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LO-Fusion VCF by FEEDBACK [Build UPDATE]
MUFF WIGGLER Forum Index -> Music Tech DIY  
Author 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.

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

Output with switch in 2Pole position.

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

VCO inserted in FREQ CV input.

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

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

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

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 wink

(And next feedback-module you create is the Sum-Module for your Preamps, yes? wink )
strat-1
Swyndger wrote:
Sounds good ... Thank you, Strat!

Kit2 is ordered as soon as I can order it wink

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


OK, I will start to work on the SUM module smile
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 smile
strat-1
Swyndger wrote:
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 smile


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
Swyndger wrote:

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

Swyndger wrote:

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
Swyndger wrote:
(((( 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
strat-1 wrote:
Swyndger wrote:
(((( 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
Swyndger wrote:
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.

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