My new design, the slackfilter

slacker · May 23, 2006, 05:44:44 PM

Quote from: Eb7+9 on May 23, 2006, 02:21:31 AM
I notice in the conversion to Voltage-Control the op-amp in the first circuit is replaced by a two stage bipolar amplifier ... you can probably squeeze more Q out of your topology by changing back the two-device gain stage to a variable (higher gain) op-amp stage ...

Yeah I've thought of trying an opamp version just to see if the sound or performance is any different.

Quote
or, for now ... in your improved schematic there's a non-inverting op-amp with a x5 gain at the output, to test this theory out you could take the resonnance feed after the op-amp instead of after that transistor stage and see how much Q increase you get ... you can rig a second pot at the output of the op-amp for the variability ... if that works then try replacing the 220k by 2meg pot in the feedback of that op-amp - at some point your filter should morph into an oscillator and give you a nice sine wave ... from your nice clips it sounds like you might still have some gain to go before you reach that point ...

The filter will already self oscillate in lowpass or highpass mode with the resonance control above about 80%. Bandpass won't quite self oscillate. I tried your idea and it does increase the Q, needs some playing about with to find the right pot value because it gets completely out of control very quickly. I've also tried increasing the value of the resonance control like in the design christian posted and that works nicely.

birt · June 03, 2006, 09:23:36 AM

those 2 BC109's are acting as variable resistance to ground right? why are there 2? can you just use one of both? or use difefrent modulations on both?

christian · June 03, 2006, 10:37:43 AM

Quote from: birt on June 03, 2006, 09:23:36 AM
those 2 BC109's are acting as variable resistance to ground right? why are there 2? can you just use one of both? or use difefrent modulations on both?

They basically give you mirrored image of the voltage, so that once they go thru the diodes (which also respond mirrored to the mirrored voltages), they cancel each other out. This is mainly a concern if youd had high-frequency oscillator modulating the VCF or some "thumping".

This is a standard thing to do in a "synth-world" to keep the control voltages out of the audio path.
For guitar fx with controlled circumstances like having just envelope follower as a CV for the filter you wouldnt need this, but to make this work with a single transistor, youd probably had to change a lot more stuff here.

ch.

birt · June 03, 2006, 11:10:33 AM

good to know. i was maybe going to put this one in my big filterbox as the 4th filter but i'll stick to three i think

christian · June 03, 2006, 11:18:59 AM

Quote from: birt on June 03, 2006, 11:10:33 AM
i was maybe going to put this one in my big filterbox as the 4th filter but i'll stick to three i think

:D:D

Calamardo · July 27, 2006, 08:09:34 PM

A little question: can someone tell me how work the two buffers in the lfo section? are both necessary?, im confused about it cause pysho lfo looks different.

slacker · July 28, 2006, 08:11:36 AM

The way my LFO works is a bit different to the psycho LFO. To get the triangle waveform I took the signal from the input of the inverter, so the voltage goes smoothly up and down as the capacitor to ground charges and discharges. This signal needs to be buffered before it can drive anything, otherwise the signal gets loaded down and the LFO stops working.
You only really need the buffer that's attached to the depth pot. The other buffer just drives an LED that flashes in time with LFO.
If you just wanted the square wave or random mode you could probably leave the buffers off completely as the 40106 can easily drive the transistor or even LEDs.

Calamardo · July 28, 2006, 09:50:22 AM

Thanks for replie

soggybag · July 28, 2006, 10:09:46 AM

What are you using for a power supply?

slacker · July 28, 2006, 10:49:16 AM

It runs off 9volt DC. I use a wallwart but a battery should work, don't know how long it would last though. I'll have to measure the current draw.

Stompin Tom · July 28, 2006, 12:23:07 PM

Cool design and cool sounds, Slacker.

Did you say vero? For this thing? That'd be awesome, but I imagine it'd be huge.

slacker · July 28, 2006, 01:43:46 PM

yeah I built it on vero, I did it on 3 separate boards so they're not too big. It would be fairly big on one board but seen as how you need a box big enough for 6 pots, 4 switches and a stomp switch I don't think size matters too much.
I really must get the vero layouts finished and posted here, I'll try and make time next week.

Stompin Tom · July 28, 2006, 02:10:13 PM

Wow. That'd be great. Thanks in advance!

Jack · August 02, 2006, 08:39:17 AM

I am on the edge of my seat.

R.G. · August 02, 2006, 09:22:34 AM

Quotethose 2 BC109's are acting as variable resistance to ground right? why are there 2? can you just use one of both? or use difefrent modulations on both?

Not exactly. The BC109's are acting as a differential current source. What makes this thing tick is that if you replace the diodes with resistors and ignore the BC109 modulation scheme, it's a fairly common active filter with three modes. The diodes act as variable resistors. The input to the gains stage comes from the center of the diodes, the input and output are mixed at the junction of the two 2.2nF caps. The BC109s only supply a variable current to change the diode resistances.

QuoteThey basically give you mirrored image of the voltage, so that once they go thru the diodes (which also respond mirrored to the mirrored voltages), they cancel each other out. This is mainly a concern if youd had high-frequency oscillator modulating the VCF or some "thumping". This is a standard thing to do in a "synth-world" to keep the control voltages out of the audio path.

Correct. The diodes are set up so that one side (the right hand BC109) is fixed, and as the input control voltage increases, it causes more current to shift to the left hand BC109. This forces more current through the diodes. With no control voltage, most of the current flows in the right hand BC109 and bypasses the diodes.

The voltages on the diodes are symmetrical about the sensitive input to the gain stage, and cancel out as noted. Same idea as was used in the Magnatone vibrato amps.

Tuemmueh · April 20, 2008, 07:01:35 AM

Did you ever finish the veros?

or somehow continue developing the circuit? Might be very interesting ...

dellamorte · April 20, 2008, 10:20:49 AM

bump:)

slacker · April 20, 2008, 10:31:27 AM

Sorry I never finished the vero layouts, I don't think I've even got them any more. They were a complete mess anyway, because I built different sections of it at different times.

dellamorte · April 20, 2008, 10:41:11 AM

awww

i was really looking forward to this one, i dont imagine im the only one either

dirk · April 21, 2008, 02:22:06 AM

Every filter design can have separate low band and highpass inputs, just like it can have separate low and highpass outputs.
Its just adding and subtracting the inputs and outputs.

Cool thing is to use it as a spectral crossfader. And you can use different filter outputs to change the filter curves.

Lots of experimental fun to have with those filters. And its a shame not more synths have this.

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My new design, the slackfilter