help designing quiet buffer/preamp - which IC?

[tempus]

Hey all

I'm thinking of building a buffer/preamp for the piezo bridge on my electric guitar. It would obviously require a high input impedance, so a TL071 would handle that nicely as a buffer. I would also need some gain there though (I want to bring it up to line level), but I'm not sure which of these 3 scenarios would give me the lowest amount of noise:

1. single TL071 with input impedance set to, say, 5M with feedback resistor to set gain to the appropriate amount (if I can get a high enough value resistor)
2. TL072 with one opamp configured as a buffer, and the other set for the required gain
3. TL071 configured as a buffer followed by a low noise opamp like a 5534

Which would give me the lowest noise? Any other (possibly quieter) solutions?

Thanks

[marcelomd]

Hi,

If you insist on a FET opamp, try the TLC2272, a "newer" TL072. It's lower noise and rail to rail.

Also, the lowest noise (affordable) opamp I know is the NJM2068. Better performance and lower power than NE553x.

[tempus]

Thanks for your reply Marcel.

The only reason I'm considering the FET opamp is that I don't think a non FET opamp will be able to give me the input impedance I need. My pickup specs the impedance as 1.3M at 100Hz.

I'll look into these other opamps though.

[PRR]

> lowest noise (affordable) opamp I know is the NJM2068

Low hiss for low-Z sources, 1K-30K impedance.

High hiss for high impedances like Piezos. I bet a TL072 will do better. The TLC2272 is pretty spiffy at 0.6 fA/√Hz. OSI computes at 15Meg, which is in the best ballpark for Piezo.

[Rob Strand]

1. single TL071 with input impedance set to, say, 5M with feedback resistor to set gain to the appropriate amount (if I can get a high enough value resistor)

You don't want an inverting configuration.  They won't be lowest noise.

Lowest noise:
- JFET input opamp,
- non inverting configuration,
- gain in first stage (not two stages) with low valued resistors in feedback network

[marcelomd]

> lowest noise (affordable) opamp I know is the NJM2068

Low hiss for low-Z sources, 1K-30K impedance.

High hiss for high impedances like Piezos. I bet a TL072 will do better.

True. Never used them as first stage. I thought, but did not write about using a JFET buffer before the gain stage.

Still, TLC2272 might be the better option.

[antonis]

The only reason I'm considering the FET opamp is that I don't think a non FET opamp will be able to give me the input impedance I need. My pickup specs the impedance as 1.3M at 100Hz.

By bootstrapping you can get very high input impedance..!! 
(both for BJT & FET input op-amps..)

Split single bias resistor into 2 separate resistors (2X100k should be OK for noise preventing purpose) and connect a 100nF (or bigger) cap between the junction of those resistors and op-amp output/inverting input..