Using Opamps


Linkwitz has been using the OPA2134 dual opamp in all of his analog circuitry for many years. I heard his Orion speakers pushed hard with a very high quality recording of an orchestra, and thought they had the best midrange and treble of any speaker I've ever heard. Since I fully respect his technical abilities I've been using the OPA2134 opamps for the last decade or so, with no regrets at all. While there may be opamps that look better on paper, I think it's a waste of time to pretend that any other opamp is going to sound audibly any better. It's going to be about the circuit as a whole, not the opamp. TL072 opamps are probably fine for most people, and are cheaper. 5532 opamps are great too, but have a lower input Z (bipolar input transistors as opposed to Fets), which could throw off high Z filter circuits a bit, but they're real good otherwise.

The 5 areas that many opamp circuits fall short on are:

  1. I always put a passive Rf filter in front of any opamp circuit as a whole (for signals entering the chassis), so I don't ask the opamps to do something they can't do well. I usually set this passive Rf filter circuit (an R and a C to Gnd) at about 80kHZ (maybe a 1K R in series and a 2nF cap to gnd. for example).
  2. In most feedback amplifier circuits, a cap is usually placed across the feedback resistor to help keep the circuit from oscillating. That's the short story. The "loop gain" of the circuit must be less than one, at the frequencies where negative feedback becomes positive feedback, due to all reactances involved. Phase margin is difficult for many to understand, yet is critically important to get right and verify. If oscillation does occur, it can blow up your speakers and/or poweramps.
  3. Another important thing is to put a 100 - 200 ohm R in series with the output of any opamp circuit (outside of the feedback loop) to minimize the effect of the load reactance messing with the phase margin of the opamp circuit.
  4. Put 0.1uF power supply bypass caps within an inch of each opamp (PS to Gnd.) (exact value not critical). Closer is better. This insures a more predictable and consistent phase margin over frequency.
  5. In a multi-stage circuit (preamps, active tone controls or active crossovers for ex.), if the peak signal is below a volt or two, and you've got a +/- 15VDC power supply, You've got plenty of headroom, so you can put a gain of maybe 4 in the first stage, and resistively drop it back down at the output, to push down hiss noise and push-pull crossover distortion effects (effectively). This may not be necessary, but pushing down noise floor never hurts.


For cap values less than about 10uF, I use polypropylene or polystyrene caps if possible. They have extremely low dielectric distortion. Teflon is technically even better but is expensive bulky and unnecessary. You don't need the expensive versions. Metal film resistors hiss less and should be used in front end of any low signal level circuitry.

These tips will make a much bigger improvement in circuit operation than the difference between an OPA2134 and any opamp that anyone thinks is better for audio. Not using these techniques properly is IMO why many people think they hear a difference in opamps that are supposedly better than an OPA2134. They might have intermittent or spurious supersonic oscillations occurring with some opamps and not others, and not realize it. Rf energy fed into an opamp (or any audio circuit) is likely to get detected and/or cause slewing related distortions which will cause intermodulation (I.M.) sum and difference distortion products. The sum products can cause further slewing, and the difference products are likely to generate noise and distortion in the audio frequency range. Digital sources are more likely to have some Rf energy in them, so the Rf filter at the input makes good sense. In the guitar amp world, guitars make great antennas for picking up all kinds of Rf energy, and delivering it to the amp circuit to be detected, creating significant non-musically related distortions. All of my amplifier circuits have passive RC rolloff components at the input, before getting to the first amplifying device.