[sdiy] More on Xpander modes was Re: HP from LP ?

paula at synth.net paula at synth.net
Wed Aug 9 08:21:05 CEST 2017

Well done David,
I'd be interested to see your schematic and hear the modes when you're 
done :)


On 2017-08-09 05:03, David Moylan wrote:
> Good news: I checked the connection for leads taking off from each
> filter pole.  I had one in the wrong place (pole 3) which explains my
> poor filter responses :)  All the High Pass modes sound like High Pass
> now with no need to do anything else.  By ear, there doesn't seem to
> be any need to tighten the capacitor tolerance or increase the
> capacitor value (at least, in my unit).
> I was already taking the outputs off the buffer outs of the IR3109.
> If I recall correctly I thought that would be the best option to
> retain the specific sound of the IR3109 and since the feedback
> resistors are connected there the DC component might be better.
> I do find BP4 mode to be a little too quiet, might try to adjust the
> resistor values to bring the level up.  Notch and Phaser modes still
> sound great but definitely more notch-y!  Fingers crossed I'll have
> time for video tomorrow.
> Dave
> On 08/02/2017 02:16 AM, Tom Wiltshire wrote:
>> Humm, I think I might have been thinking of this discussion on 
>> Electromusic:
>> http://electro-music.com/forum/topic-59982.html
>> It doesn’t unequivocally back up what I said though.
>> Tom
>> ==================
>>         Electric Druid
>> Synth & Stompbox DIY
>> ==================
>>> On 1 Aug 2017, at 23:49, David Moylan <dave at westphila.net> wrote:
>>> Not sure, do you recall where you saw that?
>>> On 08/01/2017 03:26 PM, Tom Wiltshire wrote:
>>>> Isn’t it often the case that control feedthrough is worse on 
>>>> OTA-based filters (like the IR3109) when you run them with more 
>>>> control current?
>>>> That might be one explanation for the small-valued caps - it helps 
>>>> get the high cutoffs with only a fairly small control currents.
>>>> Tom
>>>> ==================
>>>>         Electric Druid
>>>> Synth & Stompbox DIY
>>>> ==================
>>>>> On 1 Aug 2017, at 22:09, David Moylan <dave at westphila.net> wrote:
>>>>> Neil, a good point.  Also, in this particular filter implementation 
>>>>> the caps are 220pF.  I wonder how much stray capacitance could 
>>>>> affect things.  It would only take a few pF to start racking up 
>>>>> percentage points.
>>>>> I'm providing non inverting buffers on my board (don't need to 
>>>>> invert the stages since IR3109 does that already).  I did so 
>>>>> because I thought it might make the board easier to "shoehorn" into 
>>>>> other circuits and I needed a buffer close by to the first stage 
>>>>> cap, which I moved on to my PCB so that it could easily be switched 
>>>>> in/out.  So I just used a Quad opamp and provided 4.  Since I had 
>>>>> the on board buffers I think I soldered the flying leads directly 
>>>>> on the caps rather than post IR3109 buffers.  That would make them 
>>>>> more susceptible to stray capacitance from the leads, which 
>>>>> additionally aren't currently as short as they could be.
>>>>> So I have a few things to experiment with:
>>>>> 1. Increase cap values to largest value that won't adversely affect 
>>>>> filter range, and probably tighten tolerance on those caps as well. 
>>>>> Largest value I've seen in a schematic is 470pF (thanks, Florian!). 
>>>>> Wonder if it could go to 1000p.
>>>>> 2. Move the flying leads to IR3109 buffer outs and shorten as much 
>>>>> as possible.
>>>>> I have to be away for a few days suddenly so video will probably be 
>>>>> pushed until the weekend.
>>>>> On 08/01/2017 01:15 AM, Neil Johnson wrote:
>>>>>> "David Moylan" wrote:
>>>>>>     This is due to the fact that achieving HP responses relies on
>>>>>>     nulling out terms in the transfer function via mixing the 
>>>>>> different
>>>>>>     poles.  If you're not exact you don't get a full null and the
>>>>>>     attenuation of low frequencies suffers (looks more like a 
>>>>>> shelf
>>>>>>     instead of continuing roll off of low frequencies and usually 
>>>>>> has a
>>>>>>     little dip in the middle).  I'm using 1% tolerance resistors, 
>>>>>> but am
>>>>>>     about to sub in some 33.2k for 33k to get even closer to the
>>>>>>     theoretically perfect values.
>>>>>>     Even if you had perfect resistors I would imagine any 
>>>>>> mismatching
>>>>>>     between the OTA stages would also cause the attenuation to 
>>>>>> suffer,
>>>>>>     but I haven't modeled that.
>>>>>> The integrator capacitor tolerance will likely swamp any 
>>>>>> improvements in resistor tolerance.  For example the 2164 is 
>>>>>> quoted as having worst gain matching of 0.2dB which is just over 
>>>>>> 2%, yet capacitors will likely be 5% - you can get C0G down to 1% 
>>>>>> if you pay for them. So 1% resistors are already better than 
>>>>>> anything else in the signal path.
>>>>>> Neil
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