Discrete Operational Amps

The API 2520 discrete op-amp

Op-amps have been an important part of audio electronics for a long time. I remember reading as a kid about the “new” 741 types, and how op-amps would reduce electronic design to little more than building blocks. Than never exactly happened with audio circuits, but they did pretty much alter the landscape, and you’d be hard-pressed for find anything electronic in your studio that doesn’t use them.

The Melcor 1731 op-amp… somewhat rare

The “operational” part of op-amps refers to the inputs. There are two, an inverting and a non-inverting input. By taking some of the amp’s output and looping it around to one of the inputs, the amp can be made to do all sorts of handy things.

The venerable 709 opamp

There were some folks who realized early on that conventional op-amps required circuitry compromises, and that they could build their own operational amplifiers out of discrete parts (transistors, resistors, capacitors, and diodes) that better suited their needs, and outperformed the 309s and 741s that were available back in the day. The Jensen 990 and the API 2520 come to mind. There may have been others, but these two are the only ones that I can recall that have a long record of commercial availability and good performance. (Both companies are still producing audio equipment, and have a reputation for very high-quality, great sounding gear) These were used in a number of very high-end desks at a time when most desks were custom-built. Op-Amps for audio circuits have improved very dramatically since the days of the 741, of course, but there are still cases where discrete op-amps work very well.

So how do I get one of these little buggers? If you could find an affordable one used somewhere, you’d be extremely lucky… an API 2520 sold for $133 on eBay recently, and I saw no Jensens or Melcors. But there are some alternatives. My favorite is a tiny circuit board offered by

Four DOA2520 discrete op-amps… two have been completed, and two are about halfway done.

Classic Audio Products of Illinois. It’s an API2520 clone using available parts… some of these old circuits call for germanium transistors, which are impossible to find. I ordered one of the full kits ($17.50) and a few bare circuit boards ($2.30 each… a bargain).

Assembling these is not for the beginner or the faint of heart… they are very small, so that the finished op-amp has the same pin spacing and footprint as the original. Fortunately, these are very high-quality little boards. The holes are through-plated, so they take up solder very nicely. But they are so tiny that you need to be very careful when placing the parts to be certain that the right leads are going in the right holes. I have a stereo dissection microscope that I use to solder these boards… some sort of strong magnifier is necessary. Fortunately the boards come with a downloadable PDF instruction manual that guides you through the placement of the parts.

The circuit boards, like the originals, are pretty tiny.

Tip one is to use a pin jig to solder the pins. This is just a small block of wood with holes drilled in it to hold the pins straight while they are being soldered. I held the board over the block still with a spring clamp, and marked the location of each pin with a large needle. The needle pricks were drilled with a drill press. Now I place the pins in the holes, then place the board on top of the pins and solder from the top. Since the holes are through-plated, this works nicely, ensuring parallel pins. The pins on my first op-amp are a little wonky because I soldered it before I made my little wooden block pin jig.

Tip two is to solder the large transistors Q7 and Q8 just after soldering the pins, rather than following the recommended stuffing order. There are some nearby resistors that can be too tight to these transistors. By placing them first you can position the resistors in their holes with a little better fit. This is only cosmetic, but you may as well have these look nice. It goes without saying that you’ll need a good iron with a very fine tip.

A good stereo microscope isn’t a requirement, but it helps. Some kind of magnifier will be required to solder these.

I haven’t placed these in any equipment yet… just like the op-amps themselves, the preamps that use them are quite expensive. But just like the op-amps, there are kits and circuit boards available, and I’ll get started on some of these when time permits.

For the truly adventureous, you can roll your own from scratch. The circuit board for the Melcor DOA  can be seen here. Discussions about these and other discrete operational amps can be read on the Gearslutz forum, a fantastic resource for old studio gear lovers, and several folks have successfully reproduced classic circuits. Some of the difficulties include finding “unobtanium,” a favorite word of mine meaning electronic parts that are no longer manufactured, with no modern replacements. (germanium transistors, for example).

Update: Here are some other discrete op amp links:

1. http://www.passdiy.com/pdf/diyopamp.pdf– a great article about discrete op amp design from Nelson Pass.
2. http://www.forsselltech.com/downloads/design_discussions/JFET%20Opamp.PDF Fred Forssell’s article, “A Simple Class A JFET Operational Amplifier.”
3. http://www.forsselltech.com/media/attachments/Class_A_JFet_Opamp.PDF Another op amp schematic from Fred Forssell
4. http://www.johnhardyco.com/pdf/990.pdf John Hardy’s article, “990 Discrete Op Amp.” Several circuits here, including mic, phono, tape head preamps, a summing amp, and the MPC-1 preamp.
5. http://www.diyrecordingequipment.com/directory/ Several discrete op amp designs are reviewed in the Project Directory. Click on “Other Gear” and you’ll see a heading for “Discrete OpAmps.”
6. http://seniordesign.engr.uidaho.edu/2004_2005/tucson/Files/Final%20Report.pdf High Voltage, High Slew Rate Op Amp Design from Apex Microtechnology (Warning- high math hazard, may cause brain lock.)
7. http://www.eisenaudio.com/diy500/tables/opamps/ Eisen Audio’s Discrete Op Amp page, has a large number of photos and descriptions of many opamps for the API 500 series circuits.
8. http://www.sg-acoustics.ch/analogue_audio/discrete_opamps/index.html Samuel Groner’s discrete op amp page