[sdiy] Linearized 2164 VCA Warning

[Stewart Pye]

The OPA2277 looks like a good choice for a low offset, low drift dual op 
amp. According to the datasheet it has an input bias current 
cancellation scheme that eliminates the need for a bias current 
cancellation resistor that is normally required for bipolar input op 
amps. The slew rate is only 0.8V/us though.

Cheers,
Stew.

Harry Bissell wrote:
> the op-07 would not likely be a good swap for the TL07x series, its dog-slow
> and has a way-lower input impedance. iirc, the op-07 is like .3V/us vs
> 13v/us for the TL07x
>
> That said I'd use it to swap out a 741 if I wanted excellent DC performance.
> In fact I'm fond of not using faster parts than I need. I like the 4558 for most 
> synth work that does not need ultra high input impedance
>
> H^) harry
>
>
> ----- Original Message -----
> From: David G. Dixon <dixon at interchange.ubc.ca>
> To: 'Andre Majorel' <aym-htnys at teaser.fr>, synth-diy at dropmix.xs4all.nl
> Sent: Wed, 29 Dec 2010 03:15:37 -0500 (EST)
> Subject: Re: [sdiy] Linearized 2164 VCA Warning
>
> Hey Andre, I was just finishing up some work for my Parisian client when I
> got your message (coincidence?)...
>
>   
>>> Given the exact nature of the problem, I'd rather add a
>>> two-cent resistor and a couple of traces than replace a
>>> one-dollar opamp with a five-dollar one to achieve exactly the
>>> same end.
>>>       
>> Precision op amps also have lower tempcos and lower long-term
>> drift. Whether that matters in this circuit, I don't know.
>>     
>
> It doesn't.  It's a VCA!
>
>   
>> They don't have to be expensive either. The LT1013 and OP07 are
>> pretty cheap. But the cheap ones have low slew rates that might
>> limit audio-rate modulation.
>>     
>
> The one and only time I ever tried to replace a TL072 with an OP07, it got
> so hot I had to shut the synth off (this was the current servo in my 2164
> Expo VCO, and the current was bloody small).  I pried it out of the socket,
> put the cheap-ass TL072 back in, and it's been oscillating happily ever
> since (in tune and rock steady, I might add)!
>
> I'll stick with the cheapos, thanks!
>
> _______________________________________________
> Synth-diy mailing list
> Synth-diy at dropmix.xs4all.nl
> http://dropmix.xs4all.nl/mailman/listinfo/synth-diy
>
>