plutoniq9 wrote:
>
> In this case, how the 4558 is used, using a opamp with a lower S/N
> ratio and higher slew rate is better. If you check the schematics,
> the chorus circuit already uses Active Low Pass filters at the input
> and output of the BBD (MN3209).
Yes, noisy transitor filters. Even with opamp, those filters aren't
steep
enough. I know what's your idea, I considered replacing it with 072
but left it for later.
> The output of the Chorus circuit is
> then mixed with the Bypassed audio into the Final Op Amp stage (IC2
> NJM4558).
>
Maybe the best thing would be to leave bbd going thru 4558
and add 072 to mix original signal with signal after 4558.
> I thought myself that they may have left the compensation caps off
> intentionaly to filter out artifacts of the Poly 800 (ie. Hiss,
> Computer Noise ect.), but in replacing IC2, I noticed none of this.
>
Conpensation caps around inverters/summers are to prevent HF
selfoscillation of fast/hi input Z opamps. You don't need them
on 4558. It's kinda integrating filter for high freq, but
with smallers cap values, higher will filter audio in a bad way
(original signal would be changed).
> The 4558 Opamp, think of it at a dual 741 (1us slew rate BTW), it's
> got lousy specs.
ha-ha :)
You'd probably get killed if you stated this in some guitar effect
forum. Some 4558 brands are considered sacred there.
No, it's not lousy, and no it's not dual 741, it's better than it,
I think 1458 or something is closer to 741. 4558 is old workhorse,
all around dual opamp. Most of the praised old monosynths are
full of them. Some used it cause it was cheap, but somewhere it
was crucial for best sound, like in certain vocoders. Hey, look
inside old Oberheim SEM state variable filter, it's got 741s,
and it looses it's magic without them!
> The slew rate of the NE5532 is 8 times that of the
> NJM4558 & the noise floor is dropped. Why not go with the high
> quality shite, the Poly 800 is capable of some cutting sound with
> that filter, wouldn't you prefer at full 20hz-20khz sound
> reproduction coming from her outputs?
To clarify some things. 5532 is a overkill here. Plus it draws to
much current. Problem is that this chorus cct is more or less
lo-fi and that's it. BBDs are inperfect, they are sorta analog
samplers (thou if you dig inside the process itself you'll
see that all the input signals are _digitalized_ ). Noisy and
full of "sampling" artefacts, plus feedthru from HF clock driver,
plus inperfect match of two delay lines (did you know that there
are actually two lines for each bbd block?). All of it makes it
very trashy, especially at longer delays. Don't get me wrong
I like my ibanez stompbox bbd echo, but it has it's limits.
Some manufacurers knew why they used slow opamps in the output,
slew rate is like a brick wall, opamp can't spit out changes
faster than specd, so 'garbage' sampling artefact are filtered
more or less. Making higher noise floor might just make the
sound of bbd's noise more promitent. BBD is just a lower quality
shite.
Filter output, OTOH, is different story. Yes you want better opamp
for it, that's why I made full chorus bypass and made direct
out from filter. Currently I kicked chorus totally from poly,
maybe I'll put it back, but right now I'm playing with it,
considering separate module as I've added lfo frequency and
level pots, tri/square and range switches, resonance across
chorus (kinda flanger sounds). I need to experiment with different
timing clk caps for real lo-fi sound, maybe manual cv control
too. Those mods IMHO are much more attractive than opamp
upgrading, but as always YMMV, put IC socket and find the best
sounding chip...
Also, regarding 5532, you should note that it needs lower resistors
around it if you need full performance from the chip. Lower than 10k,
otherwise performance is degraded, and it's not better than 082 or so.
Poly800 uses 47k or so...
--
marjan
me : Marjan Urekar
e-mail: urekar.m@...
s-diy : http://surf.to/marjansystems
music : http://go.to/forcemajeure
