Op Amp with Ideal Input/Output Impedances

[JFace]

I know the ideal op amp doesn't exist, but I'm looking for a high input impedance (greater than 1M ohm) and a low output impedance (flat or nearly flat with rising frequencies). I am using it as an input buffer for a guitar signal that will drive the current sensing pins of both a VCA and rms converter. These pins require a generally flat output impedance for high frequencies, otherwise, you get oscillation. This is generally acquired using a classic like the NE5532, but I'm hoping to find something with a higher input impedance as well. I have been reading datasheets, but unfortunately most of them do not have this spec or a graph. Does anybody know of an Op Amp (or otherwise) that could accomplish these goals?

[bool]

OPA2134 should do it

[Processaurus]

FET input opamps are great for the high impedance input stage of guitar processors.  I couldn't tell if the OPA2134 has FET inputs.

Did you rule out the trusty TL072?

Remember there is a 250k or 500k volume pot (acting as a resistor to ground) across the guitar pickup, so extreme high impedance inputs over 1M or 2M won't do very much in that scenario, except add noise.

[PRR]

Pencil the opamp's open-loop output impedance as 300 Ohms.

Now, how much NFB do you have? You say "buffer", I will ass-ume unity-gain. With typical 10MHz GBP opamps, you have >1,000:1 excess gain over the guitar band. Output impedance will be <300/1,000 or 0.3 Ohms.

What goes into these unspecified VCA and RMS? There are MANY variants. The ones I know best are the dBx/THAT types. For in-console use they take a 22K resistor in series to the input. Compared to 22K, 0.3 Ohms is nothing.

However. This type VCA and the 22K value are aimed at "in console" use, where signal level runs up to 10V (but more often 1V). Unity-gain off a guitar will be much lower level. You may get into hiss, or obscure offset problems. I would *think* that you want adjustable gain (possibly adjusted on bench then set) before a VCA.

Like Ben, I think it really is hard to beat the TL072 for most hi-Z audio things. OPA2134 may be better? I do know the '072 can run 10Meg or 100Meg input resistor for audio applications (find the input current specs and multiply by many million; this must be much-much less than the supply voltage.) As has been said, 1Meg or maybe 3Meg is diminishing returns for guitar uses. (And won't raise hiss when guitar or short-jack is present.)

Or K.I.S.S. If you have a known-happy plan based on '5532, put a '072 in front.

[karbomusic]

OPA2134 has a FET input. I've used them a few times for buffers and am quite happy with them though that doesn't really answer your question. I want to think the output impedance is quite low but not my area of knowhow.

[JFace]

Yes, this application is using the THAT 4301. I wanted to keep my explanation simple at the risk of scaring off readers due to a wordy explanation. I read a post on this forum about the "One Knob Squeezer" implementation of a THAT chip, and the designer implied that the TL072 is not ideal for the application due to rising output impedance with frequency. I breadboarded the application circuit from the 4301 datasheet, and used a TL071 as the input buffer (non inverting, gain of 1). The functionality was there, but the noise was unacceptable. It is likely from the low signal level as PRR suggested. I since unpopulated the board, and went with a optical compressor design. The 4301 is certainly a good chip, but it will take some creativity to implement into stompbox use.

[PRR]

I don't see anything in the 4301 data about buffer impedance. THAT has many-many pre-cooked plans worth reading. The 4301 is pretty-near their dedicated VCA and RMS chips in one, so the base chip info is good background.

The 4301 suggested circuits need buffer (only 6.6K input naked) and probably a gain of about 5 to come up from guitar level. Which also makes the hiss relatively 5 times lower, a good bunch.

Always been fond of optos myself.