Any comments on my circuit?

[Balthazar]

Hi

I am trying to make this filter stomp box, which is a low pass, high pass and band pass filter with adjustable Q, which means an adjustable resonant peak at the cutoff frequency. I've gotten a lot of great help here on the forum, and I've chosen to use a state variable filter, which is a filter with 3 op-amps (plus one to buffer the signal before it), and a charge pump which provides bipolar +/- 9V. The filter is an IC, Texas Instruments UAF42. I've never made a stomp box before and am not so well versed in electronics, so I'd be happy if anyone can tell me if the circuit is any good. Here is the circuit:



The stuff around the UAF42 is taken right out of their tech sheets, so I'm pretty sure I got that right. You can see more about that here:

http://www.diystompboxes.com/smfforum/index.php?topic=100829.0

The stuff I'd like comments on are in the lower right, and upper right corners: The input and output coupling stages. I've seen in the forum that a resistor to let the coupling capacitors discharge when disconnected may help reducing pops when switching. That's what Rbleed1 in the lower right, Rhp1 etc... are for. At first I thought there was no point in output coupling, because no DC offset is expected. Then I was advised by the helpful PRR that there may be tiny offsets anyway, which will give annoying pops when switching. Therefore I chose to use output coupling anyway. I also added a capacitor R-out at the end, because I was advised it may take care of some possible oscillation issues. Anyway the UAF42 chip is spec'd to handle short circuit at the output just fine. I'd be happy for comments on these choices and also the choices of component values.

For the power bit (lower left), I have decided to use polymer caps, because their specs seem to be a dream for reducing switching noise. I didn't really try to find large values, those are actually small values for polymer caps. Yes yes, you'll call me paranoid and wasting money, since the switching noise will be out of the audible range anyway. But those polymers aren't so awfully expensive anymore, they go for about $1 a piece on ebay, and if you buy 10-25 pcs you can get down to a quarter dollar a piece if I remember right. I chose a cheaper type called United Chemi-Con from Mouser (well, a bit cheaper than Sanyo). Because there is more uF than most stomp boxes to fill up when switching on power, there will be a bit more surge current. Therefore I thought I'd chose a slightly more civilized way of turning it on than the stereo jack method. I figured that method might send current into the output of the previous box, and cause improper contact during insertion. But the surge will be for a super short time anyway. I guess I'm paranoid as I said.... anyway, I'll use an input jack with an isolated "make" contact, Switchcraft has one and a cheaper brand too.

I hope this wasn't so long nobody bother reading or comment...... I hope you'll bother to comment at least if you see something that could be improved, or just to say if it looks OK.

[Balthazar]

A bit more reference about the power section:

http://www.diystompboxes.com/smfforum/index.php?topic=100742.0

[craigmillard]

Hey Balthazar,

Did you ever get this up and running? Just got some samples of the uaf42 and this looks like what im after as a circuit:)

Anyone else used the UAF42 chip

[DIMstompboxes]

Bump this up
I got UAF42 chip too its a universal active filter. Interested on OP status and anyone to share thier comment on this project.
Does it need to have a charge pump? I think this may run on a standard 9volts. Yet also it need dual OPAMP one section for Input and the other for Output buffer isn't?