  If you need them tested to a higher rail to rail voltage, I might be able to do that also for function only in the flasher circuit
...
I have copied their data sheet and will make it available as a pdf.  I also have the CTS hybrid drawing showing the internal layout/pinout and transistor numbers.”
 
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 can’t remember where I read it, but it seemed a credible source to me at the time (maybe S. Dorsey or Remy Ann David comment somewhere). Supposedly the 325 uses iron cored inductors. Seems physically questionable though. I have a couple but i’m reluctant to dissect one as i’d like to use them.  I’m keeping an eye out for a cheap or defective one though because i’m really curious what’s in there too. If I find one, i’ll open it up.

BT
 
rackmonkey said:
Was looking for information on the 4009 op amp and came across this interesting nugget that’s part of an eBay listing for one:

...
I have copied their data sheet and will make it available as a pdf.  I also have the CTS hybrid drawing showing the internal layout/pinout and transistor numbers.”

Did you contact the seller to see if they would share the pdf?
 
Thanks rackmonkey  :)  Great stuff !!

CJ of opamps lol ;D

2n2105/2n4036 is probably correct if its a pnp/npn output stage.  This was a common
pairing those days for +/- 24v goodies.  BTW..  The  Q8  AM10 used this pair at +/- 28v !! 

Another common pairing for +/- 24v was 2n2905A/2n2219A.  That was used in the
Sphere SPA-62  and others.

Best

GARY
 
mjrippe said:
Did you contact the seller to see if they would share the pdf?

Yes. I bought one of the 4009s (he's sending me 2 for 1) and I've asked for his permission to share the schematic. The datasheet is incomplete. I've asked if he has the whole thing. He's 72 years old and says that due to lack of interest in them he'll make a deal on the whole lot. Says he'd like them to go to someone who appreciates what they are. Will post when I hear back.

It's just a differential pair of 2n2484's for the input (low noise BJT used in lots of pre's such as Quad Eight AM10 and many other Q8 modules) into a 3494 emitter follower output. The emitter of the 3494 also feeds the base of a BC548 that does something to the feedback. What exactly is beyond my eyeballing skills. That's it.
 
Datasheet for the 4009. I plan to pull the other 425 apart this week and trace out a schematic, which will include the 4009 internals.

BT
 

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  • OpampLabs_4009_Datasheet.pdf
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