[sdiy] What is this capacitor in the JP8?

[Rob Keeble]

Hi
The ba6110 enables the resonance feedback to include some of the input signal, so the filter compensates for filter volume drop off as q goes up. It's a lower noise and better designed OTA than the ca3080 but the sip footprint made it the right product for my design. In the AM8109 I haven't implemented cv control of q but it's obviously possible. 

Filters sound better with film caps, with 5% polypro getting 90% of the way on a 1% polystyrene and a lot cheaper. Ceramics are better on a harsher diode filter but even then I'd use a Mylar film even if 20%. This is reflected in customer feedback on the AM8109 where there is a distinctive nice sound quality.

You can use higher signal path caps up to say 10uf, I do this in other OTA filter chip modules. In the arp filters the 220nf caps on the input side really hurt the sound and cause hf cut off. They are in their to reduce dc offset.

Best Regards
Rob Keeble
AMSynths
www.amsynths.co.uk

> On 14 Oct 2013, at 18:57, Jack Jackson <jackdamery at hotmail.co.uk> wrote:
> 
> Hi Rob, yeh I noticed you did. I was also wondering why you use a BA6110 on the resonance VCA? Is it any different to a CA3080 or so? I saw for the 4 220pf caps around the IR3109 you specify polystyrene or polypropolene instead of the ceramics used in the original. How come?
> 
> P.s. I messaged you about this already via fb.
> 
> Cheers,
> Jack
> 
> ----------------------------------------
>> From: rob.keeble at amsynths.co.uk
>> Date: Mon, 14 Oct 2013 15:33:57 +0100
>> To: synth-diy at dropmix.xs4all.nl
>> Subject: Re: [sdiy] What is this capacitor in the JP8?
>> 
>> Hi
>> I use high quality 1uf polarised capacitors here, as in the AM8109 Eurorack module. Can be any voltage above 16v, I use25v to keen the size down.
>> 
>> Rob
>> 
>>> On 14 Oct 2013, at 14:35, Neil Johnson <neil.johnson71 at gmail.com> wrote:
>>> 
>>> Hi,
>>> 
>>> Jack Jackson wrote:
>>>> Thanks. Is the 50v an important factor here, or was it just specified so people wouldn't use something too low like 16v for example? I've found some cheap 100v ones here.
>>>> http://www.hificollective.co.uk/catalog/rbd030-047uf-100vdc-elna-bipolar-capacitors-p-9449.html
>>> 
>>> It depends. Maybe that's what they had in stock? Maybe that's what
>>> was on special at the time? You need to understand the circuit to see
>>> what the maximum voltage across it could be, including startup and
>>> shutdown conditions, and possible latchup. The voltage rating will
>>> also affect the case size, and also other electrical parameters like
>>> ESR.
>>> 
>>>> Also why they choose to use an electrolytic over a polyester?
>>> 
>>> Cheaper?? Smaller?? Adds to the J8 "sound"?? Dunno. Try replacing it
>>> with a polyester and hear what the difference is.
>>> 
>>> Neil
>>> --
>>> http://www.njohnson.co.uk
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