PS:  I just tried your example a little modified to compare a and k  
cutoff and noticed that there is a slight glitch in the wave form,  
worse for k-rate.
   Will investigate a little more

Quoting jpff@cs.bath.ac.uk:

> Well you could try the butlp with a-rate cutoff frequency to see if  
> it still happens.  That code is in git now.
> ==John ff
>
> Quoting Iain McCurdy <i_mccurdy@hotmail.com>:
>
>> I've noticed that butlp produces pretty pronounced artefacts when  
>> the cutoff frequency is moving in any way quickly as the example  
>> below demonstrates. Obviously things are dramatically improved when  
>> kr = sr but at lower control rates all of Csound's other low-pass  
>> filters sound significantly better. I am wondering whether butlp is  
>> missing out on some internal interpolation.
>> I'm testing on 6.01 OSX10.6.
>>
>> <CsoundSynthesizer>
>> <CsOptions>
>> -odac -d
>> </CsOptions>
>> <CsInstruments>
>>
>> 0dbfs=1
>> nchnls = 2
>> ksmps = 16
>>
>> alwayson 1
>>
>> instr 1
>> koct lfo 3,4,1
>> asig vco2 0.5,400,4,0.5
>>
>> asig butlp asig,cpsoct(7+koct)
>>
>> ;asig clfilt asig,cpsoct(7+koct),0,2
>> ;asig tonex asig,cpsoct(7+koct),3
>> ;asig moogladder asig,cpsoct(7+koct),0
>> ;asig bqrez asig/3,cpsoct(7+koct),1
>> ;ahp,asig,abp,abr statevar asig, cpsoct(7+koct), 1
>>
>> outs asig,asig
>> endin
>>
>>
>> </CsInstruments>
>>
>> <CsScore>
>> </CsScore>
>> </CsoundSynthesizer>
>>
>>
>> Send bugs reports to the Sourceforge bug trackers
>> csound6:
>>            https://sourceforge.net/p/csound/tickets/
>> csound5:
>>            https://sourceforge.net/p/csound/bugs/
>> Discussions of bugs and features can be posted here
>> To unsubscribe, send email sympa@lists.bath.ac.uk with body  
>> "unsubscribe csound"
>
>
>
>
>
> Send bugs reports to the Sourceforge bug trackers
> csound6:
>            https://sourceforge.net/p/csound/tickets/
> csound5:
>            https://sourceforge.net/p/csound/bugs/
> Discussions of bugs and features can be posted here
> To unsubscribe, send email sympa@lists.bath.ac.uk with body  
> "unsubscribe csound"