heptaphasis - 3HP 7-phase LFO

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charonme
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heptaphasis - 3HP 7-phase LFO

Post by charonme »

Last edited by charonme on Sun May 05, 2019 12:17 pm, edited 1 time in total.
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Heptaphasis - DIY 3HP 7-phase quadrature sine LFO
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charonme
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Post by charonme »

PCBs ordered at pcbway! :hyper:
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Post by oberkorn »

thank you. will get a couple of these next time I order PCBs
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Post by charonme »

:chug:
but wait till mine arrive so that I can test whether it even works :)
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Post by cackland »

Nice work. I've recently been looking at circuits that phase shift an incoming LFO, rather than generate.

Was thinking of using 2164 vca, but considering the LM13700.
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Post by joem »

charonme wrote::chug:
but wait till mine arrive so that I can test whether it even works :)
Do let us know! I'm intrigued!
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Post by cackland »

Any update on this project? Working / improvements / redesigns...
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Post by charonme »

still waiting for the PCBs from china, I will update as soon as they arrive
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charonme
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Post by charonme »

tonight I'm soldering

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Post by cackland »

Nice. Hopefully no oversights and everything goes to plan :)
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Post by charonme »

well the THT schottkys barely fit in their holes, the THT solder pads seem too small to comfortably solder the parts and the SMD solder pads are larger than eagle led me to believe, so they cover parts of the silkscreen and some letters are not that easy to read

also this looks dangerously close together: Image
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Post by guest »

the silkscreen seems consistently off. perhaps an error with the manufacturer? you should have a look at your gerbers in another program to see what they look like. you can upload them to oshpark and they have a free view that lets you look at them before purchase.
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Post by charonme »

well... it doesn't work, the oscillations die off when lowering the frequency with the pot. using polystyrene THT caps instead of the 470nF SMD X7R helped only a little
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Post by cackland »

At what frequency do the oscillations stop?

Wonder if the yusynth version had the same problem at the same frequency.
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Post by ersatzplanet »

No room for the 8th phase?
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Post by BugBrand »

charonme wrote:well... it doesn't work, the oscillations die off when lowering the frequency with the pot. using polystyrene THT caps instead of the 470nF SMD X7R helped only a little
It is an unfortunate thing with such quadratures - I've encountered such frustrations and believe that in part it is down to how clean the board is (flux could lead to stray leakage). I know that when I was prototyping I had to do a lot of cleaning - so next tests I'm going to try with water-soluble flux (easier to remove than regular 'no clean').

I'd be interested to hear comments on other possible issues...


PS - I haven't seen uses like the bits around TR8 & TR9 - are these meant for starting up - pushing things high so oscillations start?
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Post by guest »

are T8,9 backwards in your schematic? shouldnt it be the collectors that go to the summing junctions?
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My take

Post by frijitz »

My switchable 6/8 phase LFO is here:
http://www.electro-music.com/forum/topic-19841.html
and also at my site.

Runs over a range of 0.02 Hz to beyond 3 kHz. It’s configured as a limit-cycle oscillator (damping on each stage) rather than as an oscillating filter.

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Post by nigel »

ersatzplanet wrote:No room for the 8th phase?
Asked and answered in the original thread. There's no room for another socket on the front panel.
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Post by charonme »

guest wrote:are T8,9 backwards in your schematic? shouldnt it be the collectors that go to the summing junctions?
I don't know much about transistors, so maybe, but it works well on the breadboard and in falstad simulation like this.
nigel wrote:
ersatzplanet wrote:No room for the 8th phase?
Asked and answered in the original thread. There's no room for another socket on the front panel.
maybe in the next revision with a 4hp panel and two columns of output jacks. Or a jumper on the pcb could turn the frequency CV input jack into the 8th phase output
BugBrand wrote:I haven't seen uses like the bits around TR8 & TR9 - are these meant for starting up - pushing things high so oscillations start?
yes, the capacitor-transistor design comes from guest. The circuit around T8 introduces a rough nanosecond disturbance to which the filter responds with starting the oscillations quickly and the circuit around T9 increases the frequency to max for a couple of seconds so that the oscillations stabilize themselves quickly

I'll be debugging it this weekend
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Post by frijitz »

charonme wrote:
BugBrand wrote:I haven't seen uses like the bits around TR8 & TR9 - are these meant for starting up - pushing things high so oscillations start?
yes, the capacitor-transistor design comes from guest. The circuit around T8 introduces a rough nanosecond disturbance to which the filter responds with starting the oscillations quickly and the circuit around T9 increases the frequency to max for a couple of seconds so that the oscillations stabilize themselves quickly
In the work I have done on this system, if it is running at a high frequency and you quickly turn the frequency down, then there is no settling time required. The signals are always at the correct relative phase and just start running slower. So it seems to me all you need for startup is a voltage pulse at the CV input. Sure, drive it with a transistor switch if you want, but I say try just a series RC from V+ to the base of the input transistor.

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Post by charonme »

frijitz wrote:In the work I have done on this system, if it is running at a high frequency and you quickly turn the frequency down, then there is no settling time required. The signals are always at the correct relative phase and just start running slower. So it seems to me all you need for startup is a voltage pulse at the CV input. Sure, drive it with a transistor switch if you want, but I say try just a series RC from V+ to the base of the input transistor.
if my LFO had a high enough max frequency then yes, this would work, but my LFO's top frequency is just <20Hz, so without the pulse through T8 it would look like this: Image
So it appears that the frequency rise alone is insufficient to reach the stable amplitude (set to +-5v)
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Heptaphasis - DIY 3HP 7-phase quadrature sine LFO
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Post by guest »

on your previous design you had diodes in series with the transistors. maybe that would help? perhaps try removing R38 and seeing if the low frequency performance improves.
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Post by frijitz »

charonme wrote:
frijitz wrote:In the work I have done on this system, if it is running at a high frequency and you quickly turn the frequency down, then there is no settling time required. The signals are always at the correct relative phase and just start running slower. So it seems to me all you need for startup is a voltage pulse at the CV input. Sure, drive it with a transistor switch if you want, but I say try just a series RC from V+ to the base of the input transistor.
if my LFO had a high enough max frequency then yes, this would work, but my LFO's top frequency is just <20Hz,
Ah, I see.

But I have a couple of comments/questions:

Why are you using such a huge value for the integrating caps? Yves uses 10nF and I usually use 1nF. (And where do you even get a 470nF polystyrene part?)

If you use a smaller value then you can still bring the operating freq range down <20Hz by adjusting R51. (Note that Yves has a trimmer for that.) Then you would be able to start up with a pulse at the input to push the system into proper quadrature.

This design has pretty low distortion. I just measured it again and found the 3d harmonic down by ~40db and nothing else significant. If you can tolerate a bit more distortion, then you can lower R27 to provide a larger positive feedback.

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Post by charonme »

I have 20Hz as the top end of my frequency range. I want it to go down to ~20 minutes

wait why did I say polystyrene :( Sorry for the mistake, I have these polyester caps

I need very low distortion because the 45° (+n*180°) is created by adding 0° and 90°, this wouldn't work as well if the added shapes were not exact sines

btw what is the function of R27 (or that whole negative feedback loop)?
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