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 Forum index » DIY Hardware and Software
Roland filters clones with LM 13700
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halfshavedyaks



Joined: Jul 12, 2018
Posts: 28
Location: UK

PostPosted: Fri Jul 13, 2018 4:06 am    Post subject: Reply with quote  Mark this post and the followings unread

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: 36
Location: Coventry, UK
Audio files: 1

PostPosted: Mon Jul 16, 2018 11:25 am    Post subject: 703a rez test Reply with quote  Mark this post and the followings unread

Short answer is yeah, looks like the res setting has a fairly dramatic effect on the final level. Here are some level measurements, done in ye olde Cool Edit Pro. Done for min, half res and full res at a middling-ish cutoff setting. First number is sample value.

Code:

hpf
min   -  8430 / -11.78db
half  - 11094 /  -9.4db
max   - 26808 /  -1.73db

bpf
min   -  4728 / -16.7db
half  -  4957 / -15.64db
max   - 11451 /  -7.81db

lpf
min    - 5624 / -15.23db
half  - 11015 /  -9.01db
max   - 30395 /  -0.39db


Was fairly careful about my clone, pored over pictures of the original circuit board and track layout but could never exactly confirm the wiring of the res pot against the schematic, so I went with what the drawing said.

Anyone got a real System 700 to check this against? Thinking that maybe someone on the analogue-heaven mailing list might have one.

Alternatively just short out that side of the pot, reckon you'll get more resonance if anything.

Have attached my boring test file too.


703ab-res-test-hpf-bpf-lpf.aif
 Description:
703a rez test

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 Filename:  703ab-res-test-hpf-bpf-lpf.aif
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ringroad



Joined: Feb 28, 2012
Posts: 36
Location: Coventry, UK
Audio files: 1

PostPosted: Mon Jul 16, 2018 11:37 am    Post subject: 703a input cap polarisation Reply with quote  Mark this post and the followings unread

On the is-the-input-cap-polarised front, here's a picture of a 703a board I saved from an ebay auction (the board went for way more than I wanted to pay for it).

I reckon the purple electrolytic on the bottom left is C8 (in the res feedback path?), and the top right is C1 (the input cap) - the latter looks polarised


703a-board-detail.jpg
 Description:
703a board detail
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703a-board-detail.jpg


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halfshavedyaks



Joined: Jul 12, 2018
Posts: 28
Location: UK

PostPosted: Mon Jul 16, 2018 1:15 pm    Post subject: Reply with quote  Mark this post and the followings unread

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: 28
Location: UK

PostPosted: Mon Jul 16, 2018 1:20 pm    Post subject: Reply with quote  Mark this post and the followings unread

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: 36
Location: Coventry, UK
Audio files: 1

PostPosted: Mon Jul 16, 2018 1:49 pm    Post subject: Reply with quote  Mark this post and the followings unread

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: 28
Location: UK

PostPosted: Mon Jul 16, 2018 2:15 pm    Post subject: Reply with quote  Mark this post and the followings unread

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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