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halfshavedyaks
Joined: Jul 12, 2018
Posts: 65
Location: UK
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 Posted: Fri Jul 13, 2018 4:06 am Post subject:
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| ringroad wrote: | I had a bit of a faff with the resonance pot on my stripboard version of the 703a/b, but my own clone PCB version seemed to behave as you'd expect.
Though this is from memory - my clone isn't hooked up at the moment, desk is chaos at the moment due to a new toy. I had a quick flick through my test recordings but it didn't really prove anything.
If I get a chance over the weekend I'll hook it up and do a test. (Though it'd probably be better if someone with a State 700 piped up)
I guess the schematic would be very similar to the original, just with an LM13700 instead of the CA3080s. One side of the dual resonance pot has an effect on the level going in, fwiw. |
thanks!
I have seen another reference to this behavior being normal for the FC clone but why?
looking at this schematic: http://www.florian-anwander.de/roland_filters/SYSTEM700_703abc.jpg
It looks to me as if half the resonance pot is controlling the input gain, and it's turning it up as the resonance goes up which is exactly the behaviour I am unhappy about. I'm assuming that the partially dotted line between points 7,8 and 15 does not indicate a connection as that would makes no sense.
if that input stage gain variation is affecting the resonance it isn't clear to me how.
I'm also unsure why the input coupling cap isn't bipolar and if it's value is implicated in the filter response beyond that of any audio coupling cap.
I'm inclined to try a fixed gain at the input and see how it behaves. |
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ringroad
Joined: Feb 28, 2012
Posts: 38
Location: Coventry, UK
Audio files: 1
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ringroad
Joined: Feb 28, 2012
Posts: 38
Location: Coventry, UK
Audio files: 1
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halfshavedyaks
Joined: Jul 12, 2018
Posts: 65
Location: UK
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| Posted: Mon Jul 16, 2018 1:15 pm Post subject:
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Yes, I shorted the pot and replaced the 15k resistor in the feedback loop with 68k and now my FC State 700 behaves as I would expect with consistent level. If you don't replace the resistor you'll get a consistent but low level.
I can only imagine that Roland was compensating for behaviour in the original OTA that doesn't apply to a LM13700.
The good news for anyone cloning this with a LM13700 is you don't need the dual pot.
with the input cap - I accept that it IS polarised, I'm just not sure why it is OK for it to be.. |
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halfshavedyaks
Joined: Jul 12, 2018
Posts: 65
Location: UK
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| Posted: Mon Jul 16, 2018 1:20 pm Post subject:
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does the type of the 470pF caps that actually do the filtering matter to the tone? anyone tried different caps?
what type did roland use? |
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ringroad
Joined: Feb 28, 2012
Posts: 38
Location: Coventry, UK
Audio files: 1
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| Posted: Mon Jul 16, 2018 1:49 pm Post subject:
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I made some notes on this a while back - this is all from pictures/schematics, I've never actually had any system 700 bits, so aiui
version one
C4 and C6 are mylar film, 1.5nF
IC2, IC4 and IC6 are LM301
R18 and R24 are 10K
C2, C5 and C7 are required
The collector of Q2 is pulled down to ground by a 10M 1/2 watt resistor
version two
C4 and C6 are polyester film, 470pF
IC2, IC4 and IC6 are CA3140
R18 and R24 are 22K
C2, C5, C7, and 10M 1/2 watt resistor on Q2 aren't needed |
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halfshavedyaks
Joined: Jul 12, 2018
Posts: 65
Location: UK
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| Posted: Mon Jul 16, 2018 2:15 pm Post subject:
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OK so the FC clone is version 2
I started with whatever ceramic caps I had and then swapped to wima FKP3 polyprops, but I improved power filtering and some other things at the same time so can't be sure what caused what, but I think the filter sounded smoother afterwards.
I might try mica or polystyrene, though I think the difference will be small. |
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