Echo North
Well-known member
One of the key features of the Lola Microphone Preamplifier (http://hairballaudio.com/lola/) is the inclusion of two 990 op-amps. Deane Jensen’s now famous JE-990 paper describing the design, testing, and theory behind a powerful ultra low noise operation amplifier, was first introduced in the February 1980 AES journal. The design started with the LM394 dual super matched transistor as the input device. The LM394 is actually a monolithic device consisting of two pairs of 50 transistors connected in parallel to eliminate random variations in the individual devices. In addition to being designed for low noise, the JE-990 is a powerful op-amp capable of adequately driving a 75Ω load.
Many commercial versions of the 990 design are available for purchase today. The John Hardy Co JH-990 (http://www.johnhardyco.com/990OpAmpDetails.html) is certainly the best and most well respected version. Luckily, it’s also one of the most affordable. An equally authentic version is Steve Hogan's (http://www.soundsteward.com/) TSS-990-C which is unpotted but not in 2520 format. Most of the other designs available for purchase fall short of the original design and are “990″ op-amps in name only. Shortly after the release of the Lola, I began considering offering a DIY JE-990 that would be affordable, authentic, and easy to build. Or at least as easy as soldering over 30 components on a one inch square PCB can be. Certainly it would need to a 2520 footprint and I would need to source an affordable LM394 substitute. Luckily much of this work was completed a few years ago by MNATs when he designed a DIY JE-990 2520 layout (mnats.net/JE-990.html). Many here have built this version and much info can be found in the original support thread (http://www.groupdiy.com/index.php?topic=14422.0). I contacted MNATs about reworking the PCB for a different input component to replace the discontinued LM394 and he agreed to give a shot.
The common replacement for the LM394 is the Analog Devices MAT12. The MAT12 is the successor to the MAT02 and is available in a TO-78 thru-hole package. A very elegant component, but not a cheap one. They can cost $15-$30 depending on quantity. Luckily there is another option. Analog devices also offers the SSM2212 which is the surface mount version of the MAT12 (same die) and offers even better performance specs. Even better, it can be purchased for a fraction of the cost of the LM394.
Pictured is the result. You can see the SM2212 is mounted on the bottom. What you may not see are the two 0.1uF SMD power supply bypass capacitors at the supply pins. In true MNATs fashion the layout is, on its own, a work of art. CR3 is a diode connected transistor matching Q3 and thermally coupled using a piece of heat shrink. The output transistors are thermally coupled to biasing diodes using the clip described in the JE-990 paper. The thermally coupled biasing diodes provide enough current limiting stability for the op-amp to drive a shorted load indefinitely with no heat sink. All of the components used are those quoted in the JE-990 paper or their modern equivalent.
The DIY-990 arrives with the bottom layer SMD parts (SSM2212 and two bypass capacitors) already soldered to the PCB. The six PCB pins are also pre-soldered. All that remains are 14 resistors, 11 diodes, 8 transistors, 3 capacitors, and 2 inductors. To make it easier an assembly guide is available for download (http://www.Hairballaudio.com/images/990/DIY-990-Manual.pdf).
Information on the fully assembled JE-990 and DIY-990 kit can be found at:
http://www.Hairballaudio.com/990/
The PCBs are made and assembled in the USA.
Many commercial versions of the 990 design are available for purchase today. The John Hardy Co JH-990 (http://www.johnhardyco.com/990OpAmpDetails.html) is certainly the best and most well respected version. Luckily, it’s also one of the most affordable. An equally authentic version is Steve Hogan's (http://www.soundsteward.com/) TSS-990-C which is unpotted but not in 2520 format. Most of the other designs available for purchase fall short of the original design and are “990″ op-amps in name only. Shortly after the release of the Lola, I began considering offering a DIY JE-990 that would be affordable, authentic, and easy to build. Or at least as easy as soldering over 30 components on a one inch square PCB can be. Certainly it would need to a 2520 footprint and I would need to source an affordable LM394 substitute. Luckily much of this work was completed a few years ago by MNATs when he designed a DIY JE-990 2520 layout (mnats.net/JE-990.html). Many here have built this version and much info can be found in the original support thread (http://www.groupdiy.com/index.php?topic=14422.0). I contacted MNATs about reworking the PCB for a different input component to replace the discontinued LM394 and he agreed to give a shot.
The common replacement for the LM394 is the Analog Devices MAT12. The MAT12 is the successor to the MAT02 and is available in a TO-78 thru-hole package. A very elegant component, but not a cheap one. They can cost $15-$30 depending on quantity. Luckily there is another option. Analog devices also offers the SSM2212 which is the surface mount version of the MAT12 (same die) and offers even better performance specs. Even better, it can be purchased for a fraction of the cost of the LM394.
Pictured is the result. You can see the SM2212 is mounted on the bottom. What you may not see are the two 0.1uF SMD power supply bypass capacitors at the supply pins. In true MNATs fashion the layout is, on its own, a work of art. CR3 is a diode connected transistor matching Q3 and thermally coupled using a piece of heat shrink. The output transistors are thermally coupled to biasing diodes using the clip described in the JE-990 paper. The thermally coupled biasing diodes provide enough current limiting stability for the op-amp to drive a shorted load indefinitely with no heat sink. All of the components used are those quoted in the JE-990 paper or their modern equivalent.
The DIY-990 arrives with the bottom layer SMD parts (SSM2212 and two bypass capacitors) already soldered to the PCB. The six PCB pins are also pre-soldered. All that remains are 14 resistors, 11 diodes, 8 transistors, 3 capacitors, and 2 inductors. To make it easier an assembly guide is available for download (http://www.Hairballaudio.com/images/990/DIY-990-Manual.pdf).
Information on the fully assembled JE-990 and DIY-990 kit can be found at:
http://www.Hairballaudio.com/990/
The PCBs are made and assembled in the USA.
