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

Tom Wiltshire tom at electricdruid.net
Wed Aug 2 00:26:47 CEST 2017

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.


       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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