Point me towards a buffer???

Hola!

I'm looking for a simple buffer to put on the frontend of a INA217 based pre. I've got the pad, phase, phantom hoopla all sussed out, I just thought i might like a guitar/bass input as well. I was hoping something simple like an op-amp at unity to fix the impedance issue. I'm not sure if i could simply run the output of the op-amp to the non-inverting input of the 217. I honestly don't know if that will work or if it will oscillate or perhaps do nothing...? I have searched the meta but i haven't found just a simple buffer i can add. Any ideas? As always, thanks a ton.
Ben.

http://www.allworldautomotive.com/images/userphotos/14865_19772.jpg

.....awesome, thats perfect. Thanks!

go to AMZ
he's got teh buffaz. 
single jfet buffaz teht Iz bilt n soundz gud and dual supply jobz teht surely wood sound gud.

That one will probably oscillate...

Ptownkid said:

http://www.allworldautomotive.com/images/userphotos/14865_19772.jpg

Click to expand...

http://www.soundskulptor.com/pdf/di02-schematic.pdf

My opinion is that you will waste your time and resources designing a buffer that goes --before-- your INA217 chip. I would rather take the following route:

Design a simple variable gain unit with 1MOhm impedance, based around an OPA132 and make an insert (better said "switching") point --after-- your INA217 chip (it's pin 6.).

This way, you would be able switch between a mic input (INA chip) and instrument input (OPA chip), with separate gains and hopefully significantly better S/N ratio.

ChrioN said:

That one will probably oscillate...

Click to expand...

Hah!

tv said:

My opinion is that you will waste your time and resources designing a buffer that goes --before-- your INA217 chip. I would rather take the following route:

Design a simple variable gain unit with 1MOhm impedance, based around an OPA132 and make an insert (better said "switching") point --after-- your INA217 chip (it's pin 6.).

This way, you would be able switch between a mic input (INA chip) and instrument input (OPA chip), with separate gains and hopefully significantly better S/N ratio.

Click to expand...

Aren't you somewhat exaggerating the signal degradation? If you were right, one could not produce a decent recording with a DI in front of a mic pre.
The OPA132's input noise density of 8nV/sqrtHz makes it a poor competitor for a 2SK170 (only 1nV/sqrtHz). Although it would not be a major factor. A TL072, with 18nV/sqrtHz performs very well as a DI input stage. A typical electric instrument signal is at ca. -20dBu, the TL072's 2.5uV EIN still leaves 90 dB S/N.
A configuration with a 2SK170 as the SoundSkulptor interface delivers a typical signal at -40dBu, with 0.3uV of noise. Amplified by the INA127 at 40dB gain, the output is 0dBu, with 40uV of noise, yielding 86 dB S/N. Not that bad...
In the same conditions, the OPA132, used with only 20dB gain, would deliver its 0dBu output with 21uV of noise, making it 10 dB cleaner at -96dB S/N. Better, yes, but no tie-breaker.

Given the choice (if feasible), a no-brainer, esp. if component cost was brought into equation as well. Imho; ymmv.