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Discrete Ladder Filter stopping VCOs outputting
MUFF WIGGLER Forum Index -> Oakley Sound Systems  
Author Discrete Ladder Filter stopping VCOs outputting
eljay
Just finished testing and calibrating my Discrete Ladder Filter, I tested and calibrated it using an Oakley RPSU as a bench power supply, everything seemed fine. However, when I put the module in one of my Oakley racks and powered up I discovered that the oscillator output from my VCOs was no longer present, I mean nothing coming out and yet LEDs were lit where present signifying power and by providing an audio input to the filter from a different rack, the filter itself is working.

I have checked cables, voltage supplies, all the obvious things and have confirmed that with the Discrete Ladder FIlter disconnected the VCOs work, with the DLF plugged in to the Dizzy the output from the VCOs stops. I'm assuming there is some sort of grounding problem but I would appreciate some help in tracking it down. I have already checked the +15 and -15V and 0V outputs on the dizzy seem unaffected when the DLF is connected. Many thanks in advance.
Synthbuilder
I've got no idea... help

But I wonder whether the outputs stop or they've gone super or subsonic. Although the DLF doesn't connect to the Oakley Bus to effect the pitch of the VCOs so that's still puzzling.

I'm not sure how a problem with the 0V (grounds) could cause it either. With the power off it might be worth checking continuity between the panels of the SVCOs and 0V on the PSU - they should be connected and this should not differ when the DLF is plugged in.

Tony
eljay
This is definitely giving me something to think about. The behaviour is quite consistent, as soon as the module is plugged in to the dizzy it stops the output from any of the three VCOs in the same rack, I haven't tested the impact on any of the other modules yet. I can't identify an obvious grounding issue although there is quite an audible thump when a cable is plugged into the output socket.

Powered from an independent source the filter appears to work but maybe/perhaps not correctly or what I was expecting. If there is no power to the filter the input signal can still be heard unmodified from the output socket, is this correct? If I remove the socket 4 interconnect cable this feature disappears so I'm assuming the thru connectivity is on the main board.
eljay
A further update : Without power to the module I have input a sine wave to the input socket, I can see this on my scope on both the +ve and -ve power rail pins and the 0V pin, not the panel ground. Clearly a major short somewhere, I'll keep looking.
Synthbuilder
Just a thought... have you checked the power supply rails are reaching the DLF correctly? That is, measure the voltages on any of the op-amps on the board and see if they are +15V and -15V at pins 8 and 4 respectively. Measure with respect to 0V on the Dizzy or PSU. Then measure again with respect to pin 3 of U3 - which also should be 0V but might not be.

Tony
eljay
All power rail testing shows -15V at pins 4 and 15V at pins 8, compared with both 0V and U3 pin3.
I can run the filter and an SVCO simultaneously off my RPSU bench supply, no impact to the SVCO when the filter is connected. The power transistors do get pretty hot despite having small Filtrex heat sinks attached, but perhaps that's to be expected.
The filter appears to work functionally, i'e CV inputs do what they are supposed to, resonance pot works, the Filter self-oscillates.
So now I'm wondering if there's a problem with my rack PSU that I haven,t noticed before. I have a total of 14 modules plus an SE330 and HVM Voice module powered off a PA-30 configured PSU, is it possible I'm exceeding the power threshold of the PSU?
UPDATE:
I have demonstrated to myself that I have hit a PSU limit, if I remove an SVCO (or I assume any other module) the Filter works alongside the other modules in the rack without affecting them. If I plug the SVCO back in, the problem re-appears.
As an aside I have noticed there is a repetitive low -level noise coming from the DLF output socket either with or without any other inputs connected. This low-level noise can be minimised by turning the Drive up but never goes away completely. Is this expected behaviour?
eljay
In Conclusion:
I've added up the value of the modules and HVM/SE330 unit powered from this PSU (I probably should have thought more about this before). All in all the total comes to 843mA +ve rail and 665 mA -ve rail. Taking out an SVCO brings this down to about 800mA +ve and 667 -ve rail, give or take. The PSU with a PA-30 is rated at up to 780mA per rail so I guess I'm right on the limit of the PSU and the one additional module has taken it over the top.

I have a second rack so could look to balance out the load more efficiently across the two racks, I do seem to have a number of power hungry modules in this particular rack (DLF and Flanger take 155mA between them on the +ve power rail). Each rack has 20U of module space plus an additional IU high 19" wide module.

I have a spare RPSU PCB for just this eventuality so will build that and power the HVM/SE330 rack unit from that, saving about 220mA+ve and 190mA -ve.

Other than that, is it possible to draw any more power from the PA-30 using the Oakley PSU?
Synthbuilder
I'm not sure it's a power supply limit if the voltages are still up at +/-15V. One would normally expect the voltages to start to drop as the PSU goes into current limit.

Can you measure the voltage across each of the two big resistors on the PSU board. If it's around 0.6V on either one then you are indeed reaching the current limit.

Regarding the DLF - it may be helpful to find out how much current that one is taking. It is quite a greedy module, taking 80mA from the +15V and around 55mA from the -15V. It's a good check that all is working well if you get these sorts of figures.

When you say low level noise, do you mean hiss? If so, that is to be expected as the ladder topology is not a low noise circuit. It shouldn't be normally that noticeable with any signal going through it though.

Tony
Synthbuilder
Ah, we posted across each other. Yes, the power supply is in current limit then. Definitely get the SE330 and HVM on their own supply.

eljay wrote:
Other than that, is it possible to draw any more power from the PA-30 using the Oakley PSU?


No. The limit is set by the abilities of the PA-30. The only way you can get more from the Oakley PSU is to use a mains transformer. It's what I do but you really need to be happy with high voltage wiring to do this.

Tony
eljay
Thanks for the feedback Tony, I should have planned a little better!

Interesting to observe the behaviour of the oscillators as the current limiting comes in. At least I'll know where to look if something similar happens again.

The low-level noise is more a hum than a hiss and is a bit like hmm hmm hmm every couple of seconds. You wouldn't notice it if the Filter were in use and you need the volume on your amp up to hear it but I can imagine if you were recording the output from the filter and there was a quiet patch this may be an issue.

I would be happy to try out a mains transformer solution if using the PA-30 becomes a limiting issue, I know for H&S reasons you don't provide detailed advice but if you happen to know any websites that could assist me in understanding what is involved please PM me.

PS - Looking forward to some new modules, I've got room for a couple more once the power issue is resolved.
eljay
A test of the Discrete Ladder Filter, a simple Square Wave input with a bit of VRG modulation on the Filter and attenuated 1V/Octave input to CV1 from the midiDAC. Manual twiddling of the knobs to see what it does - I like it!

[s]https://soundcloud.com/user-531906894/dlf-sample[/s]
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