On Thu, Aug 11, 2011 at 3:23 AM, V G wrote: > On Wed, Aug 10, 2011 at 5:45 PM, Robert Rolf wro= te: > >> Or just change out the op amp in the existing amplifier for one with a >> much better noise figure. One MEANT for low noise operation, rather than >> cheapness. >> >> There are op amps designed specifically for ultra low noise audio >> operations with low internal bias currents and low shot noise as a >> result. LT1115 comes to mind, AD8597 series, OPA827, etc. >> >> Saves a lot of time is all you do is swap out the noise maker for one >> that isn't so intrinsically noisy. >> Have you looked at what is in there currently? I'll bet its a general >> purpose, super cheap, op amp. >> > > LOL! So I opened up the device and it's using a PCM2902 ( > http://focus.ti.com/docs/prod/folders/print/pcm2902.html) inside. The > opamp is an LMV824 quad opamp (http://www.national.com/pf/LM/LMV824.html) > > What do you guys think of the opamp? Is it suitable for this purpose? Is = it > a noisy opamp? > > What do you guys think of the LT1028 (I hear lots of people love this one > for audio) and LT1115? > > The LMV824 noise spec is 24nV/rt(Hz) @ 1KHz > > The LT1028 is 0.85nV > > The LT1115 is 0.9nV > > So yeah, it's a pretty unsuitable opamp in the guitar interface box. Woul= d > it be possible that the noise spec on the LMEV824 is causing the noise > problem? I was reading up on the noise spec calculations and how to use > them, but I forgot. How /bad/ is 24nV/rt(Hz) for audio purposes? > > I would ideally like to use 4 of LT1028 opamps but they're expensive to b= uy > 4 of. What other opamps would you guys recommend? 4-in-a-chip would be ni= ce, > but I would guess that quality is compromised for quantity for these opam= ps. > Actually, I think I just found a good one: LME49740 ( http://www.national.com/pf/LM/LME49740.html) 2.7nV/rt(Hz) What do you guys think?. Not too expensive ($5 ish). Also, the pinout and package seems to b= e the same, so possibly a drop in replacement? My only concern is that the LME49740 does not state that it has rail to rai= l outputs like the LMV824 does. It does, however, specify that the voltage supply range is +/-2.5V to +/-17V, so is it safe for me to assume that the outputs swing rail to rail? --=20 http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .