LO-Fusion VCF by FEEDBACK [Build UPDATE]

[strat-1]

UPDATE : We uploaded all the files needed for the build of LO-Fusion

www.feedbackmodules.com




LO-FUSION is a remake of the classic Polyfusion from the 70s with some improvements.
It is a low pass OTA VCF based on the well known LM13700 ic.
We chose to make this circuit because it's v/oct stable, and the trimmers allow to calibrate it properly.
It has a switch for 2 pole and 4 pole configuration, each with it's own character and timbre.
This design produces nice sounding filter fm results.

For the audio demos we used two oscillators slightly detuned as the sound source.

Output with switch in 4Pole position.

https://soundcloud.com/strat1/lo-fusion-4pole

Output with switch in 2Pole position.

https://soundcloud.com/strat1/lo-fusion-2pole

VCO inserted in FREQ CV input.

https://soundcloud.com/strat1/lo-fusion-fm-modulation

LPfet through VCA. ADSR to VCA and another ADSR for LPfet.

https://soundcloud.com/strat1/lo-fusion-adsr-modulation

There are 2 type of kits available :

KIT1 : 20EUR (2PCBs+FrontPanel)
KIT2 : 35EUR (Pcbs+FP, 4x2N5457, 5xminijacks, 1x50KA POT, 3X50KB POTS, 2xboard connectors, 4xCream Knobs)

COMPLETE MODULE : 90eur

KIT1 :



KIT2 :

[Swyndger]

Sounds good ... Thank you, Strat!

Kit2 is ordered as soon as I can order it

(And next feedback-module you create is the Sum-Module for your Preamps, yes? )

[strat-1]

Sounds good ... Thank you, Strat!

Kit2 is ordered as soon as I can order it

(And next feedback-module you create is the Sum-Module for your Preamps, yes? )

OK, I will start to work on the SUM module

[secrethero303]

Sounding good. Looks like you have been quite busy with all the new modules coming our way. Looking forward to making some orders when they're released.

[Swyndger]

Hey Strat!

I have some issues with this Filter, as I have with the HI-Fusion.

Both of them do absolutely nothing.

I've build both following the values on the PCB, searched for mistakes, replaced C1 printed on PCB as 47uF with a 100uF from the schematic, checked the voltage of every IC, resolderd almost everything and replaced most of the ICs. Still both filter stay quiet.

Now I started to follow the signal path, and I can measure it on U1A output (pin 6) but not on conector 2-2.

EDIT: Sorry, it seems, that I used the schematic of the HI-Fusion for the LO-Fusion, but still there is a signal un the U1A output, but no signal after R6, so I have to recheck everything again ... forget the rest of the post!

(((( So this is nonsens: On the schematic U1A pin 6 is directly connected to CON2 pin 2-2, but in the PCB I cann follow the path to R3 and R6(!?!, 20k printed on PCB, 100k in the schematic), and R6 goes to CON2 2-2. But following the schamatic R6 should belong to the Frequency CV input. I am puzzled! )))


Thank your for your help, and thank you for your great modules, I've build all of them, and except these filters (and the multi-dimension, but that is another thread) all of them work wonderfull

[strat-1]

Hey Strat!

I have some issues with this Filter, as I have with the HI-Fusion.

Both of them do absolutely nothing.

I've build both following the values on the PCB, searched for mistakes, replaced C1 printed on PCB as 47uF with a 100uF from the schematic, checked the voltage of every IC, resolderd almost everything and replaced most of the ICs. Still both filter stay quiet.

Now I started to follow the signal path, and I can measure it on U1A output (pin 6) but not on conector 2-2.

EDIT: Sorry, it seems, that I used the schematic of the HI-Fusion for the LO-Fusion, but still there is a signal un the U1A output, but no signal after R6, so I have to recheck everything again ... forget the rest of the post!

