[sdiy] LAG circuit de-tuning VCO question

[Dave Kendall]

Hey Oren - many thanks  - I reckon you've solved it :-)

I swapped out the aluminium electrolytic for a wima poly cap of 680nF, 
and a 1uF poly box cap (rapid own brand) and both worked fine - no 
problem!
I would never have suspected the difference would be so dramatic - 
either alu electros are simply not good enough,  or maybe I have a bad 
batch...

Thanks again!

cheers,
Dave
(I love this list ;-)



On Nov 5, 2011, at 00:18, Oren Leavitt wrote:

>
>
> On 11/4/2011 6:28 PM, Dave Kendall wrote:
>> Hi All.
>>
>> I have a playing with a simple expo lag circuit consisting of;
>>
>> 1V/octave input to an op amp wired as unity gain voltage follower, the
>> output of which goes through a 1M Linear pot wired as variable 
>> resistor,
>> then into a second voltage follower with a 1uF cap to GND at its 
>> input.
>> I notice that without even changing the input CV, setting the pot to 
>> max
>> de-tunes the output by up to several 100 cents, judging by ear.
>>
>> The effect is more pronounced the higher the input CV - around 0V to 
>> 1V
>> input, it's hardly noticeable.
>>
>> Is this normal, and is there a way to remove the effect? it makes it
>> pretty useless for accurate VCO use. One can mitigate the effect a bit
>> using smaller resistors/caps, but then the lag time is very short.
>>
>> Any ideas anyone?
>>
>> cheers,
>> Dave
>>
>
> Capacitor leakage - maybe?
>
> - Oren
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