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 Forum index » DIY Hardware and Software » PAIA Equipment
PAIA2720-3B VC Bandpass Filter_Augmentations
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gasboss775



Joined: Jan 02, 2016
Posts: 188
Location: Scotland

PostPosted: Tue May 16, 2017 6:40 am    Post subject: PAIA2720-3B VC Bandpass Filter_Augmentations Reply with quote  Mark this post and the followings unread

I tried out the PAIA 2720-3B VC Bandpass Filter on the breadboard. I wasn't initially that impressed with its range. However I swapped the diode for a red led and drove it from an exponential current source, the result was a great increase in the range of the filter. I also changed the way in which the Q control was implemented, resulting in, in my opinion a more natural control contour.

I also made a second version with a simpler current source using just one NPN transistor. The results were still very impressive.

The most interesting characteristic of these filters is the way they behave when overdriven. With high Q settings, the LED becomes modulated by the very signal that it is controlling and things freak out somewhat, the LED flickers too in response to the large signal swing!

The original is included for comparison.


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gasboss775



Joined: Jan 02, 2016
Posts: 188
Location: Scotland

PostPosted: Wed May 17, 2017 5:38 am    Post subject: Reply with quote  Mark this post and the followings unread

I tried out the PAiA 2720-3L Lowpass filter. Again it worked better with LEDs instead of diodes and a current source to drive the LEDs. I chained two Lowpass filter sections together to make a 4 pole filter. It was quite good but would really benefit from having a Q / resonance control. I'm going to have to figure out if this can be done.
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gasboss775



Joined: Jan 02, 2016
Posts: 188
Location: Scotland

PostPosted: Sat May 20, 2017 5:20 am    Post subject: Better Current Source Version Reply with quote  Mark this post and the followings unread

This version of the 2720-3B bandpass filter has a linear current source that doesn't have any tempco problems.


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gasboss775



Joined: Jan 02, 2016
Posts: 188
Location: Scotland

PostPosted: Sun May 21, 2017 3:39 am    Post subject: Reply with quote  Mark this post and the followings unread

I was just trying out the circuit from my last post again and it would seem that 68K is a better choice for R12 as it allows the filter to be completely closed.
I suspect that there might need to be a bit of experimentation with some of the resistor values to accommodate different LED's. Specifically, R9 & R12.

Last edited by gasboss775 on Sun May 21, 2017 7:15 am; edited 1 time in total
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gasboss775



Joined: Jan 02, 2016
Posts: 188
Location: Scotland

PostPosted: Sun May 21, 2017 7:14 am    Post subject: Reply with quote  Mark this post and the followings unread

I built 2 of the PAiA 2720-3B bandpass VCFs and designed a little two transistor mixer and output stage with 10 V peak to peak maximum output.

The sinewave generators represent the 2 filters, the 10K resistor on the right represents the output load.


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n.d



Joined: Dec 15, 2011
Posts: 49
Location: Argentina

PostPosted: Fri Feb 16, 2018 6:14 pm    Post subject: Reply with quote  Mark this post and the followings unread

Nice work!
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gasboss775



Joined: Jan 02, 2016
Posts: 188
Location: Scotland

PostPosted: Fri Mar 09, 2018 12:12 pm    Post subject: Reply with quote  Mark this post and the followings unread

n.d wrote:
Nice work!


Thanks dude.

I have experienced some problems with instability ( it bursts into oscillation ) that seem to be somewhat related to circuit layout ( I've only ever built it on a breadboard )
I did find that putting a 100n ( 0.1uF ) capacitor between the collector or Q2 and ground helped a bit. Does anyone else have any suggestions about making this behave. I was thinking of using either an FET optocoupler or a vactrol instead of the diode to tune the filter in order to avoid the circuit using the same ground as the control signal.
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