(((( So this is nonsens: On the schematic U1A pin 6 is directly connected to CON2 pin 2-2, but in the PCB I cann follow the path to R3 and R6(!?!, 20k printed on PCB, 100k in the schematic), and R6 goes to CON2 2-2. But following the schamatic R6 should belong to the Frequency CV input. I am puzzled! )))


Thank your for your help, and thank you for your great modules, I've build all of them, and except these filters (and the multi-dimension, but that is another thread) all of them work wonderfull

I will do my best to write the manuals in the next days.
I was busy in developing two new projects and keep the pace with the orders in the same time.

[strat-1]

UPDATE : We uploaded all the files needed for the build of LO-Fusion VCF

[strat-1]

I've build both following the values on the PCB, searched for mistakes, replaced C1 printed on PCB as 47uF with a 100uF from the schematic, checked the voltage of every IC, resolderd almost everything and replaced most of the ICs. Still both filter stay quiet.

The changes I made in the manual :
-47u input cap to 100u cap. This will allow to pass more lows into the filter.
-R2 should remain empty. This resistor is to lower the input level.
-C10, C11 are changed from 220nf to 1uf.
-Changed R9 from 300Kto 240K for a better FREQ pot range

Now I started to follow the signal path, and I can measure it on U1A output (pin 6) but not on conector 2-2.

(((( So this is nonsens: On the schematic U1A pin 6 is directly connected to CON2 pin 2-2, but in the PCB I cann follow the path to R3 and R6(!?!, 20k printed on PCB, 100k in the schematic), and R6 goes to CON2 2-2. But following the schamatic R6 should belong to the Frequency CV input. I am puzzled! )))

Try the calibration procedure from the manual.
Also. see if the pages with board layers from the end of the manual can help you.
I've built a lot of filters using the manual and they are all working.

[strat-1]

(((( So this is nonsens: On the schematic U1A pin 6 is directly connected to CON2 pin 2-2, but in the PCB I cann follow the path to R3 and R6(!?!, 20k printed on PCB, 100k in the schematic), and R6 goes to CON2 2-2. But following the schamatic R6 should belong to the Frequency CV input. I am puzzled! )))

I forgot to ask. Did you used 2n5457 fets ? J112 are not working in this design.

[Swyndger]

(((( So this is nonsens: On the schematic U1A pin 6 is directly connected to CON2 pin 2-2, but in the PCB I cann follow the path to R3 and R6(!?!, 20k printed on PCB, 100k in the schematic), and R6 goes to CON2 2-2. But following the schamatic R6 should belong to the Frequency CV input. I am puzzled! )))

I forgot to ask. Did you used 2n5457 fets ? J112 are not working in this design.

Sorry for answering so late. I hadn't much time for SDIY recently.
Yes, I used 2n5457.
I removed R2, but did not do the other changes right now.
Calibratiom is not possible, except of the DC offset for the input as I have no output signal.

Still the main issue is the following:

I have the signal from the input (VCO) on output of U1A, a pulse with 6Vpp for example.
But after R6 (20kOhm) and so CON2 PIN2-2 the signal is gone.
Even if I bridge R6 something puls down the signal to Zero.
If I open the conection CON2 PIN2-2 the signal is on the CTRL-Board, so the issue should be un the main board.
Removing the LM13700, still no Signal.
Measuring resistance between GND and CON2 PIN2-2 I get something about 66 Ohm. As R26 ist just 100 Ohm, this should be OK. So no short.

But something pulls the signal down as soon as it reaches the main board.
And as the HI-fusion has the same issue, it isn't just a bad soldering.

So I had no time to search further, maybe in a week or so ...
... Could the 2n5457 be the problem. The eight I used for HI- and LO-fusion where the last I had. Maybe I got a bad batch. Or the 2n3906? I matched them, but who knows?

[strat-1]

Hey Strat!
I have some issues with this Filter, as I have with the HI-Fusion.

I just finished to build a couple of LO-Fusion VCFs for selling as built modules, so I used the opportunity to make some measurements and do a troubleshoot file.
I hope this helps you, and others in finishing the module.

