Best Oakley filter for Maplin synth.

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eggpie
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Best Oakley filter for Maplin synth.

Post by eggpie » Sun Aug 23, 2020 1:55 pm

Hi all.
Question mainly for Tony, unless there's anyone who has tried this.
I've been rebuilding a ruined 5600S which has turned out really nicely.
A new cabinet in solid walnut, new front panel, rebuilt electronics even with new pcbs where needed.
I'm 99% finished now and really pleased with the result. It sounds better than I anticipated, especially the oscillators.
One weak point however are the two filters, which are a bit tame.

I am thinking of replacing them with Oakley vcfs to give the synth some real oomph.
To match the front panel legending for the filters, I have narrowed the choice to either a SVF or Journeyman.
Basically I need 1.Frequency pot 2.Resonance pot. 3.LP/HP switch. 4.OP vol pot.

I know this will be tricky mechanically because the SVF and JMan pcbs are bigger than the Maplin VCFs.
However, electronics wise, will these filters be happy running from the +/-14v of the Maplin PSU?
Would the signal and CV levels be compatible between the two systems, or would I need to change any
component values in the 5600S or the Oakley filters?
Looking at the specs in the 5600S build manual, there doesn't seem to be any specific voltage levels for signals or cvs.

I have also considered the Eurorack versions but this looks even trickier machanically.

Do you think this would be a viable and straightforward conversion?
Or more hassle than it's worth?

I have an SVF and a Jman already in my all Oakley modular, so I suppose I could just wire one up and see what happens.

Thanks in advance for any ideas or suggestions.

Cheers, Tom.
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Synthbuilder
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Re: Best Oakley filter for Maplin synth.

Post by Synthbuilder » Mon Aug 24, 2020 2:52 am

eggpie wrote:
Sun Aug 23, 2020 1:55 pm
One weak point however are the two filters, which are a bit tame.
Yeah, they are a bit. Having said that they are quite unusual which lends the 5600S a unique tone. But, yeah, if it were mine I'd change the filters.
I have narrowed the choice to either a SVF or Journeyman.
I'd make one of the filters a 24dB/octave filter. The SVF and TSL would make a nice combination. The problem is that you don't have the LP/HP function on the TSL. However, you could have the switch select between the TSL's internal -24dB and -6dB outputs. These are available on the CW and CCW pins of the now unused Shape pot. If so I'd increase the value of C23 and C26 to 2u2, and add a 220K resistor to 0V from each output to stop any clicks when the switch is changed.
will these filters be happy running from the +/-14v of the Maplin PSU?
Yes. You'll not need to make any changes to either the SVF, Journeyman or TSL.
Would the signal and CV levels be compatible between the two systems, or would I need to change any
component values in the 5600S or the Oakley filters?
Not too sure about that one. I don't know what the internal signal and CV levels are in the 5600S. Any changes could be done on the Oakley boards though, and it'll only be a two or three resistor changes if they are required. I would guess that CVs will be fine, but the audio level may need to be tweaked.

Once you've got it running, let me know what the VCO output levels are and I can advise on any changes if they are actually needed.
Do you think this would be a viable and straightforward conversion?
I'd do it if it were mine. But a lot of folk don't like the idea of changing things on 'vintage' synths.

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eggpie
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Re: Best Oakley filter for Maplin synth.

Post by eggpie » Mon Aug 24, 2020 4:02 pm

Hi Tony.
Thanks for your reply and info. Great suggestion with the TSL. That pcb is even bigger though, not sure if I have the room for it!
I'll still try though, I have one of those too.
Like I said, I may just try each filter by wiring them in remotely (powered by the 5600S) and see how they sound.
Signal level-wise, the oscillator outputs are hard wired to mixers 1,2 and 3 so they are easy to attenuate.

I'll let you know what happens when I give it a go. I just wish it was as easy as building 5U modules; the wiring to and from ridiculous veropins is not easy!

Thanks again, Tony.
Rgds. Tom.

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Pav
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Re: Best Oakley filter for Maplin synth.

Post by Pav » Tue Aug 25, 2020 9:51 am

I am interested in any mods recommended as I have my 5600s PCBs out of the case for re capping and rewiring..starting to wish I hadn't started. Lots of wires. Anyway I'm calibrating vcos now on the bench without mixer. Voltages according to manual "could" go as high as 12v if key modulation is patched into a hot source
Mostly I see 5.2v during calibration. My question is I'm seeing a one volt overshoot on the rise of sawtooth and also similar on reverse ramp. Is this a feature of design that doesn't im pact clarity or an artifact of testing or a degradation in a resistor perhaps that is fixable? My question can be treated as rhetorical as I do not wish to hijack the thread. But any feedback welcome. I'm envious of your patch matrix I regret not purchasing in hindsight, but by using euro sockets for patching, I can leverage any of my Oakley filters.
rgds Pav

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Re: Best Oakley filter for Maplin synth.

Post by Synthbuilder » Tue Aug 25, 2020 10:36 am

Pav wrote:
Tue Aug 25, 2020 9:51 am
I'm seeing a one volt overshoot on the rise of sawtooth and also similar on reverse ramp.
The overshoots could be one of two things. Firstly, your scope may produce a brief spike, or even ringing (which is a very brief burst of damped oscillation), if the rise or fall times of input signals are fast enough. Secondly, and I think this is the more likely, that the overshoot is caused by the triangle to sawtooth generator in the VCO. It uses an analogue switch, the 4416, which is identical to the more common 4016 but the switching logic is inverted on two of the stages. When the switch changes state the internal control signal will bleed onto the analogue parts (it's called charge injection). This will often cause a small voltage spike on the switch's output and input lines. It can be quite a nuisance. But in this case, it's all part of the charm of the synth.

Which is more I can say of the internal wiring in those things. :bang:

I think I have said this before - but replace all the zener diodes in the power supply to stop them from taking out the whole power supply.

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eggpie
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Re: Best Oakley filter for Maplin synth.

Post by eggpie » Tue Aug 25, 2020 1:36 pm

Hi Pav/ Tony.
I also had a sawtooth anomaly in just one of my Maplin oscillators. Turned out to be half of the CD4416 was shot and wasn't shaping the sawtooth correctly. Luckily I have a small stash of them. Weirdly, it still sounded ok.
Pav.... I may carry on this subject elsewhere, perhaps in the 'music tech diy section, as this post is becoming more Maplin than Oakley!
Hope your project is going well, I have a couple of minor issues too, perhaps we can put our heads together.
I'll let you know if I post elsewhere.
Cheers. Tom.

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Re: Best Oakley filter for Maplin synth.

Post by Pav » Tue Aug 25, 2020 3:19 pm

Thanks Tom,Tony,
A dedicated thread for 5600s would be useful yes.
Re zeners
I had replaced the original zeners with fresh zeners before you mentioned this. Not because they were faulty. Never had any issues with them, presumably as I never shorted the rails. There is a sensing circuit that shuts the rails down if too much current is drawn, but I'm guessing that's not sufficient.
rgds Pav

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