[sdiy] The sound of various Op-Amps

[harrybissell]

Hi Michael

Michael Boracci wrote:

> Mr. Bissell
>  This is why I had posed the question about the sound of certain Op-Amps, It
> seems almost vital to have these components in some of your modules and
> processors. I am seeing from the response that in some cases it may be
> better to accept the faults of the less glamorous devices in order to savor
> the unique qualities that made some of the most admired synthesizers and
> processors.

Just to mention a funny (true) story... many years ago I took apart my
360 Systems "Slavedriver" (guitar interface) and decided to replace the
4558 opamps (which I somehow associated with being a cheap part
like the 1458)... with "something better".  After unsoldering all six preamp
chips... I got the data book out and found that I didn't HAVE any better
chip to replace them with. So I put in sockets and stuck the same chips back
in where they live to this day.  :^P

Now I replace a chip only when I'm certain that it is the WRONG chip for
the circuit.  Usually, most designers of commercial products use the right chips

so its very seldom that I can make any improvements

>
>
>  With regard to slew rate, is there a point where there is no additional
> return in apparent audio quality? In Tape-Op magazine I read an article
> about how some of the pro-audio preamps use video grade Op-Amps in an effort
> to achieve the holy grail of unadulterated sound. It seems like a bit of
> overkill to have that kind of speed when you are only dealing with audio
> signals.

I'd agree with that opinion. I don't believe in 'monster cable' and other
questionable
audio mods.  Some people feel that by using extreme bandwidth op amps they can
keep the 'phase shift' over the audio range at a minimum... increasing the
'je-ne-sais-quoi'
factor.  (fill in your favorite funny word there).   I believe that if I cannot
hear a difference,
it does not matter.

Its well known that (static) phase shift is not clearly audible.  I have a
guitar synth project
using a phase shifter as a vibrato circuit... and to 'turn it off' I just stop
the modulation (no
bypass).  That has to massacre the phase response... all the right harmonics are
present...
in all the right amplitudes.. at all the wrong places  (the waveform is
completely mangled!)
and it does not sound any different (or not different enough to bother with a
true bypass).

Another place that 'fast' slew rate becomes important is when really high gain
is expected from
one stage.  Because of the gain / bandwidth product of most of the plebian
opamps I use...
I use two stages at moderate gain instead one one really big gain stage. I have
not found
the TL07x series to be 'too slow' for anything except a good comparator.  I also
worry that
the video opamps might accidently amplify "Radio Free Europe" by mistake

(I suppose there hasn't been any RFE since the 60's... so if you like fill in
the favorite
radio station of your choice...  personally I like to have only MY music through
my gear...
something that has been a problem in more than one of my designs :^)

One intriguing thing about some of those video opamps is the possibility (on
some) of
differential outputs as well as inputs.  I think I could make use of that... if
only they worked
on +/-15V supplies, but that was ANOTHER thread :^P

>

H^) harry