[sdiy] dual prec. op amp options?

[Seb Francis]

Also, unless you are running the VCA directly from something like a Gate 
signal or square wave then most likely the max. slew rate of the source 
will be much slower than the VCA CV stage.

But hey, there's nothing wrong with over-engineering if you can afford it :)


Seb Francis wrote:
> 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
>>>>       
>>
>>   
>