So :

Turn FREQ on max (CW) and RESO at min (CCW)
Feed a 8vpp or 10vpp saw on input, and adjust ATT pot util you have an 8vpp saw on point 1.
You should have approx the same reading on point 2. The first op-amp works as non-inverting amplifier.
If something is wrong in this step, check U1 (UA741) for defects, shorts etc.
You should have approx the same reading on point 3, but inverted. Each LM13700 op-amp works as an inverting amplifier.
If something is wrong in this step, check u6 (LM13700).
You should have approx the same reading on point 4. Each 2N5457 work as a buffer.
If something is wrong in this step, definitely the 2N fets are not working.
Do the same for each LM13700 stage 5,6, because they work as inverters, you should have inverted waveform.
You can't have a reading on point X, because on inverting op-amp circuits, the negative input goes to 0V.
Check if U4 is working by taking a reading in point 7.

Now is time to check the freq control circuit.
By turning the FREQ pot from min to max, check if you -3.6Vdc to +600mvdc on point 8.
By turning the FREQ pot from min to max, check if you -9.8Vdc to -9.2vdc on point 9.
The LM13700 OTAs are controlled in current, but having this slight dc variation on point 9, shows you the transistor circuit is working.

[Swyndger]

Hello Strat! Thank you, very, very much!

Following your troubleshoot instructions:
I checked the singal on TP1 and 2 and it was good.
But no signal on TP3.
Changed the LM13700 to some of another batch, but no difference.
Checked (again) the voltage of U6, and (again) the values of the resistors arround U6 etc. but everything is fine. Had allready replaced the FET.
So I was just about to give up.

But then I thought I could check the frequence control as described, too.
And I had -10,3V on TP8 the whole time. Nothing changed while turning the FreqPot.
Replaced U2, but no difference.
Output of U2A seemed to be Ok, cand remember the exact voltage, but it changed while turning the FreqPot.
So I checked (again) all the values of the components around U2/Frequency Control ...
... and I saw what stupid mistake I have made: R14 was 51k (like R15) and it should be 5k1! So the whole filter was shut the whole time, and so I never could hear or measure anything.

Replaced R14 with the right one, and the filter works.

Tomorrow I will replace it in the HI-Fusion, too - as I've build both togerther, I made the same mistake twice -, make the changes you have made in the build manual, calibrate them and then I will have two new filters.

Once again: Thank you, Strat!

[vibralux]

God damn it. I just received my Lo/Hi Fusion and Fet Filters and these sound sooo different than anything else I have. And I have plenty: Belgrad, Dual Borg, Korgasmatron, Filter 8, Wasp, RK6. This is truly unique.

[humax5600]

Finished today.
Works perfectly and sounds fantastic !

DSCI1483.jpg (91.38 KiB) Viewed 1970 times

[strat-1]

I think the made a movie demo for ENV-310 is also relevant as a demo for LO-FUSION VCF

"ENV-310 fed into LO-FUSIONs freq input and cv input of VCA-100. In second part of the movie LFO-10 triangle output it's modulating the other freq input of LO-FUSION VCF. Signal source is the square output of TWO59 VCO.
Ambience is made with 1-BIT sigma delta DELAY.
VCA-100 is not in the system, is a work in progress module made on a testboard, soon to be released."



Modulating LO-FUSION VCF both FREQ and RESONANCE cvs with one LFO-10.

[23Gears]

I am building this at the moment. On page 7 of the build manual, it seems the placement of the two resistors 240kOhm and 300kOhm are swapped compared to the silkscreen o the board. For me, R17 is marked 300K and R9 240K. I assume the silkscreen is correct and the manual incorrect?

[Kosmikos]

I am building this at the moment. On page 7 of the build manual, it seems the placement of the two resistors 240kOhm and 300kOhm are swapped compared to the silkscreen o the board. For me, R17 is marked 300K and R9 240K. I assume the silkscreen is correct and the manual incorrect?

Ha, just ran in the same issue, and the schematics seems to indicate that the silkscreen is correct and the build manual has them swapped.

[Kosmikos]

I finished the build and completed the calibration, I really like the character of this filter. It brings things alive especially with a bit of resonance.

However I’ve noticed a fair amount of input bleeding through the filter when closed, more so when the input knob is set to a high value. Do they all do this?