Op amp for Bose 901 equalizer series IV

Hi All!!

I am trying to have some fun with op amp swapping with an old bose 901 equalizer series IV. I also have a series III which is fully discrete. The series IV euqualizer came with LF353, pretty bad. I mounted machined socket so I could swap them. I then installed RC4558, bad highs, super harsh. I moved to TL072, improved things, smoother, but loss of volume. Changed to NE5532, getting better. Do you have any other suggestion to mount a "better" opamp? I included schematics of one channel.
Thank you!!
Screenshot_20230715-140032_Samsung Notes.jpg
AX tech editor
Joined 2002
Paid Member
If you have a 'scope you should really look at the output that there's no oscillations.
Most modern opamps are much faster then the older types and may oscillate in a circuit not meant for them.
It will often work, but there are no hard and fast rules.
The harsness you mention could well be oscillation.
If you don't have a 'scope, try an RF detector build from a diode and a cap, lots on Google.



  • rfdet.PNG
    16.7 KB · Views: 52
  • Like
Reactions: 1 user
Joined 2007
Paid Member
You also need to look at the power supply. It is unusual and limited in current delivery with neither rail regulated with respect to ground although each is stable with respect to the other. Note #2 on that diagram recommends TL072 and tbh I would take those over the 4558 (which is imo not good). That supply will not support four NE5532's or similar power hungry opamps.

If you noticed a difference in volume with TL072's then something is wrong. There should be no change in level with any suitable opamp fitted.
  • Like
Reactions: 1 user
An OPA 1642 would draw only slightly more power supply current (~1mA per IC) than a 4558 and would be a massive improvement in performance and is still relatively inexpensive (~$2 a pop).
I would also change all the polarized electrolytic capacitors for Nichicon ES MUSE bipolar caps.