Soundlab - LFO High freq bleed to output

[Loaded]

Hi everyone,

This is my first post on the forum. I finished Soundlab board and wired all the pots to a cardboard box to test everything and get my head around the custom panel design and patches. Everything appears to be working fine apart from few minor issues, and I am already very proud of it.

While good with soldering iron, meter and oscilloscope, I know very little of electronics theory. Just started to learn about RC circuits in more detail, and I am not sure if this behavior I am about to describe is normal by design or not. When I first heard sounds out of my Soundlab, I got very impressed and in turn extremely picky about it but I do understand it was not meant to be uber hi-fi modular synth device, so I gotta ask if this is behavior is normal or not to get that out of the way.

Any help or pointers would be greatly appreciated!

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1) LFO bleed problem

When I switch LFO to high mode and turn frequency knob all the way up, it gets very high in frequency and I can actually hear it's sound loud and clear, with all the mixer knobs turned fully down. This is happening with any cable / amplifier configuration I tried so it is got to be something inside the circuit. I am currently running it of a good bench power supply, but I get the same with two 9v batteries.

If I place my scope probe near the LFO's PCB section without even touching it, I can see this waveform, with peek of around 15mV and it seems to be strongest around the wire connected to S18-3 point on the board. It looks like a tiny brother the IC7 pin-5 waveform. I unplugged my audio cable and moved it around in space and noticed that this same sound gets extremely loud when I place my audio cable jack near the same wire (unplugged). However, when cable is plugged to the output jack this wire is far away from it and I can still hear the sound equally loud.

Only knobs that seem to affect this sound are filter Cut-Off Frequency and Main Output. If cut off frequency knob is turned all the way down, nothing can come through (even oscillators cranked fully up via mixer) but this sound does get through and it is even louder. When I start moving this knob clockwise, at about 5% or so i can hear a 'thump' sound (similar when stomping on some true bypass guitar pedals, both in LP/BP modes) and high pitch sound gets a bit quieter and bit lower in frequency. Otherwise Cut-off works normally. Lastly if I turn the Output knob fully clockwise to 100% this sound gets very quiet, basically to a level I could live with. This is with all the mixer knobs turned fully down, but the sound I am describing is as loud as an oscillator set to 40-50% via Mixer if Cut-Off is say at 50% and Output jack not turned fully up. If I turn Output jack to 100% then this sound gets down to 10%-20% loudness.

I understand that primary purpose of LFO is to be Low Frequency Oscillator and maybe not to be pushed to audible frequencies range at the first place? Or is my Soundlab troubled by a bug somewhere? Whats your experience with this? I can try to upload some sound samples and oscilloscope photos if that would help you understand.

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2) LFO has big frequency gap between Low and High

Is this normal? Is there any way to get around this. I found myself wanting frequencies that are in this gap the most, like slower vibrato and similar efects. Highest low is too low, and lowest high is too high on my Soundlab. Is this normal too? If yes, are there any instructions online on how to get around this problems? I could not find any. Will it help if i do 'Plus' mods?

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3) MIXER + Patching

I would like to fully patch it, and I got all the patch points noted on MFOS website already running sweet. Ignored bunch of routing switches and I do understand all the points on the schematics as described by Ray. However, I was also wondering if it is possible to patch it in such a way to bypass the filter and plug say OSC2 Square straight into VCA, while leaving OSC2 Saw going to the filter. If yes, please show me which point would be good for OSC to VCA-IN on the schematic / pcb / w. diagram.

Or maybe have one 'external sound' patch point for VCA (to get tremollo type efects) and one for Filter (autowah type efects). Is this possible?

I figured where to patch Mixer inputs (mixer knobs), but Mixer is by default connected to the Filter, and I failed to find any helpful info on MFOS website or this forum about this.

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English is my second language so I hope I did not make things too confusing. Many thanks to Ray and all on the forum for sharing your thoughts and knowledge.

Peace, out
-Loaded

[jordroid]

Hello, the LFO bleed through i think is not normal behavior, mine does get up into the audio range, but you can only hear it if it is modulating something and the mixer pots are turned up.

Also, the slow and fast rate settings on mine do have a little gap, but it is very small. Meaning max frequency when set to slow range is very similar to minimum frequency on high range.

I am sorry that i don't have a ton of practical help for you, as i have just finished my soundlab, but i thought i would let you know that the symptoms you are describing are abnormal.

The LFO wiring is a bit tweaky, hopefully it will turn out to be something not too hard to track down, i wish you luck and will help out more if there is anything i can do.

[Loaded]

Hi, thanks for the reply m8. It does seem like LFO bleed is normal, because I did check all the values, even did all the "plus" mods in the meantime, and while noise got more finer sounding and LFO got referenced to ground, this bleed did not go away. Bleh, I guess if kept on lower frequencies it does no harm.

I also managed to get rid of Cut-Off frequency "thump" sound by increasing R42 value a tiny bit up to 7.5k. However this resistor was removed with Plus mod, so this sound is back again and there is a little gap of silence (no useful sounds) at the lowest cut-off range again.

Re LFO rate, there is a big rate gap on mine between two modes, so that's something I will need to look into. Probably can experiment with few cap values or something. Hope I'll figure it out.

Meantime, I also think I answered my third question, and found the spot for VCA input reading through schematic pages. Mixer is just a set of 3 "voltage dividers" going into a filter, so I think I will take those 3 pots outside this fixed arrangement, together with LFO output and Envelope output pots, and just make them as a separate section and fully patch their inputs/outputs, make them sort of as attenuators for both signal and control voltage levels. That way, I can have 5th mixer pot if I'm not using Envelope and LFO, or have few different envelope output levels if I choose to assign them that way. My soundlab is going to be fully patched drone machine so I think this will be a good call.

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As far as Plus mods go, I did them all, and in general they are all useful, but I almost think I liked original Soundlab core sounds a bit better. It sounded more like a raw bad-ass synth with lots of character. It is quite likely I'll revert few mods back. I'll mention some impressions here while at it, because it may help someone around this decision.

With Plus mods, ramp oscillators sound much more hi-fi when going inverted and about ground now, and filter seems little less wild. Noise is also much finer sounding, not as grainy as before. Envelope and LFO go about ground what is a keeper and very useful for modulation.

I'll go back to old sync method for sure, because it was more musically useful imo. I like Osc2 to go very deep square with PWM going, what is not possible with new sync where osc2 must be above osc1 in frequency to be audible. Old sync could give me some warbly cool sounds (with Osc1 frequency swept above Osc2). Also PWM resistor change is not needed, it just tames the modulation and makes it again sound less extreme and more polite.

Hope this info helps someone.

peace,
Loaded