[sdiy] dual prec. op amp options?

[phillip m gallo]

The originating post mentioned Mike Irwin's great linearized SSM2164 VCA.
The slew rate of the SSM is .7V/us so other that audio path noise due to the
amp of choice, it's the control loop where the slew rate indicates the
recovery time of a non-zenered control loop that slam's the rail when off.

Also if your envelopes don't go to 0V (or less) then off you lose some OFF
attenuation.

regards,
p

-----Original Message-----
From: owner-synth-diy at dropmix.xs4all.nl
[mailto:owner-synth-diy at dropmix.xs4all.nl]On Behalf Of sdiy
Sent: Thursday, December 21, 2006 10:01 AM
To: Seb Francis
Cc: Synth DIY
Subject: Re: [sdiy] dual prec. op amp options?


I agree if you use the VCA for volume control only. If you use it for
shaping
control voltages it's another case.

Harald


 ,Am Donnerstag, 21. Dezember 2006 18:34 schrieb Seb Francis:
> Sorry, I meant to write 50kHz no 50MHz.
>
> Even so, my point is that if a VCA takes 20us to switch fully on or off
> (i.e. using a 0.5V/us opamp for a 10V control voltage) then for a 10kHz
> input signal the entire slew time takes only 1/5 wavelength.  In
> practice it's unlikely even that the input signal will have a frequency
> this high.
>
> Personally I'd try it out in the digital domain first .. load a few
> different waveforms into Soundforge, or whatever and try some different
> fade-ins (e.g. 5us, 10us, 20us).  I wouldn't think you can hear much
> (any?) difference.
>
> Seb
>
> sdiy wrote:
> > If you put an square wave through an 741 with 0,5uS Slewrate say with 5
> > to 10kHz you can hear by ear and see at the scope that the square is not
> > longer a square. You can clearly see the ramps. Higher harmonics
> > disapear.
> >
> > f=1/T    1/20E-6 = 50E03  thats 50kHz not 50MHz.
> >
> > Well that is not relevant for volume control. So about 3V/uS Slew rate
> > and low offset temp drift is enough for the CV secton of an VCA.
> >
> > Harald
> >
> >  Donnerstag, 21. Dezember 2006 16:17 schrieb Seb Francis:
> >> Do you really need such precision and speed for a VCA?
> >>
> >> A slew rate of 0.5V/us will only take about 20us for a 10V swing -
> >> that's a single wavelength at 50MHz.  Do you really need faster than
> >> this?  And will the SSM2164 actually be this fast anyway?
> >>
> >> And are you going to notice the input offset at all?  For something
like
> >> the MXL1013/LT1013 this isn't going to change much over time, so it
will
> >> just be adjusted out when you set the initial gain.  And even if
there's
> >> no adjustment, 300uV (max) out of a FSR of 10V won't be noticeable (for
> >> pitch, yes, but not for volume).
> >>
> >> Just my £0.02 worth.
> >>
> >> Seb
> >>
> >> Fernando de Izuzquiza wrote:
> >>> Hi all,
> >>>
> >>> Searching for dual precision op amps for the control section of
> >>> (M.Irwin) SSM2164 based VCAs.
> >>> I'm looking for a modest/decent slew rate too, not just low offset.
> >>>
> >>> As single op amp I found the OP27GP, at around $2 with a slew rate of
> >>> 2.8 V/us and max input offset voltage of 100 uV (typical 30 uV)
> >>>
> >>> I'd like to find a similar specs op amp but dual (you may suggest
> >>> alternatives to the single OP27 too...)
> >>>
> >>> The OP270GP is very near (sr 2.4 V/us, max ov 250 uV), but at $6 is a
> >>> bit too much...
> >>> So, there is a cheaper alt. with similar specs?
> >>>
> >>> I see that in single format there many specs/price options, but I only
> >>> know the OP270 for a dual with still decent slew rate...
> >>>
> >>> single        -max- iov (uV)        slew rate (V/us)    notes
> >>> OP177G        60                0,3                slow
> >>> OP27G        100 (typ. 30)        2,8 (min. 1,7)        OK
> >>> LTC1150CN    10                3                great but too
expensive
> >>> AD797A        80                20                too expensive
> >>>
> >>> dual
> >>> LT1013CN    300                0,4                slow
> >>> OP270G        250 (typ. 50)        2,4 (min. 1,7)        expensive
> >>>
> >>> ...
> >>>
> >>>
> >>> Fernando

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