Doepfer

Hi psm,

Not much experience in this area, but a quick look at the 'audio' 
chapter in Jung's 'IC Op-Amp Cookbook' (1st edition *only*!) suggests 
that a key factor is 'gain bandwidth product' (you need large one), 
but also slew rate comes into play as well. Basically the more gain 
you want, the less bandwidth you get. Thus the gain stage on the 119 
acts as a low-pass filter, and as you crank the gain up, the lower 
the cut-off frequency gets. I probably won't have time to try any 
real measurements, but I did a quick simulation of the gain stage of 
the 119, and the results (very, very roughly) suggest that there 
shouldn't be much of a problem with the asymmetric input unless the 
gain is nearly max (8-9 perhaps), but with the symmetric input 
problems are likely to be noticed at much lower gain settings, 
perhaps around 4 or 5. (This is because the overall gain requirement 
for the symmetrical input is much greater.) Does this tie in with 
your use?

The book also shows how to improve standard op-amp set-ups for audio 
use (gets non-trivial very quickly - external compensation, extra 
bits 'n bobs everywhere!). As for a recommended chip, you could try 
searching the Synth DIY archives (www.buchi.de/sdiy) as I bet the 
question comes up there from time to time, or you could of course 
pose the question directly to that list (there's a link at the bottom 
of the archive page).

Tim


--- In Doepfer_a100@yahoogroups.com, "ps_minor" <pscottm@h...> wrote:
> lately i've been using my system for processing external signals 
quite often 
> and i've come to notice that my a119 degrades the sound more than 
i'd like.   
> the studio environment i'm usually in is of a high standard, neve 
console, etc...  
> unfortunately i'm sometimes limited to what i can send to the a119 
due to 
> lessened sound quality of the results.  especially w vocals and 
drums.   i'm 
> very careful w gain-staging so i don't think it's me.
> 
> out of curiousity just  patched the a119 from a send and back to 
the desk, the 
> quality drops considerably w vocals and drums, but the narrowed 
bandwidth 
> is usually pleasing on synth and guitars.
>  
> was looking at the opamps inside the a119, i believe the one that 
handles 
> audio duties is a TI tl064c, pretty run of the mill chip w low 
power draw and 
> pretty low slew rate according to info i found on TI's site.
> 
> i imagine a chip upgrade would make a big difference here as long 
as it 
> doesn't draw too much current.  not looking for anything fancy but 
an 
> improvement in bandwidth and slew rate would be nice.  anyone have 
any 
> knowledge of opamps enough to recommend a suitable upgrade?
> 
> anyone else feel this way about the a119?
> 
> thanks!
> -psm