All about the BA662 chip

Overview  The BA662 is a custom made DC controlled variable transconductance amplifier (or OTA) that Roland had manufactured by Rohm from the late 1970’s and is now obsolete. There are two types of chip with different suffices; the A version has been selected for lower offset than the B version. They are further classified by gain/gm. The BA662B can only replace another BA662B, whilst an A can be used in either application.

To quote Roland from the 100M Service Manual: “Color has great importance in circuits of some models”. However there is nothing mystical about the sound of the BA662, it is just a reasonable good OTA chip which is often carefully matched and selected.

Factory Selection The BA662 was used in places where close matching of the gain (gm) of the chip is important. Roland measured each BA662 and categorised them into 9 grades, ranging from 1 (low gm) to 9 (high) . They colour coded each chip with a small dot of paint on the top of the chip, the codes are:

  1. Brown
  2. Red
  3. Orange
  4. Yellow
  5. Green
  6. Blue
  7. Dark green or violet
  8. Grey or black
  9. White

Roland required exact matching of colour dot in some applications (e.g. portamento in the Jupiter 4), and within +/2 colours for others. This need for measurement and matching was obviously costly and led to the BA662 being replaced by precision OTA chips like the IR3109, which were also smaller. The BA662 lived on in some Boss pedals but was ultimately retired.

BA6110  The more recent Rohm BA6110 OTA chip is similar to the BA662, albeit with a slightly different pin out, and it too is now obsolete. It is 3-7dB louder in the same circuit, has low offset (3mV) and low distortion (below 0.2% typically) when the linearizing diodes are used. We use the BA6110 in many AMSynths circuits (often matched), as they are still available, work well and are low cost. Other designers have used the LM13700 or even dual transistor based clones of the BA662 (which are a lot more expensive than using the BA6110!).

BA662F The Roland 80017A chip array in the Juno 106 uses the IR3109 in SMD and 2x BA662F 14-pin SMD chips. The BA662F version has the same OTA and buffer as the SIP version but in a surface mount body with the following pin out:

  1. OTA Control Input
  2. No Connection
  3. Negative OTA Signal Input
  4. Positive OTA Signal Input
  5. No Connection
  6. Buffer Control Input
  7. Buffer Control Input
  8.  Negative Power
  9. OTA Output
  10.  No Connection
  11.  Buffer Input
  12.  Buffer Output
  13.  No Connection
  14.  Positive Power
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